Hierarchical Structured Ni3S2@rGO@NiAl-LDHs Nanoarrays: A

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Hierarchical structured Ni3S2@rGO@NiAl-LDHs nanoarrays: a competitive electrode material for advanced asymmetrical supercapacitors Dongxuan Guo, Xiumei Song, Lichao Tan, Huiyuan Ma, Haijun Pang, Xinming Wang, and Lulu Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06053 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018

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Hierarchical

structured

Ni3S2@rGO@NiAl-LDHs

nanoarrays: a competitive electrode material for advanced asymmetrical supercapacitors Dongxuan Guo,a Xiumei Song,c Lichao Tan,*a,b Huiyuan Ma,*a Haijun Pang,a Xinming Wanga, Lulu Zhanga

a, School of Materials Science and Engineering, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, No. 4 Linyuan Road, Xiangfang District, Harbin 150040, China E-mail address: [email protected], [email protected] b, Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, No. 145 Nantong street, Nangang District Harbin 150001, China c, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 73 Huanghe Road, Nangang District, Harbin 150090, P R China

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Abstract In this account, well-aligned hierarchical nickel sulfide@reduced graphene oxide@nickel aluminium layered double hydroxides composite (denoted as Ni3S2@rGO@NiAl-LDHs) supported on Ni-foam substrate is successfully designed and constructed via successive hydrothermal process. Ni3S2 nanorod arrays grown on Ni-foam could provide large open space and short ions diffusion path, graphene with high specific surface area and excellent conductivity can effectively transfer charges, NiAl-LDHs has large contact area with electrolyte, thus enabling fast and reversible redox process, which could improve the specific capacitance. As a consequence, the Ni3S2@rGO@NiAl-LDHs fulfills superior specific capacity, pleasurable chargedischarge rate and outstanding lifespan. Moreover, an advanced asymmetrical device is assembled by employing Ni3S2@rGO@NiAl-LDHs and rGO@Fe3O4-C, which delivers high specific capacity (201.3 Fg-1) and exceptional energy density (71.7 Whkg1).

The well-aligned Ni3S2@rGO@NiAl-LDHs could provide a promising conception

constructing hierarchical structural materials in the area of supercapacitor. Keywords: Ni3S2; rGO; NiAl-LDHs; Asymmetric supercapacitor; Hierarchical structure

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Introduction As the power source metric of civilization, energy has deep implications for the life of humanity. Currently, the consumption of fossil fuel has accelerated dramatically global energy and ecological crisis.1-4 Recently, supercapacitors have attracted strong interest owing to ultrahigh power density, environmental benignity, and long lifespan.5,

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Typically, supercapacitors can be classified into pseudocapacitors (PCs) and electrical double-layer capacitors (EDLCs) according to working mechanism.7 Generally, the working mechanism of PCs is reversible redox process, and EDLCs are related with charge accumulation.8-11 Taking pseudocapacitive materials into consideration, Ni3S2 has spurred great interest as an attractive platform to construct various functional materials because of high specific capacitance, excellent redox process and eco-friendly.12-16 For example, Lou et al. fabricated Ni3S2 nanosheets on CNTs backbone through a multi-step route. Impressively, a satisfying specific capacity of 514 Fg-1 was delivered at 4 Ag-1.17 Zhang et al. constructed Ni3S2@Ni(OH)2 core-shell nanostructures on three-dimensional graphene, which revealed a favorable specific capacity of 1037.5 Fg-1.18 However, the large-scale application of Ni3S2 is still hindered by the poor conductivity (~10-17 S/cm) accompanied with short diffusion distance of electrolyte into pseudocapacitor electrodes,19 only the external part can be involved into redox process while the interior could scarcely contribute to the total capacitance, which would inevitably lead to an unsatisfactory rate capability and cycling stability. To circumvent these drawbacks and then widen the practical application of Ni3S2, 3

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it is highly feasible to construct hierarchical construction with the combination of two/three types of materials on 3D conductive matrix as binderless electrode.20-28 Nickel sulfides with hierarchical structure on Ni-foam could possess many incomparable advantages such as fast ions/electrons transport, low contact resistance and intensive cycling stability compared to single-phase sulfides.29-32 Furthermore, the hierarchical configuration can efficiently lower the surface energy, which may result in a high level of reversibility.33, 34 Herein, combing all these ideas together, the hierarchical Ni3S2@rGO@NiAlLDHs assembled on Ni-foam is engineered and prepared through a three-step hydrothermal reaction. The well-aligned hierarchical Ni3S2@rGO@NiAl-LDHs yields beneficial electrochemical behavior due to the following strengths: (i) the vertical aligned Ni3S2 nanorods directly supported on conductive matrix could avoid the utilization of nonconductive polymeric binders; (ii) graphene figures as support to assemble NiAl-LDHs nanosheets and the second conductive matrix for electrons transfer; (iii) the further introduction of NiAl-LDHs enables a high contact area with electrolyte, which could improve the specific capacitance of electrode materials. As a consequence, the hierarchical composite shows outstanding electrochemical performance. To evaluate the electrochemical behavior of Ni3S2@rGO@NiAl-LDHs as the positive electrode in asymmetrical supercapacitor, we also design a suitable negative

electrode,

rGO@Fe3O4-C,

using

Ni3S2@rGO@NiAl-LDHs//rGO@Fe3O4-C

a

hydrothermal

asymmetrical

process.

supercapacitor

The fulfills

superior energy density and excellent cycling stability. The well-aligned 4

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Ni3S2@rGO@NiAl-LDHs could provide a promising conception constructing hierarchical structural materials in the area of supercapacitor.

Scheme. 1 The fabrication procedure of Ni3S2@rGO@NiAl-LDHs on Ni-foam.

Results and Discussion The growth procedure could be illustrated in scheme. 1. Initially, Ni-foam serves as conductive matrix to grow Ni3S2 nanorod arrays via hydrothermal reaction (Fig. 1a). Afterward, the second hydrothermal process is implemented during which the thin graphene nanosheets can be uniformly coated on the as-grown Ni3S2 nanorods (Fig. 1bc). Finally, NiAl-LDHs is introduced through in situ growth procedure in which dense NiAl-LDH nanosheets self-assemble on graphene from a top view (Fig. 1d-e). Notably, the composite has a smooth surface with 2.8 µm in height (Figure 1f). Such hierarchical structure could provide more electroactive sites and rapid ions/electrons transport.

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Fig. 1 SEM images of (a) Ni3S2, (b)-(c) Ni3S2@rGO and (d)-(e) Ni3S2@rGO@NiAl-LDHs. (f) Cross-sectional SEM image of Ni3S2@rGO@NiAl-LDHs.

The morphology of the as-prepared Ni3S2@rGO@NiAl-LDHs composite is measured by transmission electron microscopy (TEM). Ni3S2 nanorods and rGO nanosheets can be seen from Figure 2(a), and the interplanar spacing can be demonstrated to be 0.41 nm from inset, corresponding to (101) plane of Ni3S2. Moreover, ultrathin NiAl-LDHs nanosheets grow vertically (Figure 2(b)), the interplanar spacing can be demonstrated to be 0.26 nm from inset, corresponding to (012) plane of NiAl-LDHs.35, 36

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Fig. 2 TEM images of the Ni3S2@rGO@NiAl-LDHs at different magnifications.

Typical X-ray diffraction (XRD) analysis of Ni3S2, Ni3S2@rGO and Ni3S2@rGO@NiAl-LDHs on Ni-foam is illustrated in Fig. 3a. Three typical strong peaks could be attributed to Ni-foam.37 Besides, the peaks at 21.7, 31.1, 37.8, 49.7 and 55.2o are ascribed to the single crystalline Ni3S2 (JCPDS No. 44-1418).38 Another series of peaks observed at 11.6, 23.1, 34.9, 39.4, 60.8 and 62.4o indicates the presence of NiAl-LDHs (JCPDS No.15-0087).39 Additionally, the peak derived from graphene is found simultaneously with relatively low intensity owing to the low mass content.40 The as-prepared samples are further characterized by FTIR analysis. Fig. 3b demonstrates the FT-IR spectrum of Ni3S2, Ni3S2@rGO and Ni3S2@rGO@NiAl-LDHs. The broad peak at 3446 cm-1 is attributed to the O-H, the stretching vibrations at 1355 and 827 cm-1 can be assigned to CO32-.40 Besides, the metal-oxygen (M-O or O-M-O) vibrations are recorded below 800 cm-1 in the low wavenumber region.41

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Fig. 3 XRD patterns and FT-IR spectra of Ni3S2, Ni3S2@rGO and Ni3S2@rGO@NiAl-LDHs on Nifoam.

XPS spectroscopy is adopted to examine the Ni3S2@rGO@NiAl-LDHs electrode (Fig. 4). Fig. S1 confirms the coexistence of Ni, S, O, C, and Al. The peaks located at 874.4 and 856.6 eV are attributed to Ni 2p1/2 and Ni 2p3/2 (Fig. 4a), indicating the existence of Ni2+.42, 43 Two peaks situated at 68.5 and 74.1 eV in Al 2p XPS spectrum are assigned to Al 2p3/2 and Al 2p1/2, respectively (Fig. 4b). Besides, the peaks centered at 161.8 eV and 168.6 eV in S 2p region are attributed to S2- and high oxidation state sulfur (Fig. 4c).44 In Fig. 4d, one major peak of C 1s is located at 284.5 eV (C-C), and two minor peaks are situated at 286.5 eV (C-O) and 287.8 eV (C=O).45

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Fig. 4 XPS patterns of Ni3S2@rGO@NiAl-LDHs.

The obtained Ni3S2@rGO@NiAl-LDHs (4 mg/cm2) on Ni-foam, together with Ni3S2@rGO (2 mg/cm2) and Ni3S2 (1.5 mg/cm2), is selected as electrode in a standard three-electrode system. CV profiles of Ni3S2@rGO@NiAl-LDHs are investigated (Fig. 5a). As expected, great accessibility of electrolyte ions through the electrode surface leads to a pair of pronounced redox peaks, owing to the valence state changes of Ni2+/Ni3+.46 In addition, these quasi-symmetrical profiles retain well with the scan rates, indicating the desirable rate capability.47 Furthermore, the CV profiles of these samples at 15 mVs-1 are illustrated in Fig. 5b. The Ni3S2@rGO@NiAl-LDHs demonstrates more intense current density and enveloped area than the other two counterparts, suggesting an enhanced activity and specific capacitance. The intensive electrochemical behavior 9

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is ascribed to more active sites and rapid electrons transport. The galvanostatic discharging curves of Ni3S2@rGO@NiAl-LDHs are provided in Fig. 5c, which delivers specific capacitances of 2026, 1816, 1658, 1395, 1221 and 1010 Fg-1 at 1, 2, 3, 5, 8 and 10 Ag-1, respectively. For comparison, the specific capacitances of

Ni3S2,

Ni3S2@rGO,

Ni3S2@rGO@NiAl-LDHs,

rGO@NiAl-LDHs

and

Ni3S2@NiAl-LDHs against various current densities are compared (Fig. 5d). The maximum specific capacity of Ni3S2@rGO@NiAl-LDHs is higher than those previously recorded Ni3S2-based or NiAl-LDH-based composites (Table. 1).

Fig. 5 CV profiles of Ni3S2@rGO@NiAl-LDHs (a); CV profiles of Ni3S2, Ni3S2@rGO and Ni3S2@rGO@NiAl-LDHs at 15 mVs-1 (b); The galvanostatic discharging profiles of Ni3S2@rGO@NiAl-LDHs at different current densities (c); Specific capacity of Ni3S2, Ni3S2@rGO,

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Ni3S2@rGO@NiAl-LDHs, rGO@NiAl-LDHs and Ni3S2@NiAl-LDHs at various current densities (d).

EIS tests are measured to figure out the kinetics of electrode materials (Fig. 6a). The impedance spectra is made up of a semicircle at high frequency region and a straight line at low frequency region. The slope of the straight line reveals the diffusion resistance (RW). The intercept of the semicircle at Z’’-axis represents the resistance of the electrochemical system (Rs).53 The semicircle diameter reflects the charge transfer resistance (Rct) of the redox reaction, which is related to the porous structure of electrode material.54 As a result, Rct values of Ni3S2, Ni3S2@rGO, Ni3S2@rGO@NiAlLDHs, rGO@NiAl-LDHs, and Ni3S2@NiAl-LDHs are 0.26, 0.29, 0.22, 1.1 and 0.33 Ω, respectively. The smallest Rct of Ni3S2@rGO@NiAl-LDHs demonstrates a rapid charge transfer process. In addition, the Rs values of each them are 0.72, 0.37, 0.48, 0.82 and 0.91 Ω, respectively. The result can be ascribed to the high specific surface area of graphene, which endows the Ni3S2 nanorods and NiAl-LDHs nanosheets with rapid charge transfer. Moreover, Ni3S2@rGO@NiAl-LDHs has a steeper slope along Z’’ axis, reflecting its efficient ions diffusion during redox reaction. Continuous GCD tests are operated at 1 A/g to test the cycling stability of Ni3S2@rGO@NiAl-LDHs (Fig. 6b). A capacitance retention of 87.7% over 10000 cycles is witnessed, showing that hierarchical nanostructures can buffer the volume expansion effectively. The feature of the Ni3S2@rGO@NiAl-LDHs after cycling measurement is further investigated (Fig. S2). Obviously, morphology is well-preserved even after cycling.

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Fig. 6 Electrochemical EIS of the Ni3S2, Ni3S2@rGO, Ni3S2@rGO@NiAl-LDHs, rGO@NiAlLDHs and Ni3S2@NiAl-LDHs (a); cyclic performance of Ni3S2@rGO@NiAl-LDHs (b). Table 1 Comparison of capacitances between the Ni3S2@rGO@NiAl-LDHs and Ni3S2-based or NiAl-LDH-based composites. Material

Specific

Current

Capacitance

Density

(Fg-1)

Electrolyte

Refs

(Ag-1)

Ni3S2@3DrGO

1886

1

2 M KOH

48

3D Ni3S2

1370.4

2

6 M KOH

49

NiAl-LDHs@rGO

1630

1

6 M KOH

50

NiAl-LDHs@CNT

1500

1

2 M KOH

51

NiAl-LDHs@MnO2

1554

1

6 M KOH

52

Ni3S2@rGO@NiAl-LDHs

2026

1

6 M KOH

This work

To explore the possibility of Ni3S2@rGO@NiAl-LDHs as the positive electrode in asymmetrical supercapacitor (ASC), reduced graphene oxide@ferroferric oxide-carbon (rGO@Fe3O4-C) composite is also synthesized via a hydrothermal process as a negative electrode. As illustrated in Fig. 7a and b, it is clear to see that Fe3O4-C nanoparticles

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are well attached to graphene nanosheets. Besides, the XRD analysis demonstrates that all the typical peaks coincide with Fe3O4 (PDF#26-1136), rGO and carbon (Fig. 7c). Fig. 7d demonstrates the CV profiles of rGO@Fe3O4-C. The shapes of these CV profiles indicate that the capacitance results from the EDLC via ions adsorption and the pseudocapacitance via the redox couple of Fe2+/Fe3+.55,

56

No significant change is

detected in the shapes of CV curves, thus suggesting good rate capability. The typical GCD curves are plotted in Fig. 7e, and the rGO@Fe3O4-C electrode demonstrates a favorable specific capacity of 226 Fg-1 at 1 Ag-1 from discharging curve. EIS measurement is further conducted to investigate the kinetics of rGO@Fe3O4-C electrode (Fig. 7f). The low Rct (about 0.12) indicates fast electron transfer. Therefore, we believe that the as-synthesized rGO@Fe3O4-C is an outstanding negative electrode.

Fig. 7 (a)-(b) SEM images of rGO@Fe3O4-C; (c) XRD pattern of rGO@Fe3O4-C; (d) CV profiles of rGO@Fe3O4-C at different scan rates; (e) GCD profiles of rGO@Fe3O4-C at different current densities; (f) Electrochemical EIS of rGO@Fe3O4-C.

Typically, the two as-prepared electrodes with a separator are immersed into 2 M 13

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KOH solution (Fig. 8a). The mass ratio between positive and negative electrode is determined by charge balance. A couple of redox peaks demonstrate the redox reaction between electrode and electrolyte (Fig. 8b), and CV profiles retain the primary shape with the scan rates, which indicates the excellent rate performance of ASC. The ACS delivers 202, 170.4, 152, 129 and 93.8 Fg-1 at 1, 2, 3, 5 and 10 Ag-1, respectively (Fig. 8c). Fig. 8d exhibits Ragone plot along with the reported results. Remarkably, the ASC exhibits a relatively high value of 71.7 Whkg-1 at 801.6 Wkg-1, significantly higher than other reported devices.57-61 Long-term lifespan is a decisive factor for actual applications, and the as-assembled ASC is subjected to 5000 continuous GCD cycles from 0 to 1.6 V. Noticeably, the ASC still delivers a relatively superior specific capacity retention of 91.8% (Fig. S3), reflecting its excellent electrochemical stability. Such glorious electrochemical performance of the as-fabricated Ni3S2@rGO@NiAlLDHs//rGO@Fe3O4-C device can be ascribed to the excellent pseudocapacitive properties of Ni3S2@rGO@NiAl-LDHs as a positive electrode and the enlarged potential range of rGO@Fe3O4-C as a negative electrode.

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Fig. 8(a) Electrochemical tests of Ni3S2@rGO@NiAl-LDHs//rGO@Fe3O4-C ASC in 2 M KOH; (b) CV profiles of ASC at different scan rates; (c) GCD profiles of ASC at different current densities; (d) Ragone plots of ASC.

Conclusion In conclusion, well-aligned hierarchical Ni3S2@rGO@NiAl-LDHs on Ni foam has been developed through a three-step hydrothermal process. The as-prepared hierarchical electrode reveals intensive electrochemical properties, including excellent specific capacity and favorable lifespan. Moreover, an advanced asymmetrical device based on Ni3S2@rGO@NiAl-LDHs and rGO@Fe3O4-C is fabricated, which yields remarkable electrochemical performance. The well-aligned Ni3S2@rGO@NiAl-LDHs could provide a promising conception constructing hierarchical structural materials in the area of supercapacitor.

Supporting information 15

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Experimental section, material characterizations, electrochemical measurements, and additional characterization and measurement.

Acknowledgments This work was financially supported by the NSF of China (21671049, 51702071, 5172063), the Heilongjiang Science Foundation (QC2018066), the China Postdoctoral Science Foundation (2016M601414, 2018T110278), and Heilongjiang Postdoctoral Foundation (LBH-TZ1706).

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Ni3S2@rGO@NiAl-LDHs is prepared via hydrothermal process, the whole process is simple, safe and pollution-free, conforming to the principles of sustainability in the chemistry. 254x172mm (150 x 150 DPI)

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