Controlled synthesis of NiCo2S4 hollow spheres as high-performance electrode materials for supercapacitors

Controlled synthesis of NiCo2S4 hollow spheres as high-performance electrode materials for supercapacitors

Accepted Manuscript Controlled synthesis of NiCo2S4 hollow spheres as high-performance electrode materials for supercapacitors Kangqiang Liang, Weidon...

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Accepted Manuscript Controlled synthesis of NiCo2S4 hollow spheres as high-performance electrode materials for supercapacitors Kangqiang Liang, Weidong He, Xiaolong Deng, Hong Ma, Xijin Xu PII:

S0925-8388(17)33911-7

DOI:

10.1016/j.jallcom.2017.11.153

Reference:

JALCOM 43846

To appear in:

Journal of Alloys and Compounds

Received Date: 11 May 2017 Revised Date:

20 October 2017

Accepted Date: 12 November 2017

Please cite this article as: K. Liang, W. He, X. Deng, H. Ma, X. Xu, Controlled synthesis of NiCo2S4 hollow spheres as high-performance electrode materials for supercapacitors, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.153. 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.

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Controlled synthesis of NiCo2S4 hollow spheres as high-performance electrode materials for supercapacitors

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Kangqiang Liangb, Weidong Hea, Xiaolong Denga, Hong Mab, Xijin Xua*

School of Physics and Technology, University of Jinan, Jinan, P. R. China

b

School of Physics and Electronics, Shandong Normal University, Jinan 250014, P. R.

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China ∗

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Corresponding authors: (Xijin Xu) [email protected]

ACCEPTED MANUSCRIPT ABSTRACT: Nickel cobalt sulfides (NiCo2S4) with different morphologies have been synthesized and used as pseudocapacitive materials. Particles, sheets and hollow spheres can be easily synthesized with different components of reagents or chemical

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reaction time. Benefits from the hollow structure, the electrolyte can diffuse easily into active material and result in a lower resistance. The electrochemical performances of NiCo2S4 depend on the morphologies, among which the hollow

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NiCo2S4 spheres exhibit the highest specific capacitance of 756 Fg-1 at 1 Ag-1, and

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longer cycle stability. This method may give some suggestion to synthesis of hollow particles and enhancement of electrochemical capacitors.

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Keywords: NiCo2S4, Supercapacitors, Hollow spheres, Solvothermal

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1 Introduction Supercapacitors have received increasing attention as next-generation energy storage devices because of their high power densities, fast charge-discharge rate, long cycling life, and safe operation [1-3]. It is a kind of energy storage between battery

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and capacitor, which has better cyclicity, higher specific power than battery, and larger capacity than traditional capacitor [4]. Supercapacitors are generally divided into two classes according to the mechanism of electrical storage, i.e. electric double-layer capacitors

(EDLC)

and

faradaic

pseudocapacitors.

Compared

to

EDLC,

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pseudocapacitor has much higher capacitance, in which metallic oxides [5-9] or sulfides [10,11,12] are considered as an important category of electrode materials. A

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series of methods, such as electrolytic deposition [13], chemical vapor deposition [14], hydrothermal or solvothermal method [15], sol-gel method [16], have been applied to efficiently fabricate the supercapacitors’ electrode materials. Recently, Lou et. al synthesized NiCo2S4 ball-in-ball hollow spheres with specific capacitance of 1036 Fg-1 at 1.0 A g-1 and high energy density of 42.3Wh kg-1 at a power density of 476

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Wkg-1 [17]. Wang et al. prepared NiCo2S4 nanoplates using Ni(OH)2 and Co(OH)2 as precursor and got a specific capacitance of 437 Fg-1 at 1 A g-1 [18]. Dong et al. controlled the morphology of NiCo2S4 nanoparticals by changing the ratio of

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component of solvent and achieved high specific capacitance of 1048 Fg-1 at the current density of 3.0 A g-1 [19].

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Hollow micro-structures deliver excellent performance due to their intriguing structural features, such as large surface area, low density [20]. A lot of efforts have been devoted to developing methods for the rational synthesis of various hollow structures. Typically, NixCo3-xS4 hollow nanoprisms [21] and Ni-Co sulfide nanoboxes [22] were synthesized and manifested enhanced electrochemical performance. The formation of onion-like Ni based sulfide particles has caused one’s attention, not only the structure but the transformation methods (ion exchange) [23]. Most of the researchers synthesized transition metal sulfides with hollow structures through two major methods, i.e. two-step sulfuration [24,25] and template method[26].

ACCEPTED MANUSCRIPT In this work, we report the formation of hollow NiCo2S4 spheres via a one-step facile solvothermal process using ethylene glycol (EG) as solvent. The time-dependence of the morphology and their electrochemical performances are carefully studied.

2. Experimental Section

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2.1 Synthesis process

All chemicals (Ni(NO3)2⋅6H2O, Co(NO3)2⋅6H2O, ethylene glycol (EG), thiourea, sodium acetate (NaAc)) are analytical grade without any further purification. The

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particles were synthesized as followed: 0.2 g NaAc was dissolved in 60 ml EG, followed by addition of Ni(NO3)2⋅6H2O (1 mmol), Co(NO3)2⋅6H2O (2 mmol) and

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thiourea (5 mmol). The solution was stirred for at least 40 min to form a pink transparent liquid. Subsequently, the as-prepared solution was transferred into an 80 mL Teflon-lined stainless-steel autoclave and was heated to 180 °C and maintained for 3 h, 6 h, 9 h, 12 h, (named as S3, S6, S9, S12), respectively. After the autoclave was naturally cooled to room temperature, the synthesized products were collected by centrifugation, washed several times with distilled water and absolute ethanol, dried at

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60 °C in oven. For comparison, another sample was synthesized under the same conditions for 6 h but only without NaAc (named as C6#).

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2.2 Material Characterizations

The morphologies of NiCo2S4 nanostructures were characterized by a field

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emission scanning electron microscope (FESEM, FEI QUANTA FEG250). The phase and structure were measured with X-ray diffraction (XRD, D8-Advance, Bruker, using Cu Kα, λ=0.15418nm). The chemical structures were examined using X-ray photoelectron spectroscopy (XPS , Thermo ESCALAB 250Xi) and all binding energies were referenced to the C 1s peak (284.8 eV). The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were carried out on a Tecnai G2 F20 at an acceleration voltage of 200 kV. 2.3 Electrochemical measurements The working electrodes were prepared as follows: 80 wt. % of sample was mixed

ACCEPTED MANUSCRIPT with 10 wt. % of acetylene black in an agate mortar until a homogeneous black powder was obtained. Then, 10 wt. % of polytetrafluoroethylene (PTFE) was added into this mixture. The resulting products were applied to a piece of carton fiber paper and were dried for several hours at 60 °C in vacuum. Each electrode contains about

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2.0 mg of electro-active material with the geometric surface area of about 1.0×2.0 cm2.

A typical three-electrode experimental cell was used to measure the electrochemical properties of the working electrodes. The NiCo2S4 served as the working electrodes,

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while a platinum foil electrode and a Ag/AgCl electrode were used as the counter and reference electrode, respectively. All the electrochemical measurements were carried

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out in 1.0 M KOH aqueous solution as the electrolyte. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out on the CHI660 electrochemical work station (Chenhua, ShangHai).

3. Results and discussion

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SEM images in Fig.1 clearly show the influence of NaAc on the final morphologies. The interconnected NiCo2S4 particles (Fig. 1a,b) are synthesized without the addition of NaAc. The diameters of these particles range from hundreds of nanometers to

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several micrometers as shown in Fig. 1b, and no hollow structures can be observed in this case. As NaAc is added, the morphologies of NiCo2S4 change a lot. The particles

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are composed of sheets structures (Fig. 1c, d) are formed at 3 h. When the reaction time extends to 6 h, hollow spheres with a diameter of hundreds nanometers are formed as shown in Fig. 1c,1h. From the enlarged FESEM images (Fig. 1h,i,j), the cracked spheres are also clearly observed, from which we can know that the thickness of shell is tens of nanometers. It can conclude that the reaction time and NaAc play a key role in porous structures and EG influences the morphology of sphere [27, 28]. The Ostwald ripening [29], a phenomenon where small nanoparticles dissolve and are re-deposite onto larger particles, should be accounted as an important factor for the hollowing process. Considering of the XRD patterns (Fig.3), it is noted that during the

ACCEPTED MANUSCRIPT relocation of the solid material with Ostwald ripening, the solid material will be crystallized to be more thermodynamically stable and thus transformation from sheets to crystallized hollow spheres are obtained [30]. TEM images in Fig. 2a-c provide an intuitive way to investigate the interior

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structure of the spheres by showing notable contrast difference between the hollow and solid parts. These hollow NiCo2S4 spheres exhibit a unique hollow architecture, and the average diameters of the outer shells are about 300nm with an average shell thickness of around 15nm. A lattice spacing of 0.22nm is observed in Fig. 2d, which

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is in good agreement with interplanar spacing of NiCo2S4 (400) plane. The lattice distances of 0.16 nm and 0.33 nm correspond to (440) and (220) planes of XRD patterns (Fig.3) shows

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NiCo2S4(JCPDS card No. 43-1477), respectively [31].

that all the peaks for all the samples can be indexed into cubic NiCo2S4 phase (JCPDS card No. 43-1477). In this case, the interplanar spacing can be calculated using Bragg’s law: nλ = 2d sin θ

(1)

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where n is the order of reflection, λ is the wavelength of incident X-ray (Cu Kα = 0.154nm), d is the interplanar spacing, and θ is the angle between the incident X-ray and the scattering planes. The calculated spacing between layers (400) is about

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0.23 nm, in consistent with our HRTEM observation (0.22nm in Fig.2c and d). X-ray photoelectron spectroscopy measurements were performed to study the

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elemental composition of S6 (Fig.4). The spectrum of NiCo2S4 (6 h) affirms that the atomic ratio of Ni:Co:S is 1.3:2.1:4.0, close to 1:2:4, which is consistent with the stoichimetric ratio of NiCo2S4. The Ni 2p spectrum at 853.2 eV (Fig.4) is comparable to the Ni2+ species. The binding energy at 856.4 eV corresponds to the spin-orbit characteristic of Ni3+. The spectrum of Co 2p can be ascribed to two spin-orbit doublets and two shakeup satellites. The first doublets at 778.5eV and 793.6eV and the second at 781.7eV and 797.9eV could be assigned to Co3+ and Co2+. Compared to the XPS spectra of others [32], the binding energies of Co 2p shifted to 778.5eV and 793.6eV, suggesting a strong interaction. No residuals or impurities are detected. The results clearly indicate that NiCo2S4 have been successfully synthesized, in which

ACCEPTED MANUSCRIPT Ni(NO3)2⋅6H2O, Co(NO3)2⋅6H2O, and thiourea are used as Ni2+, Co2+, and S2- sources, respectively. The equation can be as follows [27]: 1 Ni 2+ + 2Co 2 + + 4 S 2 − + O2 + H 2O = NiCo2 S 4 + 2OH − 2

(2)

3.2 Electrochemical performance

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Fig. 5a displays the cyclic voltammograms for different samples recorded in the range of 0 to 0.5 V vs. Ag/AgCl at the scan rate of 10 mVs-1. The shapes of the CV curves reveal that the charge-storage mechanism is distant from that of double-layer capacitance which is close to an ideal quasi-rectangular shape, suggesting that the

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NiCo2S4 electrodes are faradaic pseudocapacitance. The redox processes can be

CoS + OH − ƒ CoSOH + e−

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expressed as [27]:

(3)

CoSOH + OH − ƒ CoSO + H 2O + e −

(4)

NiS + OH − ƒ NiSOH + e−

(5)

The galvanostatic charge/discharge curves of the NiCo2S4 are shown in Fig. 6a. The

Cs = I∆t / m∆V

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specific capacitances (SCs) can be calculated as following equation [31]: (6)

Where Cs is the specific capacitance of the electrode materials, I is the current density

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during the discharge process, ∆t is the discharge time, ∆V is the potential window and m is the mass of the electrode materials. It can be calculated that the SCs are 354

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Fg-1, 756 Fg-1, 321 Fg-1, 386 Fg-1 and 378Fg-1 at same current densities of 1 Ag-1 for the samples S3, S6, S9, S12 and C6#. For different samples, the Cs increased in the order S6>12h>C6#>S3>S9. This

implies that the capacitive performances deeply depend on morphologies. Fig. 6b presents the GCD curves of the NiCo2S4 (S6) electrode at different current densities from 1Ag-1 to 10Ag-1 with a potential window from 0 to 0.37 V. With the increase of current density, the values of SCs are 740 Fg-1, 697 Fg-1, 627 Fg-1, 216 Fg-1, and 65Fg-1 at the current density of 2 Ag-1, 3 Ag-1, 4 Ag-1, 5 Ag-1, and 10Ag-1, respectively. Compared with other samples, the S6 delivers the best performance of specific

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The Nyquist plots of all NiCo2S4 samples are presented to further confirm the impedance-dependent effect as listed in Fig.8. The bulk solution resistance (Rs) values of S3, S6, S9, S12 and C6# are calculated to be 1.67Ω, 1.03Ω, 1.47Ω, 1.10 Ω and 1.13 Ω , respectively, whilst the corresponding charge-transfer resistance (Rct) values

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are 1.8Ω, 0.55Ω, 0.78 Ω, , 0.63 Ω and 0.60 Ω . It is noticed that the 6h sample exhibits smaller electrochemical reaction impedance, indicating that this kind of samples have

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smaller charge transfer resistance and faster ion diffusion rate, which are consistent with the electrochemical results above.

It is well-known that cycle stability is another parameter of great importance to evaluate the performance of supercapacitors. As shown in Fig.9, it is noted that the specific capacitance is almost constant with only minor fluctuations during the long

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cycle process. Interesting, all the samples have intensity of increasing capacitance. After about 300 cycles, the specific capacitance reached to a stable value, indicating the good stability of all samples.

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4. Conclusions

In summary, NiCo2S4 with different morphologies were successfully synthesized

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through a general solvothermal method. The morphologies transformed from particles, nanosheets to hollow spheres with the addition of NaAc and the extension of the reaction time. The hollow NiCo2S4 spheres delivered a large specific capacitance of 756 Fg-1 at 1 Ag-1. These results demonstrated that the NiCo2S4 would hold great promise for high performance supercapacitor applications.

Acknowledgements The work was financially by the National Natural Science Foundation of China (Grant

ACCEPTED MANUSCRIPT No. 51672109, 21505050), Natural Science Foundation of Shandong Province for

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Excellent Young Scholars (ZR2016JL015).

ACCEPTED MANUSCRIPT Reference [1] M. Winter, R. J. Brodd, What are batteries, fuel cells, and supercapacitors? Chem. Rev. 104 (2004) 4245-4269. [2] C. Wang, K. Guo, W. He, X. Deng, P. Hou, F. Zhuge, X. Xu, T. Zhai, Hierarchical hydroxides

core/shell

nanoarchitectures

for

RI PT

[email protected]

high-performance hybrid supercapacitors, Sci. Bull. 62 (2017) 1122-1131.

[3] W. He, W. Yang, C. Wang, X. Deng, B. Liu, X. Xu, Morphology-controlled syntheses of α-MnO2 for electrochemical energy storage, Phys. Chem. Chem. Phys.,

SC

18 (2016) 15235-15243.

[4] E. Zhou, C. Wang, X. Deng, X. Xu, Ag nanoparticles anchored NiO/GO

M AN U

composites for enhanced capacitive performance, Ceram. Inter. 42 (2016) 12644-12650.

[5] M. Huang, Y. Zhang, F. Li, L. Zhang, Z. Wen, Q. Liu, Facile synthesis of hierarchical [email protected] core-shell arrays on Ni foam for asymmetric supercapacitors, J. Power Sources 252 (2014) 98-106.

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[6] B. Guan, D. Guo, L. Hu, G. Zhang, T. Fu, W. Ren, J. Li, Q. Li, Facile synthesis of ZnCo2O4 nanowire cluster arrays on Ni foam for high-performance asymmetric supercapacitors, J. Mater. Chem. A 2 (2014) 16116-16123.

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[7] W. He, C. Wang, F. Zhuge, X. Deng, X. Xu, T. Zhai, Flexible and high energy density asymmetrical supercapacitors based on core/shell conducting polymer nanowires/manganese dioxide nanoflakes, Nano Energy, 35 (2017) 242-250.

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[8] E. Zhou, C. Wang, M. Shao, X. Deng, X. Xu, MoO2 nanoparticles grown on carbon fibers as anode materials for lithium-ion batteries, Ceram. Inter. 43 (2017) 760-765.

[9] E. Mitchell, A. Jimenez, R. K. Gupta, K Ramasamy, M Shahabuddind, S. R. Mishra, Ultrathin porous hierarchically textured NiCo2O4-graphene oxide flexible nanosheets for high-performance supercapacitors, New J. Chem. 39 (2015) 2181-2187. [10] R. S. Ray, B. Sarma, A. L. Jurovitzki, M. Misra, Fabrication and characterization of titania nanotube/cobalt sulfide supercapacitor electrode in various electrolytes,

ACCEPTED MANUSCRIPT Chem. Eng. J. 260 (2015) 671-683. [11] Z. Zhang, Q. Wang, C. Zhao, S. Min, X. Qian, One-Step hydrothermal synthesis of 3D petal-like Co9S8/RGO/Ni3S2 composite on nickel foam for high-performance supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 4861-4868.

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[12] K. Krishnamoorthy, P. Pazhamalai, S. J. Kim, Ruthenium sulfide nanoparticles as a new pseudocapacitive material for supercapacitor, Electrochim. Acta 227 (2017) 85-94. [13]

V.

H.

Nguyen,

C.

Lamiel,

J.

J.

Shim,

Hierarchical

mesoporous

SC

[email protected] arrays on nickel foam for high-performance supercapacitors, Electrochim. Acta 161 (2015) 351-357.

M AN U

[14] H. Wang, C. Wang, C. Qing, D. Sun, B. Wang, G. Qu, M. Sun, Y. Tang, Construction of carbon-nickel cobalt sulphide hetero-structured arrays on nickel foam for high performance asymmetric supercapacitors, Electrochim. Acta. 174 (2015) 1104-1112.

[15] M. Sun, J. Tie, G. Cheng, T. Lin, S. Peng, F. Deng, F. Ye, L. Yu, In situ growth of

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burl-like nickel cobalt sulfide on carbon fibers as high-performance supercapacitors, J. Mater. Chem. A 3 (2015) 1730-1736.

[16] N. Wang, M. Yao, P. Zhao, Q. Zhang, W. Hu, Highly mesoporous structure nickel

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cobalt oxides with an ultra-high specific surface area for supercapacitor electrode materials, J. Solid State Electr. 20 (2016) 1429-1434. [17] L. Shen, L. Yu, H. B. Wu, X. Yu, X. Zhang, X. W. Lou, Formation of nickel sulfide

ball-in-ball

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cobalt

hollow

spheres

with

enhanced

electrochemical

pseudocapacitive properties, Nat.Commun. 6 (2015) 6694. [18] J. Pu, F. Cui, S. Chu, T. Wang, E. Sheng, Z. Wang, Preparation and electrochemical characterization of hollow hexagonal NiCo2S4 nanoplates as pseudocapacitor materials, ACS Sustain. Chem. En. 2 (2014) 809-815. [19] Y. Zhang, M. Ma, J. Yang, C. Sun, H. Su, W. Huang, X. Dong, Shape-controlled synthesis of NiCo2S4 and their charge storage characteristics in supercapacitors, Nanoscale 6 (2014) 9824-9830. [20] J. Hu, M. Chen, X. Fang, L. Wu, Fabrication and application of inorganic hollow

ACCEPTED MANUSCRIPT spheres, Chem. Soc. Rev. 40 (2011) 5472–5491. [21] L. Yu, L. Zhang, H. B. Wu, X. W. Lou, Formation of NixCo3−xS4 hollow nanoprisms with enhanced pseudocapacitive properties, Angew. Chem. Int. Ed. 53 (2014) 3711-3714.

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[22] Y. Xu, X. Gao, W. Chu, Q. Li, T. Li, C. Liang and Z. Lin, Ni-Co sulfide nanoboxes with tunable compositions for high-performance electrochemical pseudocapacitors, J. Mater. Chem. A 4 (2016) 10248-10253.

[23] B. Y. Guan, L. Yu, X. Wang, S. Song, and X. W. Lou, Formation of onion-Like

SC

NiCo2S4 particles via Sequential ion-exchange for hybrid supercapacitors, Adv. Mater. 29 (2017) 1605051

M AN U

[24] Y. Wen, S. Peng, Z. Wang, J.Hao, T. Qin, S. Lu, J. Zhang, D. He, X. Fan and G. Cao, Facile synthesis of ultrathin NiCo2S4 nano-petals inspired by blooming buds for high-performance supercapacitors, J. Mater. Chem. A 5 (2017) 7144-7152. [25] M. Yan, Y. Yao, J. Wen, L. Long, M. Kong, G. Zhang, X. Liao, G. Yin, and Z. Huang, Construction of a hierarchical [email protected] core-shell heterostructure

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nanotube array on Ni foam for a high-performance asymmetric supercapacitor, ACS Appl. Mater. Interfaces 8 (2016) 24525-24535. [26] A. Sivanantham, P. Ganesan, and S. Shanmugam, Hierarchical NiCo2S4 nanowire

EP

arrays supported on Ni Foam: an efficient and durable bifunctional electrocatalyst for oxygen and hydrogen evolution reactions, Adv. Funct. Mater. 26 (2016) 4661-4672. [27] H. Chen, J. Jiang, Y. Zhao, L. Zhang, D. Guo, D. Xia, One-pot synthesis of

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porous nickel cobalt sulphides: tuning the composition for superior pseudocapacitance. J. Mater. Chem. A 3 (2015) 428-437. [28] H. Deng, X. Li, Q. Peng, X. Wang, J. Chen, Y. Li, Monodisperse magnetic single-crystal ferrite microspheres, Angew. Chem. Int. Ed. 44 (2005) 2782-2785. [29] B. Liu, H. C. Zeng, Symmetric and asymmetric ostwald ripening in the fabrication of homogeneous core-shell semiconductors, Small 1 (2005) 566-571. [30] D. Nguyen, K. Kim, Self-development of hollow TiO2 nanoparticles by chemical conversion coupled with Ostwald ripening, Chem. Eng. J, 286 (2016) 266–271 [31] W. He, C. Wang, H. Li, X. Deng, X. Xu, T. Zhai, Ultrathin and porous

ACCEPTED MANUSCRIPT Ni3S2/CoNi2S4 3D-network structure for superhigh energy density asymmetric supercapacitors, Adv. Energy Mater. 2017, 1700983. [32] J. Huo, J. Wu, M. Zheng, Y. Tu, Z. Lan, Flower-like nickel cobalt sulfide microspheres modified with nickel sulfide as Pt-free counter electrode for

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dye-sensitized solar cells, J. Power Sources 304 (2016) 266-272. [33] X. Ma, L. Zhang, G. Xu, C. Zhang, H. Song, Y. He, C. Zhang, D. Jia, Facile synthesis of NiS hierarchical hollow cubes via Ni formate frameworks for high performance supercapacitors, Chem. Eng. J. 320 (2017) 22–28.

Moderated surface defects of Ni

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[34] Y. Zhang, M. Park, H. Y. Kim, S. Park,

particles encapsulated with NiO nanofibers as supercapacitor with high capacitance

M AN U

and energy density, J. Colloid. Interf. Sci. 500 (2017) 155-163.

[35] Y. He, L. Wang, D. Jia, Z. Zhao, J. Qiu , NiWO4/Ni/Carbon composite fibres for supercapacitors with excellent cycling performance, Electrochim. Acta 222 (2016) 446-454.

[36] D. Cheng, Y. Yang, Y. Luo, C. Fang, J. Xiong ,Growth of ultrathin mesoporous

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Ni-Mo oxide nanosheet arrays on Ni foam for high-performance supercapacitor electrodes, Electrochim. Acta 176 (2015) 1343-1351. [37] J. Wen , S. Li , K. Zhou , Z. Song , B. Li , Z. Chen ,T. Chen , Y. Guo and G. Fang,

EP

Flexible coaxial-type fiber solid-state asymmetrical supercapacitor based on Ni3S2 nanorod array and pen ink electrodes, J. Power Sources 324 (2016) 325-333. [38] Y. Jiang, Z. Li, B. Li, J. Zhang, C. Niu, Ni3Si2 nanowires grown in situ on Ni

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foam for high-performance supercapacitors, J. Power Sources 320 (2016) 13-19.

ACCEPTED MANUSCRIPT Figure Captions: Fig. 1. SEM images of the NiCo2S4 nanostructures with low magnification (a)-(e) and high magnification (f)-(j) for samples C6#, S3, S6, S9 and S12, respectively. Hollow

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spheres can be observed in high resolution SEM (h-j), corresponding to samples S6, S9, S12, respectively.

Fig. 2. TEM images of NiCo2S4 (S6) with low-magnification (a) and (c,d)

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high-magnification (b); (c) TEM images of a single hollow NiCo2S4 nanospheres,

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HRTEM image (d).

Fig. 3. XRD pattern of as-prepared samples. Compared to other samples, the samples synthesized for 6h, 9h,12h have much higher degree of crystallinity. The compare sample without NaAc synthesized at 180 °C for 6 hours is marked as C6#.

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Fig. 4. XPS plots of survey and elements scans of sample S6.

Fig. 5. CV curves of different samples (a); and at various scan rates ranging from 10 to 100mVs-1 for NiCo2S4 6h samples(b).

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Fig. 6. (a) Galvanostalic discharge and charge (GCD) curves of different samples; (b)

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GCD curves at different current densities (1-10Ag-1) for sample S6. Fig. 7. The specific capacitances of all samples at different current densities. The S6 shows the best performance at low or high current densities. Fig. 8. Nyquist plots of different samples. The sample (S6) has much lower resistance, indicating better capacitance performance and good stability. Fig.9. Cycling performances of S3, S6, S9 and S12 samples at a scan rate of 50mVs-1.

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Fig. 1. SEM images of the NiCo2S4 nanostructures with low magnification (a)-(e) and high magnification (f)-(j) for samples C6#, S3, S6, S9 and S12, respectively. Hollow spheres can be observed in high resolution SEM (h-j), corresponding to samples S6, S9, S12, respectively.

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Fig. 2. TEM images of NiCo2S4 (S6) with low-magnification (a) and (c,d)

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high-magnification (b); (c) TEM images of a single hollow NiCo2S4 spheres, HRTEM image (d).

(400)

(511)

(400)

C6# S3 S6 S9 S12

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Intensity (a.u.)

(311)

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Fig. 3. XRD pattern of as-prepared samples. Compared to other samples, the samples synthesized for 6h, 9h,12h have much higher degree of crystallinity. The compare

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sample without NaAc synthesized at 180 °C for 6 hours is marked as C6#.

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Intensity(a.u.)

Co 2p

C 1s S sp

Ni 2p Scan

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S 2p Scan

Co 2p Scan

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880 870 860 Binding Energy (eV)

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Intensity (a.u.)

1200 1000 800 600 400 200 Binding Energy (eV)

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Intensity (a.u.)

Ni 2p

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Fig. 4. XPS plots of survey and elements scans of sample S6.

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ACCEPTED MANUSCRIPT

10mv/s 20mv/s 30mv/s 60mv/s 100mv/s

0.06 0.03 0.00 -0.03 -0.06 -0.09

SC

(a)

RI PT

Current (A)

0.09

0.0

0.2 0.3 Potential (V)

0.4

M AN U

Fig. 5. CV curves of different samples (a); and

0.1

(b)

AC C

(a)

EP

TE D

at various scan rates ranging from 10 to 100mVs-1 for NiCo2S4 6h samples (b).

(b)

Fig. 6. (a) Galvanostalic discharge and charge (GCD) curves of different samples; (b) GCD curves at different current densities (1-10Ag-1) for sample S6.

0.5

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig.7. The specific capacitances of all samples at different current densities. The S6

AC C

EP

TE D

shows the best performance at low or high current densities.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 8. Nyquist plots of different samples. The sample (S6) has much lower

TE D

resistance, indicating better capacitance performance and good stability.

S3 S6 S9 S12

800

EP

Specific capacitance (Fg-1)

1000

AC C

600 400 200

500

1000

1500

2000

Cycling number Fig. 9. Cycling performances of S3, S6, S9 and S12 samples at a scan rate of 50 mV s-1.

ACCEPTED MANUSCRIPT Table 1. Literature on Ni-based electrode materials for supercapacitor application. Synthesis Methods

Morphology

Specific capacitance (Fg-1)

Electrolyte

Ref.

Hydrothermal

Cubes

750Fg,at 2Ag-1

2M KOH

33

NiO

Electrospinning

Fibers

526Fg,at 1Ag-1

3M KOH

34

NiWO4

Electrospinning

Fibers

582Fg,at 5Ag-1

6M KOH

35

NiO

Hydrothermal

Sheets

191Fg,at 1Ag-1

2M KOH

36

Ni3S2

Hydrothermal

Rods

34.3Fg,at 0.3Ag-1

3M KOH

37

Ni3Si2

Chemical Vapor

Wires

760Fg,at 0.5Ag-1

2M KOH

38

1M KOH

This work

Deposition Hydrothermal

Hollow spheres

756Fg,at 1Ag-1

AC C

EP

TE D

M AN U

NiCo2S4

RI PT

NiS

SC

Electrode materials

ACCEPTED MANUSCRIPT 1. Hollow spheres were synthesized and assembled supercapacitors. 2. NiCo2S4 particles, sheets and hollow spheres are easily synthesized.

AC C

EP

TE D

M AN U

SC

RI PT

3. Hollow NiCo2S4 spheres exhibit highest specific capacitance of 756 Fg-1 at 1 Ag-1.