Hierarchical Ni3S2 nanosheets coated on mesoporous NiCo2O4 nanoneedle arrays as high-performance electrode for supercapacitor

Author’s Accepted Manuscript Hierarchical Ni3S2 nanosheets coated on mesoporous NiCo2O4 nanoneedle arrays as highperformance electrode for supercapacitor Y. Chang, Y.W. Sui, J.Q. Qi, L.Y. Jiang, Y.Z. He, F.X. Wei, Q.K. Meng www.elsevier.com

PII: DOI: Reference:

S0167-577X(16)30537-7 http://dx.doi.org/10.1016/j.matlet.2016.04.059 MLBLUE20656

To appear in: Materials Letters Received date: 16 February 2016 Revised date: 3 April 2016 Accepted date: 8 April 2016 Cite this article as: Y. Chang, Y.W. Sui, J.Q. Qi, L.Y. Jiang, Y.Z. He, F.X. Wei and Q.K. Meng, Hierarchical Ni3S2 nanosheets coated on mesoporous NiCo2O nanoneedle arrays as high-performance electrode for supercapacitor, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.04.059 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 galley proof before it is published in its final citable 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.

Hierarchical Ni3S2 nanosheets coated on mesoporous NiCo2O4 nanoneedle arrays as high-performance electrode for supercapacitor Y. Chang, Y.W. Sui, J.Q. Qi*, L.Y. Jiang, Y.Z. He, F.X. Wei, Q.K. Meng School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, People’s Republic of China *

Corresponding author. Tel.: +86-516-83591788, Fax: +86-516-83591788. E-mail addresses: [email protected]

Abstract A novel hierarchical structure of Ni3S2 nanosheets coated on mesoporous NiCo2O4 nanoneedles arrays was designed and investigated. Due to the positive synergistic effect of the two materials resulting from the three-dimensional coated structure, the as-prepared NiCo2O4/Ni3S2 heterostructure hybrid exhibited excellent electrochemical performance with areal capacitance of 4569.1 mF·cm-2 at current density of 1 mA·cm-2 as well as remarkable cycling stability of 89.2% capacitance retention after 1000 cycles. The impressive results here may pave the way to development of high-performance electrode materials for supercapacitors. Keywords: Nanocomposites; Ni3S2 nanosheets; NiCo2O4 nanoneedles; Coated structure; Electrical properties; Supercapacitor 1. Introduction Currently, supercapacitors have been considered to be the most ideal candidates for the next generation energy storage system due to their attractive traits, such as high-power density, fast charge-discharge ability, long cycle-life, environment friendly [1, 2]. Electrode material is the key factor. Hence a great deal of effort has been performed on the synthesis of high-performance

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electrode materials [3, 4]. Spinel NiCo2O4 has attracted considerable attention in high performance lithium ion batteries and supercapacitors owing to its excellent electrical conductivity and redox chemical valences [5-7]. In order to further enhance electrical performance of NiCo2O4, NiCo2O4 has been combined with other electrode materials to achieve composites with hybrid structures [8, 9]. Among them, core/shell structure is a typical one. Various configurations with core/shell structure have been designed [10, 11]. Besides, coated structure was developed in many systems, such as Fe3O4/carbon coated silicon composite [12], Cobalt sulfide coated on NiCo2S4 [13], NiO coated CuO [14]. Recently, heazlewoodite Ni3S2 has been used in supercapacitors owing to its high theoretical capacity, good electrical conductivity and low cost [15-17]. As so far, little work has been performed on Ni3S2 coated on NiCo2O4. Herein, Ni3S2 nanosheets coated on mesoporous NiCo2O4 nanoneedle arrays were designed and synthesized by a facile two-step hydrothermal method in this paper. The high specific capacitance of the hybrid could be expected due to the synergistic effect of the combination of Ni3S2 and NiCo2O4 in alkaline electrolyte. 2. Experimental procedure The Ni foam (NF) with the size of 1×2 cm2 was cleaned using a 1M HCl solution with sonication for 20 min to remove oxide layer, and then rinsed with deionized (DI) water and ethanol. In a typical synthesis of NiCo2O4 nanoneedle arrays, 2 mmol Co(NO3)·6H2O, 1 mmol Ni(NO3)·6H2O, 8 mmol urea and 4 mmol NH4F were dissolved in a 40 ml of DI waters. After that, the above solution and a piece of NF were transferred into a 50 ml Teflon-lined stainless steel autoclave. The autoclave was then sealed and maintained at 120 °C for 6 h in an oven. Subsequently, the sample was annealed at 350 °C for 3 h at a ramping rate of 1 °C·min-1 in air to

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obtain the NiCo2O4 nanoneedles. In order to synthesize Ni3S2 coated on NiCo2O4, 2 mmol Ni(NO3)·6H2O and 2 mmol thiourea were dissolved in 40 DI water under stirring for 30 min. NF with NiCo2O4 nanoneedles was put in the solution in a 50ml Teflon-lined stainless steel autoclave, and kept at 120 °C for 4h. Subsequently, the sample was rinsed several times with DI water and dried at 70 °C for 12h in a vacuum oven. The mass loading of NiCo2O4 was 1.8 mg·cm-2 and the total mass of NiCo2O4/Ni3S2 hybrid was approximately 3.3 mg·cm-2. For a comparison, the bare Ni3S2 nanosheets and the NiCo2O4 nanoneedles were also synthesized by the same method. The crystal structure of product was characterized by X-Ray Diffraction (XRD, D8 Advance, Bruker, Germany). Hitachi SU-70 scanning electron microscope (SEM) and FEI Tecnai G2 F20 transmission electron microscopy (TEM) were used to observe the samples. The electrochemical performance of the as-prepared sample was evaluated by cyclic voltammetrode (CV) and galvanostatic charge-discharge tests with an electrochemical workstation (CHI660E, Chenhua). All measurements were carried out at room temperature in a three-electrode cell, in which the as-prepared sample were used as the working electrode, a platinum foil as the counter electrode, Hg/HgO as the reference electrode, and 2M KOH as the electrolyte. 3. Results and discussion Fig. 1 shows the wide-angle XRD pattern of the as-synthesized NiCo2O4/Ni3S2 heterostructure hybrid. As seen from Fig. 1, the three strong peaks correspond to the NF substrate. Additionally, the diffraction peaks of spinel NiCo2O4 and heazlewoodite Ni3S2 can be observed from Fig. 1, confirming the presence of NiCo2O4 and Ni3S2 in the synthesized material. There is no other additional diffraction peaks in the pattern, revealing the high phase purity of

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NiCo2O4/Ni3S2 heterostructure hybrid formed. Fig. 2 illustrates the surface morphology and hierarchical structure of Ni foam, NiCo2O4 as well as NiCo2O4/Ni3S2 heterostructure hybrid. The surface of the bare Ni foam is very clean (Fig. 2(a)). In Fig. 2(b), it is shown that dense NiCo2O4 nanoneedle arrays uniformly grow on the surface of NF. The nanoneedles varied from 15nm to 35nm in diameter and the average size was about 24nm (Fig. 2(c)). NiCo2O4 nanoneedle arrays were fabricated via similar method in previous work [18]. After the second hydrothermal process, Ni3S2 nanosheets interconnected each other with large size of 0.5-0.6μm were uniformly coated on the surface of NiCo2O4 nanoneedles, as shown in Fig. 2(d). At high magnification, Ni3S2 nanosheets with sheet-like shape were observed (Fig. 2(f)) and connected with each other to form a network. Additionally, the uniform Ni3S2 nanosheets displayed a highly open structure. For such a unique structure, the space between nanoneedlles and nanosheets may provide a green channel for electrolyte ions access to the surface of the active materials [19]. The significant difference in size between NiCo2O4 and Ni3S2 may result in the formation of the coated structure. However, from Fig. 2, the hierarchical structure of Ni3S2 nanosheets coated on NiCo2O4 nanoneedles can not be confirmed. Hence TEM was used to further investigate the morphology of NiCo2O4 and NiCo2O4/Ni3S2 heterostructure hybrid, as shown in Fig. 3. Fig. 3(a) shows typical structure of NiCo2O4 nanoneedls uniformly grown on NF substrate. Fig. 3(b) displays that NiCo2O4 nanoneedles are highly mesoporous, which may be caused by the release of gases (e.g. CO2) during the thermal decomposition process [20]. Clear lattice fringes with interplanar spacing of ~4.7Å can be seen from the high-resolution TEM image in Fig. 3(c), which corresponds to the (111) plane of spinel NiCo2O4 [1, 5]. After the second hydrothermal process,

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Ni3S2 covered on the surface of NiCo2O4 nanoneedles as a whole, as shown in Fig. 3(d). At high magnification, it can be obviously seen that Ni3S2 nanosheets coated on NiCo2O4 nanoneedles (Fig. 3(e)). Evidently, the second-step experiment led to the formation of a coated structure in comparison with the structure in Fig. 3(a). From Fig. 3(f), the Ni3S2 crystal lattice fringes of (110) plane with d-spacing of 2.88Å are apparent [19]. The supercapacitive performances of NiCo2O4 and NiCo2O4/Ni3S2 heterostructure hybrid were studied by CV and galvanostatic discharge techniques, respectively. Fig. 4(a) illustrates the CV curves of the NiCo2O4/Ni3S2 hybrid electrodes at different scan rates with potentials windows range from -0.4 V to 1.0 V (vs. Hg/HgO). A pairs of well-defined redox peak can be clearly observed in Fig. 4(a), implying the presence of a reversible Faradic reaction and pseudo-capacitance behavior. The reactions of NiCo2O4 and Ni3S2 could be presented as follows: NiCo2O4 + OH-1 + H2O  NiOOH + 2CoOOH + e-1

(1)

CoOOH + OH-1  CoO2 + H2O + e-1

(2)

Ni3S2 +3OH-1  Ni3S2(OH)3 + 3e-1

(3)

In addition, the current response increases accordingly and the shapes of CV curves do not change obviously with the increase of scan rate, indicating excellent electrochemical reversibility and good rate capability. The comparison of CV curves of pure NiCo2O4 nanoneedles, Ni3S2 nanosheets and NiCo2O4/Ni3S2 heterostructure hybrid electrodes at a scan rate of 20mV/s are shown in Fig. 4(b). It is seen that the NiCo2O4/Ni3S2 electrode exhibits the largest area at the same scan rate, suggesting the highest capacitance and electrochemical reaction activity. The greatly enhanced electrochemical behavior should be attributed to the synergistic effect resulting from the three-dimensional coated structure. The positive synergistic effect improves the ions and electrons

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transport efficiency, conductivity and structural stability [21]. The galvanostatic discharge curves of the NiCo2O4/Ni3S2 at different current densities in the voltage range of 0.1-0.52V (vs. Hg/HgO) are present in Fig. 4(c). A distinct plateaus appears during the discharge process, further verifying the pseudo-capacitance behavior. This result corresponds well with the CV curves. The areal capacitance of NiCo2O4/Ni3S2 hybrid were calculated to be 4569.1, 4187.6, 3930.2, 3292.8, 3118.6 and 2863.4 mF·cm-2 at current densities of 1, 2, 5, 10, 15, and 20 mA·cm-2, respectively (Fig. 4(e)). The areal capacitance of NiCo2O4/Ni3S2 hybrid is far higher than those of NiCo2O4 nanoneedles (1498.1 mF·cm-2) and Ni3S2 nanosheets (1664.7 mF·cm-2) at 2 mA·cm-2 (Fig. 4(d)). The cycle stability of the NiCo2O4/Ni3S2 electrode is evaluated by repeated charge-discharge measurement at high current densities of 10 mA·cm-2, as shown in Fig. 4(f). At this current density, the areal capacitance of 2923.8 mF·cm-2 (89.2% of the initial value) can be maintained after 1000 cycles, indicating the excellent cyclability of the NiCo2O4/Ni3S2 electrode. 4. Conclusion A new hierarchical Ni3S2 nanosheets coated on mesoporous NiCo2O4 nanoneedles grown on NF were successfully fabricated and used as an electrode for supercapacitor. The synergistic effect between NiCo2O4 nanoneedles and Ni3S2 nanosheets improves the ions and electrons transport efficiency, conductivity and structural stability of the hybrid. As a result, the as-prepared NiCo2O4/Ni3S2 electrode exhibited excellent electrochemical performance with areal capacitances of 4569.1 mF·cm-2 at current density of 1 mA·cm-2. Furthermore, long cycling stability of 89.2% retention after 1000 cycles at the current density of 10 mA·cm-2 was obtained. It is believed that the unique structure of NiCo2O4/Ni3S2 heterostructure hybrid could be a promising electrode

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material for supercapacitor. Acknowledgments This research was supported by the financial support provided by “the Fundamental Research Funds for the Central Universities” (2014QNA12). References [1]

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Fig. 1 XRD pattern of NiCo2O4/Ni3S2 heterostructure hybrid grown on NF.

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Fig. 2 SEM images of (a) Ni foam, (b, c) NiCo2O4 and (d-e) NiCo2O4/Ni3S2 heterostructure hybrid.

Fig. 3 (a) TEM image of Ni foam, (b) and (c) TEM and HRTEM images of NiCo2O4 nanoneedles, (d, e) TEM images of NiCo2O4/Ni3S2 heterostructure hybrid and (f) HRTEM image of Ni3S2.

Fig. 4 (a) CV curves of the NiCo2O4/Ni3S2 electrode, (b) CV curves, (c) galvanostatic discharge curves of the NiCo2O4/Ni3S2 electrode, (d) galvanostatic charge/discharge curves, (e) specific capacitance of NiCo2O4/Ni3S2 electrode, and (f) electrochemical cyclic stability of NiCo2O4/Ni3S2.

Highlights 

A novel structure of Ni3S2 nanosheets coated on NiCo2O4 nanoneedles was designed.



There exists positive synergistic effect between NiCo2O4 and Ni3S2. 9

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NiCo2O4/Ni3S2 heterostructure hybrid exhibits higher electrochemical property.

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