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Room-temperature synthesis of NiS hollow spheres on nickel foam for high-performance supercapacitor electrodes
Accepted Manuscript Room-Temperature Synthesis of NiS Hollow Spheres on Nickel Foam for HighPerformance Supercapacitor Electrodes Van Chinh Tran, Sumanta Sahoo, Jae-Jin Shim PII: DOI: Reference:
S0167-577X(17)31334-4 http://dx.doi.org/10.1016/j.matlet.2017.08.136 MLBLUE 23113
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
23 June 2017 16 August 2017 31 August 2017
Please cite this article as: V.C. Tran, S. Sahoo, J-J. Shim, Room-Temperature Synthesis of NiS Hollow Spheres on Nickel Foam for High-Performance Supercapacitor Electrodes, Materials Letters (2017), doi: http://dx.doi.org/ 10.1016/j.matlet.2017.08.136
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.
Room-Temperature Synthesis of NiS Hollow Spheres on Nickel Foam for HighPerformance Supercapacitor Electrodes Van Chinh Tran,1 Sumanta Sahoo,1 and Jae-Jin Shim*
School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeoungbuk 38541, Republic of Korea 1
These authors contributed equally to this work.
*
Corresponding author. Tel: +82-53-810-2587; Fax: +82-53-810-4631.
E-mail Address: [email protected] (J-J. Shim).
Abstract NiS hollow spheres were synthesized directly on the surface of Ni foam (NiS@NF) at room temperature via a simple electrodeposition in a KOH/thiourea solution without any Ni precursors. Morphological analysis revealed the formation of NiS hollow spheres, which consisted of nanoparticles. The NiS@NF nanocomposite exhibited a very high specific capacitance of 1553 F g-1 (2.64 F cm-2) at a current density of 2.35 A g-1 (4 mA cm-2) in 6 M KOH electrolyte. The NiS@NF nanocomposite also demonstrated excellent cycling stability (95.7% specific capacitance retention after 2000 cycles). Key words: supercapacitor; nickel sulfide; hollow sphere; electrodeposition; nickel foam
1
1. Introduction For decades, supercapacitors have attracted significant attention as a promising energy storage device because of its high power density, strong cycle stability, and fast charge/discharge rate [1, 2]. One typical type is the Faradaic supercapacitor, which stores energy through reversible Faradaic reactions of active materials and commonly exhibits higher capacitance than other types of supercapacitors [1-5]. Over the past few years, considerable efforts have been made to develop efficient and cost-effective electrode materials for Faradaic supercapacitors. Among the many redox-active Faradaic electrode materials [1-10], transitional metal chalcogenides have been studied widely because of their excellent intrinsic properties and good electrochemical performance [4]. Nickel sulfides with different phases, such as Ni3S2, NiS2, NiS, Ni3S4 etc., are considered promising candidates as electrode materials for supercapacitors. Among them, NiS has been investigated widely as Faradaic electrodes because of its low cost, superior redox activity, and high electronic conduction. Over the last few years, NiS with various morphologies have been synthesized using different chemical methods, such as hydrothermal methods [1116]. On the other hand, there are no reports on the room-temperature synthesis of NiS hollow spheres. To the best of the authors’ knowledge, this is the first report on the synthesis of NiS hollow spheres on Ni foam and its potential applications for binder-free supercapacitor electrodes.
2. Preparation of NiS@NF NiS@NF was synthesized using a modified electrodeposition process with a three-electrode cell, where a 2 X 1 cm2 cleaned NF, platinum foil, and Ag/AgCl electrode were used as the working, counter, and reference electrodes, respectively (Fig. 1). The potential window of -0.2 to 0.8 V and the scan rate of 20 mV s-1 were chosen. First, a 0.5 M thiourea solution (50 ml) was used for 10 electrodeposition cycles. Subsequently, 25 ml of thiourea was replaced with 25 ml of a 3 M KOH solution and the deposition process was continued for up to 300 cycles to obtain a proper coating of NiS on NF.
3. Results and Discussion Fig. 1 shows a schematic diagram of the three-electrode cell for the electrodeposition process. The possible electrochemical reactions involved during the formation of NiS hollow spheres are as follows: Ni + 2OH- + 2H+ → Ni(OH)2 + H2
2
(1)
NH2-CS-NH2 + OH- → HS- + CO(NH2)2
(2)
HS- → H+ + S2-
(3)
Ni(OH)2 + S2- → NiS + 2OH-
(4)
SEM of NiS@NF clearly showed the formation of NiS hollow spheres, approximately 1 µm diameter, on the NF surface (Fig. 2 b-d). The smooth surface of the bare NF (Fig. 2a) was covered with a large number of NiS microspheres (Fig. 2b). The hollow structure of the microspheres was confirmed by the open spheres, as shown in Fig. 2c. The SEM image at high magnification revealed porous NiS spheres with a stone-paved-surface structure consisting of a number of NiS nanoparticles (Fig. 2d). The TEM images revealed NiS nanoparticles to have a flakelike structure with a thickness of 5 nm (Figs. 2e and f). The porous-sphere structure is beneficial for easy and fast electrolyte transport to the active sites. The growth of NiS hollow spheres was caused by the Kirkendall effect, which is explained in details with schematic diagram (Fig. S1) in supporting information section. Fig. 3a presents the XRD pattern of the NiS@NF composite showing the crystallinity of the NiS hollow spheres. In addition to the three characteristic peaks of NF, the observed diffraction patterns with sharp peaks at 29.1o, 33.2o, 34.5o, 45.6o, and 53.3o 2θ showed good agreement with the XRD pattern of hexagonal NiS (JCPDS No. 01-077-1624) (Fig. 3a). Fig. 2b shows the deconvoluted XPS of NiS@NF with characteristic peaks for Ni 2p and S 2p. The presence of a Ni 2p3/2 peak at a binding energy of 855.8 eV confirmed the ‘+2’oxidation state of Ni. In addition, the peaks at 163.6 and 162.1 eV were assigned to S 2p1/2 and S 2p3/2, which confirmed that the dominating form of S is S2- [17]. Fig. 4a presents the CV patterns of a Ni foam substrate before and after deposition, which clearly demonstrates the enhancement in the current response and the existence of a redox pair. Compared to the bare-NF, the NiS@NF electrode exhibited a larger CV curve area at a scan rate of 5 mV s-1 (Fig. 4b). Furthermore, the CV curve of NiS@NF displayed two well-defined peaks at 0.17/0.31 V (cathodic/anodic), which can be attributed to the reversible conversion between Ni(II) and Ni(III) [13], according to the following electrochemical reaction:
NiS + OH- ↔ NiSOH + e-
(4)
In addition, the CV curves at different scan rates were similar in shape with a distinguishable redox pair, indicating the excellent reversibility of the electrode (Fig. 4c). The non-linear charge-discharge curves also indicates the Faradaic behavior of the electrode (Fig. 4d). Based on the discharge time recorded from the charge/discharge
3
profiles, the areal capacitance (Ca) of NiS@NF was calculated to 2.64, 2.23, 1.94, 1.72, 1.35, and 1.31 F cm-2 at current densities of 4, 6, 8, 10, 20, and 30 mA cm-2, respectively. Furthermore, the maximum specific capacitance (Cs) of NiS@NF was 1,553 F g-1 (specific capacity of 776.5 C g-1) at the current density of 2.35 A g-1, which is significantly higher than those of the other reported NiS-based supercapacitor electrodes (Table S1). Importantly, the electrode achieved 73% of its theoretical specific capacitance within a potential window of 0.5 V (the theoretical specific capacitance of NiS is approximately 2,126 F g-1. See Supporting Information). The Nyquist plot of NiS@NF also exhibited low equivalent series resistance (ESR) of 0.32 Ω (Fig. 4e), indicating good charge transport properties, rapid ion diffusion, and low contact resistance between the active electrode material and current collector. At the low frequency region, the vertical increase in the Warburg impedance (W) also indicated the conductive nature of the electrode [18]. The NiS@NF exhibited an excellent cycling stability with 95.7% capacitance retention after 2,000 charge/discharge cycles at a current density of 20 mA cm-2 (Fig. 4f). The initial capacitance enhancement of 104.4% was attributed to the activation process of the electrode material [13, 19]. The enhanced electrochemical performance of NiS@NF can be attributed to its hollow porous structure. The hollow spheres provide a higher surface area for the electrochemical reactions, while its porous structure provides channels for easy electrolyte ion transport and thus fast redox reactions. The hollow structure also offers a superior stability to prevent deformation during cycling operations. 4. Conclusions The NF surface was decorated with NiS hollow spheres using a simple electrodeposition process. As it is a binder-free supercapacitor electrode, the NiS@NF exhibited a high specific capacitance (1,553 F g-1 at a current density of 2.35 A g-1) and excellent cycling stability (capacitance retention of 95.7% after 2,000 charge/discharge cycles). This synthetic method can be applied to the deposition of other metal sulfides on Ni foam.
Acknowledgement This study was supported by the Priority Research Centers Program (NRF-2014R1A6A1031189), the Basic Science Research Program (NRF-2015R1D1A1A09060292), and the Korea-China International Cooperation Program (NRF-2015K2A2A7053101), all through the National Research Foundation of Korea (NRF-S. Korea) funded by the Ministry of Education and the Ministry of Science, ICT, and Future Planning, Republic of Korea.
4
References [1]
K. Zhou, W. Zhou, L. Yang, J. Lu, S. Cheng, W. Mai, Z. Tang, L. Li, S. Chen, Adv. Funct. Mater. 25 (2015) 7530-7538.
[2]
G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797-828.
[3]
D. Ghosh, C.K. Das, ACS Appl. Mater. Interfaces 7 (2015) 1122-1131.
[4]
D. Kong, C. Cheng, Y. Wang, J.I. Wong, Y. Yang, H.Y. Yang, J. Mater. Chem. A 3(2015) 16150-16161.
[5]
V.H. Nguyen, J.-J. Shim, J. Power Sources 273 (2015) 110-117.
[6]
V.H. Nguyen, V.C. Tran, D. Kharismadewi, J.-J. Shim, Mater. Lett. 147 (2015) 123-127.
[7]
C. Wei, C. Cheng, Y. Cheng, Y. Wang, Y. Xu, W. Dua, H. Pang, Dalton Trans. 44 (2015) 17278-17285.
[8]
G. Han, Y. Liu, L. Zhang, S. Zhang, J. Tang, W. Tang, E. Kan, Sci. Rep. 4 (2014) 4824.
[9]
V.C. Tran, V.H. Nguyen, T.T. Nguyen, J.H. Lee, D.C. Huynh, J.-J. Shim, Synth. Met. 215 (2016) 150-157.
[10]
N.V.
Hoa,
C.
Lamiel,
N.H.
Nghia,
P.A.
Dat,
J.-J.
Shim,
Mater.
Lett.
(2017)
DOI:
10.1016/j.matlet.2017.05.013. [11]
P. Zhao, Q. Zeng, K. Huang, Mater. Lett. 63 (2009) 313-315.
[12]
W. Li, S. Wang, L. Xin, M. Wu, X. Lou, J. Mater. Chem. A 4 (2016) 7700-7709.
[13]
X.-Y. Yu, L. Yu, L. Shen, X. Song, H. Chen, X. W. Lou, Adv. Funct. Mater. 47 (2014) 7440-7446.
[14]
Y. Zhang, J. Xu, Y. Zhang, X. Hu, J Mater Sci: Mater Electron. 27 (2016) 8599-8605.
[15]
W. Zhang, X. Yan, X. Tong, J. Yang, L. Miao, Y. Sun et al., Mater. Lett. 178 (2016) 120-123.
[16]
T. Zhu, Z. Wang, S. Ding, J. S. Chen, X. W. Lou, RSC Adv. 1 (2011) 397-400.
[17]
X. Wang, B. Batter, Y. Xie, K. Pan, Y. Liao, C. Lv et al. J. Mater. Chem. A 3 (2015) 5905-15912.
[18] D. R. Kumar, T. T. Nguyen, C. Lamiel, J.-J. Shim, Mater. Lett. 165 (2016) 257-262. [19]
J. Pu, T.T. Wang, H.Y. Wang, Y. Tong, C.C. Lu, W. Kong, Z.H. Wang, ChemPlusChem 79 (2014) 577583.
5
Figure captions Fig. 1. Schematic diagram of the synthetic route of NiS@NF. Fig. 2. SEM images of (a) the bare NF and (b-d) NiS@NF at low and high magnifications; TEM images of a NiS hollow sphere at different magnifications (g-h). Fig. 3. XRD patterns of the bare NF and NiS@NF (a); the deconvoluted XPS of Ni2p and S2p of NiS@NF (b). Fig. 4. (a) CV curves of NF before and after electrodeposition; (b) CV curves of the bare NF and NiS@NF at the scan rate of 5 mV/s; (c) CV curves at different scan rates, (d) galvanostatic charge/discharge curves as a function of the current density, (e) Nyquist plot, and (f) cycling stability of NiS@NF up to 2,000 cycles.
Figure 1.
6
Figure 2.
Figure 3.
7
Figure 4.
8
Highlights NiS hollow spheres were electrodeposited directly on Ni foam at room temperature. The NiS@NF composite showed high specific capacitance: 1,553 F g-1 at 2.35 A g-1. The NiS@NF composite exhibited good cycling stability: 95.7 % after 2,000 cycles.
9
10
S0167-577X(17)31334-4 http://dx.doi.org/10.1016/j.matlet.2017.08.136 MLBLUE 23113
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
23 June 2017 16 August 2017 31 August 2017
Please cite this article as: V.C. Tran, S. Sahoo, J-J. Shim, Room-Temperature Synthesis of NiS Hollow Spheres on Nickel Foam for High-Performance Supercapacitor Electrodes, Materials Letters (2017), doi: http://dx.doi.org/ 10.1016/j.matlet.2017.08.136
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.
Room-Temperature Synthesis of NiS Hollow Spheres on Nickel Foam for HighPerformance Supercapacitor Electrodes Van Chinh Tran,1 Sumanta Sahoo,1 and Jae-Jin Shim*
School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeoungbuk 38541, Republic of Korea 1
These authors contributed equally to this work.
*
Corresponding author. Tel: +82-53-810-2587; Fax: +82-53-810-4631.
E-mail Address: [email protected] (J-J. Shim).
Abstract NiS hollow spheres were synthesized directly on the surface of Ni foam (NiS@NF) at room temperature via a simple electrodeposition in a KOH/thiourea solution without any Ni precursors. Morphological analysis revealed the formation of NiS hollow spheres, which consisted of nanoparticles. The NiS@NF nanocomposite exhibited a very high specific capacitance of 1553 F g-1 (2.64 F cm-2) at a current density of 2.35 A g-1 (4 mA cm-2) in 6 M KOH electrolyte. The NiS@NF nanocomposite also demonstrated excellent cycling stability (95.7% specific capacitance retention after 2000 cycles). Key words: supercapacitor; nickel sulfide; hollow sphere; electrodeposition; nickel foam
1
1. Introduction For decades, supercapacitors have attracted significant attention as a promising energy storage device because of its high power density, strong cycle stability, and fast charge/discharge rate [1, 2]. One typical type is the Faradaic supercapacitor, which stores energy through reversible Faradaic reactions of active materials and commonly exhibits higher capacitance than other types of supercapacitors [1-5]. Over the past few years, considerable efforts have been made to develop efficient and cost-effective electrode materials for Faradaic supercapacitors. Among the many redox-active Faradaic electrode materials [1-10], transitional metal chalcogenides have been studied widely because of their excellent intrinsic properties and good electrochemical performance [4]. Nickel sulfides with different phases, such as Ni3S2, NiS2, NiS, Ni3S4 etc., are considered promising candidates as electrode materials for supercapacitors. Among them, NiS has been investigated widely as Faradaic electrodes because of its low cost, superior redox activity, and high electronic conduction. Over the last few years, NiS with various morphologies have been synthesized using different chemical methods, such as hydrothermal methods [1116]. On the other hand, there are no reports on the room-temperature synthesis of NiS hollow spheres. To the best of the authors’ knowledge, this is the first report on the synthesis of NiS hollow spheres on Ni foam and its potential applications for binder-free supercapacitor electrodes.
2. Preparation of NiS@NF NiS@NF was synthesized using a modified electrodeposition process with a three-electrode cell, where a 2 X 1 cm2 cleaned NF, platinum foil, and Ag/AgCl electrode were used as the working, counter, and reference electrodes, respectively (Fig. 1). The potential window of -0.2 to 0.8 V and the scan rate of 20 mV s-1 were chosen. First, a 0.5 M thiourea solution (50 ml) was used for 10 electrodeposition cycles. Subsequently, 25 ml of thiourea was replaced with 25 ml of a 3 M KOH solution and the deposition process was continued for up to 300 cycles to obtain a proper coating of NiS on NF.
3. Results and Discussion Fig. 1 shows a schematic diagram of the three-electrode cell for the electrodeposition process. The possible electrochemical reactions involved during the formation of NiS hollow spheres are as follows: Ni + 2OH- + 2H+ → Ni(OH)2 + H2
2
(1)
NH2-CS-NH2 + OH- → HS- + CO(NH2)2
(2)
HS- → H+ + S2-
(3)
Ni(OH)2 + S2- → NiS + 2OH-
(4)
SEM of NiS@NF clearly showed the formation of NiS hollow spheres, approximately 1 µm diameter, on the NF surface (Fig. 2 b-d). The smooth surface of the bare NF (Fig. 2a) was covered with a large number of NiS microspheres (Fig. 2b). The hollow structure of the microspheres was confirmed by the open spheres, as shown in Fig. 2c. The SEM image at high magnification revealed porous NiS spheres with a stone-paved-surface structure consisting of a number of NiS nanoparticles (Fig. 2d). The TEM images revealed NiS nanoparticles to have a flakelike structure with a thickness of 5 nm (Figs. 2e and f). The porous-sphere structure is beneficial for easy and fast electrolyte transport to the active sites. The growth of NiS hollow spheres was caused by the Kirkendall effect, which is explained in details with schematic diagram (Fig. S1) in supporting information section. Fig. 3a presents the XRD pattern of the NiS@NF composite showing the crystallinity of the NiS hollow spheres. In addition to the three characteristic peaks of NF, the observed diffraction patterns with sharp peaks at 29.1o, 33.2o, 34.5o, 45.6o, and 53.3o 2θ showed good agreement with the XRD pattern of hexagonal NiS (JCPDS No. 01-077-1624) (Fig. 3a). Fig. 2b shows the deconvoluted XPS of NiS@NF with characteristic peaks for Ni 2p and S 2p. The presence of a Ni 2p3/2 peak at a binding energy of 855.8 eV confirmed the ‘+2’oxidation state of Ni. In addition, the peaks at 163.6 and 162.1 eV were assigned to S 2p1/2 and S 2p3/2, which confirmed that the dominating form of S is S2- [17]. Fig. 4a presents the CV patterns of a Ni foam substrate before and after deposition, which clearly demonstrates the enhancement in the current response and the existence of a redox pair. Compared to the bare-NF, the NiS@NF electrode exhibited a larger CV curve area at a scan rate of 5 mV s-1 (Fig. 4b). Furthermore, the CV curve of NiS@NF displayed two well-defined peaks at 0.17/0.31 V (cathodic/anodic), which can be attributed to the reversible conversion between Ni(II) and Ni(III) [13], according to the following electrochemical reaction:
NiS + OH- ↔ NiSOH + e-
(4)
In addition, the CV curves at different scan rates were similar in shape with a distinguishable redox pair, indicating the excellent reversibility of the electrode (Fig. 4c). The non-linear charge-discharge curves also indicates the Faradaic behavior of the electrode (Fig. 4d). Based on the discharge time recorded from the charge/discharge
3
profiles, the areal capacitance (Ca) of NiS@NF was calculated to 2.64, 2.23, 1.94, 1.72, 1.35, and 1.31 F cm-2 at current densities of 4, 6, 8, 10, 20, and 30 mA cm-2, respectively. Furthermore, the maximum specific capacitance (Cs) of NiS@NF was 1,553 F g-1 (specific capacity of 776.5 C g-1) at the current density of 2.35 A g-1, which is significantly higher than those of the other reported NiS-based supercapacitor electrodes (Table S1). Importantly, the electrode achieved 73% of its theoretical specific capacitance within a potential window of 0.5 V (the theoretical specific capacitance of NiS is approximately 2,126 F g-1. See Supporting Information). The Nyquist plot of NiS@NF also exhibited low equivalent series resistance (ESR) of 0.32 Ω (Fig. 4e), indicating good charge transport properties, rapid ion diffusion, and low contact resistance between the active electrode material and current collector. At the low frequency region, the vertical increase in the Warburg impedance (W) also indicated the conductive nature of the electrode [18]. The NiS@NF exhibited an excellent cycling stability with 95.7% capacitance retention after 2,000 charge/discharge cycles at a current density of 20 mA cm-2 (Fig. 4f). The initial capacitance enhancement of 104.4% was attributed to the activation process of the electrode material [13, 19]. The enhanced electrochemical performance of NiS@NF can be attributed to its hollow porous structure. The hollow spheres provide a higher surface area for the electrochemical reactions, while its porous structure provides channels for easy electrolyte ion transport and thus fast redox reactions. The hollow structure also offers a superior stability to prevent deformation during cycling operations. 4. Conclusions The NF surface was decorated with NiS hollow spheres using a simple electrodeposition process. As it is a binder-free supercapacitor electrode, the NiS@NF exhibited a high specific capacitance (1,553 F g-1 at a current density of 2.35 A g-1) and excellent cycling stability (capacitance retention of 95.7% after 2,000 charge/discharge cycles). This synthetic method can be applied to the deposition of other metal sulfides on Ni foam.
Acknowledgement This study was supported by the Priority Research Centers Program (NRF-2014R1A6A1031189), the Basic Science Research Program (NRF-2015R1D1A1A09060292), and the Korea-China International Cooperation Program (NRF-2015K2A2A7053101), all through the National Research Foundation of Korea (NRF-S. Korea) funded by the Ministry of Education and the Ministry of Science, ICT, and Future Planning, Republic of Korea.
4
References [1]
K. Zhou, W. Zhou, L. Yang, J. Lu, S. Cheng, W. Mai, Z. Tang, L. Li, S. Chen, Adv. Funct. Mater. 25 (2015) 7530-7538.
[2]
G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797-828.
[3]
D. Ghosh, C.K. Das, ACS Appl. Mater. Interfaces 7 (2015) 1122-1131.
[4]
D. Kong, C. Cheng, Y. Wang, J.I. Wong, Y. Yang, H.Y. Yang, J. Mater. Chem. A 3(2015) 16150-16161.
[5]
V.H. Nguyen, J.-J. Shim, J. Power Sources 273 (2015) 110-117.
[6]
V.H. Nguyen, V.C. Tran, D. Kharismadewi, J.-J. Shim, Mater. Lett. 147 (2015) 123-127.
[7]
C. Wei, C. Cheng, Y. Cheng, Y. Wang, Y. Xu, W. Dua, H. Pang, Dalton Trans. 44 (2015) 17278-17285.
[8]
G. Han, Y. Liu, L. Zhang, S. Zhang, J. Tang, W. Tang, E. Kan, Sci. Rep. 4 (2014) 4824.
[9]
V.C. Tran, V.H. Nguyen, T.T. Nguyen, J.H. Lee, D.C. Huynh, J.-J. Shim, Synth. Met. 215 (2016) 150-157.
[10]
N.V.
Hoa,
C.
Lamiel,
N.H.
Nghia,
P.A.
Dat,
J.-J.
Shim,
Mater.
Lett.
(2017)
DOI:
10.1016/j.matlet.2017.05.013. [11]
P. Zhao, Q. Zeng, K. Huang, Mater. Lett. 63 (2009) 313-315.
[12]
W. Li, S. Wang, L. Xin, M. Wu, X. Lou, J. Mater. Chem. A 4 (2016) 7700-7709.
[13]
X.-Y. Yu, L. Yu, L. Shen, X. Song, H. Chen, X. W. Lou, Adv. Funct. Mater. 47 (2014) 7440-7446.
[14]
Y. Zhang, J. Xu, Y. Zhang, X. Hu, J Mater Sci: Mater Electron. 27 (2016) 8599-8605.
[15]
W. Zhang, X. Yan, X. Tong, J. Yang, L. Miao, Y. Sun et al., Mater. Lett. 178 (2016) 120-123.
[16]
T. Zhu, Z. Wang, S. Ding, J. S. Chen, X. W. Lou, RSC Adv. 1 (2011) 397-400.
[17]
X. Wang, B. Batter, Y. Xie, K. Pan, Y. Liao, C. Lv et al. J. Mater. Chem. A 3 (2015) 5905-15912.
[18] D. R. Kumar, T. T. Nguyen, C. Lamiel, J.-J. Shim, Mater. Lett. 165 (2016) 257-262. [19]
J. Pu, T.T. Wang, H.Y. Wang, Y. Tong, C.C. Lu, W. Kong, Z.H. Wang, ChemPlusChem 79 (2014) 577583.
5
Figure captions Fig. 1. Schematic diagram of the synthetic route of NiS@NF. Fig. 2. SEM images of (a) the bare NF and (b-d) NiS@NF at low and high magnifications; TEM images of a NiS hollow sphere at different magnifications (g-h). Fig. 3. XRD patterns of the bare NF and NiS@NF (a); the deconvoluted XPS of Ni2p and S2p of NiS@NF (b). Fig. 4. (a) CV curves of NF before and after electrodeposition; (b) CV curves of the bare NF and NiS@NF at the scan rate of 5 mV/s; (c) CV curves at different scan rates, (d) galvanostatic charge/discharge curves as a function of the current density, (e) Nyquist plot, and (f) cycling stability of NiS@NF up to 2,000 cycles.
Figure 1.
6
Figure 2.
Figure 3.
7
Figure 4.
8
Highlights NiS hollow spheres were electrodeposited directly on Ni foam at room temperature. The NiS@NF composite showed high specific capacitance: 1,553 F g-1 at 2.35 A g-1. The NiS@NF composite exhibited good cycling stability: 95.7 % after 2,000 cycles.
9
10