Hexagonal phase NiS octahedrons co-modified by 0D-, 1D-, and 2D carbon materials for high-performance supercapacitor

Electrochimica Acta 311 (2019) 83e91

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Hexagonal phase NiS octahedrons co-modified by 0D-, 1D-, and 2D carbon materials for high-performance supercapacitor Rui Zhang, Chengxing Lu, Zhaoliang Shi, Tong Liu, Tengfei Zhai, Wei Zhou* School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, 100191, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 March 2019 Received in revised form 2 April 2019 Accepted 17 April 2019 Available online 22 April 2019

It's crucial to improve the electronic conductivity, active sites and structural stability of transition-metal sulfides for better performance. Herein, hexagonal-phase NiS octahedrons were obtained through phase transition and co-modified by multidimensional carbon, i.e. 0D carbon QDs, 1D CNTs, and 2D reduced graphene oxide (NiS@C QDs-CNTs-rGO). It delivers a significantly enhanced specific capacity of 241 mAh g
1 at a current density of 1 A g
1 and capacity of 149 mAh g
1 at 20 A g
1, superior to its counterparts with other phases NiS2@CNTs-rGO (154 mAh g
1 at 1 A g
1, 52 mAh g
1 at 20 A g
1) and Ni7S6@CNTsrGO (167 mAh g
1 at 1 A g
1, 124 mAh g
1 at 20 A g
1). Furthermore, asymmetric supercapacitors (ASC) assembled by NiS@C QDs-CNTs-rGO and graphene hydrogel achieve a remarkable cycling stability (capacity retention of 82% after 5000 cycles). XPS results confirm that strong CeS bonds exist between carbon matrix and NiS NPs, which stabilizes structural stability and thus leading to excellent long-term cycling stability. The excellent electrochemical performance could be ascribed to the improved conductivity and structural stability, the co-modified 0D, 1D, and 2D carbon structures, and strong CeS bonds between active material and carbon matrix. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Nanostructure Phase transition Multidimensional carbon CeS bonds Electrochemical properties

1. Introduction As their reversible multielectron Faradaic reactions on the surface of electrode materials, pseudocapacitive or battery-type electrode have attracted much attention in the field of energy transfer and storage. It is because of their higher specific capacity and power density than electrochemical double-layer capacitors (EDLCs) [1e4]. Among various promising battery-type electrode materials, transition metal sulfides (TMSs) have been widely studied for their high electrochemical activity [5e8]. But those semiconductor materials show poor electrical conductivity, rate capability and electrochemical stability, which limits their practical application [9e11]. To overcome these disadvantages, a simple method is to introduce conductive carbon materials, such as zero-dimensional (0D) carbon quantum dots (QDs), one-dimensional (1D) carbon nanotubes (CNTs), or two-dimensional (2D) graphene sheet to form composite nanomaterials [12e15]. For example, carbon dots modification on CuS-GO composite and MnO2 nanowires can improve the active sites and stabilize the electrode materials

* Corresponding author. E-mail address: [email protected] (W. Zhou). https://doi.org/10.1016/j.electacta.2019.04.111 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

[12,13]. Ying's group introduced graphene to form graphenewrapped nickel sulfide composite, which showed significantly improved rate capability compared to nickel sulfide at high current densities [14]. Considering the crumpled property of 2D flexible graphene, researchers make an effort to fabricate 3D spatial structures by introducing extra 1D CNTs into 2D graphene, which benefits for a good dispersion of active materials and better electron transport with three-dimensional (3D) network-like structure [15]. These complex spatial architecture composed of multiple dimensional structures can improve the structural and electrochemical stability and electric conductivity [15,16]. Besides, effective linking of rational structures between main compositions are necessary to enhance structural stability and electrical conductivity of the composites. For example, Yang et al. improved the stability by functionalizing carbon spheres with graphene through p-p interaction to form a sandwich-like porous structure with stable charge transfer during long-term cycling [17]. Also, the connection between conductive substrates and active components can influence the conductivity of the electrodes. For example, the active materials of nickel sulfides can be stabilized by chemical bonds of SeC with tight connection to carbon materials [5,18]. Thus, chemical bonding modification is also an efficient route to improve the electrochemical performance of electrode

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materials. Herein, we used NiS2 octahedrons freeze-dried with 1D CNTs and 2D graphene as precursor to fabricate 3D structure, which was then heat-treated at different temperature to obtain hexagonal phase NiS that modified by carbon QDs, CNTs, and rGO (NiS@C QDsCNTsrGO), cubic phase NiS2 modified by CNTs and rGO (NiS2@CNTsrGO), and orthorhombic phase Ni7S6 modified by CNTs and rGO (Ni7S6@CNTs-rGO). The 3D composite electrode material showed much improved conductivity and stability compared to pure nickel sulfides. Interestingly, remarkable CeS bonds between carbon matrix and NiSx particles were confirmed, which also strengthened the structural stability and conductivity of the composite. We systematically tested the electrochemical performance of NiSx-based materials, using the three samples separately as electrode material. The results show that the NiS@C QDs-CNTs-rGO delivers a higher specific capacity and superior high-rate capability. In addition, we assembled asymmetric supercapacitors (ASC) based on the NiS@C QDs-CNTs-rGO and graphene hydrogel (GH), which also exhibit excellent capacity retention of ~82% after 5000 cycles. The method provides a feasible route for multidimensional carbon modification and simultaneous phase control, which is beneficial to develop high-performance electrode materials for supercapacitors. 2. Experimental section COOH-functionalized multi-walled carbon nanotubes (MWCNTs) were purchased from Nanjing XFNANO Materials Tech Co., Ltd., China, which were washed by acetone, ethanol and Deionized (DI) water for further use. Graphene oxide (GO) sheets were prepared through modified Hummer's method, which were dispersed and stored in aqueous solution [19]. 2.1. Synthesis of the precursor NiS2-CNTs-GO The NiS2 octahedrons were synthesized based on our previous method [20]. In a typical experiment, 0.237 g of NiCl2$6H2O, 0.65 g of Na2S3O4$6H2O and 0.55 g of PVP were dispersed in 38 mL of deionized water by 2 h ultrasonic vibration. The solution was poured into autoclave (50 mL), which was then sealed and kept in an oven at 150  C for 12 h, and then cooled down to room temperature. The product was washed several times with DI water and dried at 60  C. NiS2 (0.1 g), GO (20 mL, 7 mg mL
1) and MWCNTs (20 mg) were mixed in 30 mL of DI water and sonicated for 30 min. Subsequently, the mixture was quickly frozen in liquid nitrogen and transferred into the freeze dryer for 48 h. The NiS2-CNTs-GO precursor was obtained. 2.2. Synthesis of the NiS octahedrons modified by carbon quantum dots (QDs), CNTs, and rGO (NiS@C QDs-CNTs-rGO) The as-formed precursor was then annealed at 500  C for 4 h in nitrogen gas flow (50 sccm). The obtained powder was washed by DI water and ethanol several times, named as NiS@C QDs-CNTsrGO. For comparison, the same precursor was separately heat treated at 350  C and 700  C by control experiments, which produced other two different phases of nickel sulphide composites accordingly, named as NiS2@CNTs-rGO and Ni7S6@CNTs-rGO.

immersed in 1 M KOH aqueous solution overnight to exchange interior water. 2.4. Materials characterization The crystal structure and component of the synthesized nanocomposites were examined by a Rigaku Dmax 2200 XRD with CuKa radiation (l ¼ 0.154 nm). The morphologies of these samples were examined by a Hitachi 7500 field-emission gun scanning electron microscope (FE-SEM). The TEM and HRTEM were carried out by a JEOL JEM-2100F. The XPS were collected on an ESCALAB 250 electron spectrometer from ThermoFisher Scientific Corporation with monochromatic 150 W Al Ka radiation. 2.5. Electrocatalytic measurements The electrochemical properties were evaluated by an electrochemical workstation (Gamry, Interface1000, USA) in 1 M KOH aqueous electrolyte, while Pt wire and Hg/HgO electrode were used as the counter and reference electrodes, respectively. The working electrodes were prepared by mixing the active materials (80 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) binder in N-methyl-2pyrrolidone (NMP) solution to obtain homogeneous slurry. Then the resultant slurry was smeared onto nickel foam current collector (1  2 cm2) with average mass of 0.8 mg. After drying overnight at 60  C in a vacuum oven, the electrode was pressed at a pressure of 10 MPa. However, the graphene hydrogel was treated by the solvent-exchange process with KOH, which was directly used as an electrode in a wet state, without adding any binder and conductor. A small piece of graphene hydrogel was placed on nickel foil and then compressed under a 10 MPa pressure for 1 min. The asymmetric supercapacitor (ASC) was assembled with the NiS@C QDsCNTs-rGO as a positive electrode, graphene hydrogels as a negative electrode, a piece of nonwoven polypropylene membrane (NKK-MPF30AC-100) as the separator, and the stainless-steel twoelectrode coin cells (CR2032) as a sealing device. The specific mass of the cathode and anode active materials in such ASC is kept as 1 and 3 mg, respectively. In order to achieve the best electrochemical performance for ASC, the optimal mass ratio of the Ni2CoLDH@MXene and graphene hydrogel was estimated by the following equation [21].

. . mþ m
¼ C
$V
Cþ $Vþ Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS, 0.01e100 kHz) tests were performed at room temperature. The specific capacity (QS) is calculated from obtained GCD curves of the electrode based on the following equation [22,23].

Q S ¼ I$Dt=m In which QS, I, Dt and m refer to the specific capacity (mAh g
1), discharge current (mA), discharge time (h), and mass of active materials (g), respectively. The energy density (E) and power density (P) are evaluated under CV dynamic conditions from the equations below [24].

ð E ¼ I Vdt=m

2.3. Synthesis of graphene hydrogel The graphene gel is obtained by loading 2 mL GO (~2 mg mL
1) dispersion into the Teflon-lined autoclave and heated at 180  C for 12 h. After being cooled to room temperature, the hydrogel was

P ¼ E=Dt where I is the discharge current, dt is the discharge time, and m is the total mass of the positive and negative electrodes.

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Fig. 1. Schematic illustration for the synthesis of the NiS2@CNTs-rGO, NiS@C QDs-CNTs-rGO, and Ni7S6@CNTs-rGO with heat-treatment at different temperature.

Fig. 2. SEM images of (a) the NiS@C QDs-CNTs-rGO, (b) NiS2@CNTs-rGO, and (c) Ni7S6@CNTs-rGO, showing nickel sulfide NPs in 3D structures. TEM images of (d) the NiS@C QDsCNTs-rGO, (e) NiS2@CNTs-rGO, and (f) Ni7S6@CNTs-rGO, showing uniformly dispersed nickel sulphides NPs wrapped by CNTs and graphene sheets. Corresponding HRTEM images of (g) the NiS@C QDs-CNTs-rGO, (h) NiS2@CNTs-rGO, and (i) Ni7S6@CNTs-rGO taken from the TEM images above.

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3. Results and discussion The NiS@C QDs-CNTs-rGO, NiS2@CNTs-rGO, and Ni7S6@CNTsrGO were fabricated by a freeze-drying and post annealing process, as illustrated in Fig. 1. The precursor of NiS2 octahedrons were prepared by hydrothermal method according to our previous method (Fig. S1, Supporting Information) [20]. The as-prepared NiS2 NPs, graphene oxide (GO), and multi-wall carbon nanotubes (MWCNTs) were mixed in aqueous and freeze-dried to prepare 3D aerogel analog with highly porous structure. NiS2 NPs were welldispersed within these 3D network spatial structures supported by 1D CNTs and 2D GO, forming the precursor of the NiS2-CNTs-GO. Thermal treatment was separately performed at 350, 500, and 700  C on the precursor in N2 with reductive environment provided by carbon, which produced three kinds of nickel sulfide phases, corresponding to the NiS2@CNTs-rGO, NiS@C QDs-CNTs-rGO, and

Ni7S6@CNTs-rGO, respectively (details are given in the Experimental). It's notable the added carbon materials (CNTs and GO) can construct 3D structure and especially provide reduction atmosphere at high temperature to adjust the phase of nickel sulfide. Morphology and structure characterizations on the three samples are shown in Fig. 2. Fig. 2a, d, and g correspond to the NiS@C QDs-CNTs-rGO, while Fig. 2b, e, h to the NiS2@CNTs-rGO, and Fig. 2c, f, i to the Ni7S6@CNTs-rGO. From SEM images in Fig. 1aec, it can be found that these particles with an average diameter of 150 nm are well-dispersed in 3D wrinkles of rGO sheets and CNTs. Obviously, both nickel sulfide NPs and carbon materials can promote the dispersion of each other. The graphene sheets cooperating with CNTs effectively avoid the agglomeration of NPs at high annealing temperature [25]. Especially, we can observe some tiny dots scattering on the graphene sheets in Fig. 2a, which were further confirmed by TEM and HRTEM images (Fig. 2d, g) to be

Fig. 3. (a) XRD pattern of the NiS@C QDs-CNTs-rGO (NiS, JCPDS NO. 02-1280), NiS2@CNTs-rGO (NiS2, JCPDS NO. 11-0099) and Ni7S6@CNTs-rGO (Ni7S6, JCPDS NO. 24-1021). (b) Raman spectrums of the rGO-CNTs, NiS@C QDs-CNTs-rGO, NiS2@CNTs-rGO and Ni7S6@CNTs-rGO. (c) XPS spectrum of the NiS@C QDs-CNTs-rGO. (d) High-resolution Ni 2p, (e) S 2p, and (f) C 1s XPS spectrums of the NiS@C QDs-CNTs-rGO.

R. Zhang et al. / Electrochimica Acta 311 (2019) 83e91

carbon QDs. TEM images in Fig. 2def give more clear presentation that NPs and doped CNTs are wrapped uniformly by rGO sheets. HRTEM images in Fig. 2gei shows that the lattice spacing of 0.198, 0.283 and 0.292 nm are well in accordance with the (102) planes of hexagonal NiS, (200) planes of cubic NiS2, and (121) planes of orthorhombic Ni7S6, respectively. One of the dots in Fig. 2d was further enlarged in Fig. 2g with its HRTEM image at the bottom right. The lattice spacing of 0.21 nm can be attributable to the (200) planes of C QDs. To check their forming, pure rGO sheets mixed with CNTs were annealed at 500  C, similar QDs could be observed in Fig. S2 (Supporting Information). It's interesting to find out only the heat treatment at around 500  C can produce C QDs based on a series of control experiments. It could be deduced the QDs come from the decomposition of rGO or CNTs. These carbon QDs can provide extra active sites for further electrochemical process [26,27]. The component and structure of the samples were further checked by XRD. As shown in Fig. 3a, the main peaks in the three samples can be ascribed to NiS (JCPDS NO. 02e1280), NiS2 (JCPDS NO. 11e0099) and Ni7S6 (JCPDS NO. 24e1021). The peaks marked by asterisk can be ascribed to CNTs, while those marked by diamond correspond to graphene [28e30]. The appearance of graphene has also been proven by Raman spectroscopy with typical D and G bands in Fig. 3b. The intensity of D to G bands (ID/IG) is a measure of ordering and graphite degree [31]. The ratio of ID/IG is figured out to be 1.33, 1.32, 1.39 for the NiS@C QDs-CNTs-rGO, NiS2@CNTs-rGO, and Ni7S6/CNTs-rGO, respectively. The ratios are very close, indicating that the ordering degree of graphene in the three samples is similar after heat treatment. We applied X-ray photoelectron spectra (XPS) to further verify the chemical composition and chemical bonding state of the samples. The XPS survey spectrums show the presence of Ni, S, O, N, and C elements in the NiS@C QDs-CNTs-rGO (Fig. 3c). The O 1s signal could be attributed to the residue oxygen containing group or the adsorbed oxygen on the sample surface. These elements also appear in the XPS survey spectrum of NiS2@CNTs-rGO and Ni7S6@CNTs-rGO (Figs. S3a and S3b, Supporting Information). As shown in the highresolution XPS spectrum of Ni 2p of the NiS@C QDs-CNTs-rGO in Fig. 3d, the doublet peaks at 854.1 and 871.9 eV correspond to the

87

Ni 2p3/2 and Ni 2p1/2 of Ni2þ (NieS), while two weak peaks nearby at 861.5 and 880.3 eV can be ascribed to the shake-up satellite peaks [32,33]. The appearance of Ni3þ might be related to the forming of SeC bonds. The SeC bonds can modulate electronic structures of Ni, and then lead to a reduced formation energy for high-valence Ni [32,34,35]. For the S 2p spectrum in Fig. 3e, in addition to the S 2p3/2 peaks (161.8 eV) and S 2p1/2 peaks (162.5 eV) corresponding to SeNi, the SeC peak was detected at 163.8 eV (SeC 2p3/2) and 164.8 eV (SeC 2p1/2) [36,37]. It can be observed the SeC bonds is related to the main peak in S 2p spectrum of the NiS@C QDs-CNTs-rGO, indicating the tight interaction between NiS and carbon matrix. The C 1s spectrum in Fig. 3f further reveals the binding energy at 285.6 eV can be ascribed to CeS bond, while another two peaks at 284.7 and 288.8 eV, corresponding to CeC/ C]C and OeC]O, respectively [38]. The above results reveal the formation of SeC and CeS bonds, which might come from the linking between nickel sulfide NPs and the carbon matrix. Similarly, the NieS and CeS bonds can also be detected in the spectrums of NiS2@CNTs-rGO and Ni7S6@CNTs-rGO (Fig. S4, Supporting Information). For the NiS@C QDs-CNTs-rGO, in addition to the spatial 3D structure constructed by 1D CNTs and 2D rGO sheets, it also has the decoration of 0D carbon QDs and the SeC bonds linking NiS and carbon matrix. Benefiting from these compelling structural and compositional advantages, better electrochemical performances can be expected. The electrochemical performance of the asfabricated three samples was separately evaluated on a threeelectrode configuration in 1.0 M KOH electrolyte, as shown in Fig. 4. Fig. 4a displays the cyclic voltammogram (CV) curves of the NiS@C QDs-CNTs-rGO, NiS2@CNTs-rGO, and Ni7S6@CNTs-rGO at a scan rate of 10 mV s
1. A pair of redox peaks can be observed in all CV curves, which can be considered to be a reversible redox process of Ni3þ/Ni2þ based on the following equation, NixSy þ OH
/ NixSyOH þ e
[39,40]. Compared with the NiS2@CNTs-rGO and Ni7S6@CNTs-rGO, the oxidation and reduction peaks of the NiS@C QDs-CNTs-rGO are shifted right and left, respectively. The peak voltage range is wider and the peak current is larger. It indicates that the NiS@C QDs-CNTs-rGO can continuously perform redox reactions at a higher current density and a wider voltage range,

Fig. 4. Electrochemical performances of three kinds of active materials in 1 M KOH electrolyte. (a) CV curves at a scan rate of 10 mV s
1 and (b) Galvanostatic charge/discharge curves at 20 A g
1of the NiS@C QDs-CNTs-rGO, NiS2@CNTs-rGO, Ni7S6@CNTs-rGO and Ni foam. (c) CV curves at various scan rates and (d) Galvanostatic charge/discharge curves at different current densities of the NiS@C QDs-CNTs-rGO. (e) Relationship of specific capacity, current density and IR drop for the three samples. (f) EIS curves of the NiS@C QDs-CNTsrGO, NiS2@CNTs-rGO, and Ni7S6@CNTs-rGO. The inset is magnified EIS spectra at high-frequency region.

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exhibiting a higher specific capacity. As shown in Fig. 4b, the galvanostatic charge-discharge (GCD) measurements of the three electrodes were conducted at 20 A g
1. As expected, the NiS@C QDs-CNTs-rGO shows the highest specific capacity of ~149 mAh g
1, while the NiS2@CNTs-rGO and Ni7S6@CNTs-rGO only show ~52 and 124 mAh g
1, respectively. CV and GCD tests on Ni foam show no capacity contribution. Fig. 4c shows the CV curves of the NiS@C QDs-CNTs-rGO at a voltage window of 0e0.7 V at different sweeping rates of 2, 5, 10, 20 and 40 mV s
1. The shape maintains well with increasing scan rate, which indicates a good rate capability of the electrode material. The charge-discharge curves in Fig. 4d display an obvious voltage plateau between 0.25 and 0.45 V at all current density ranges, confirming the battery-type behavior. The specific capacities of the NiS@C QDs-CNTs-rGO are calculated to be 241, 206, 188, 168, 149, 134, and 120 mAh g
1 at 1, 2, 5, 10, 20, 30, and 40 A g
1, respectively. The GCD curves of the NiS2@CNTs-rGO and the Ni7S6@CNTs-rGO shown in Fig. S5 (Supporting Information), also exhibit a typical battery-type behavior with well-defined potential plateau. By comparison, the NiS@C QDs-CNTs-rGO electrode possesses the highest specific capacity among the three electrodes at any current densities. For example, when the current density is 1 A g
1, the capacity of NiS@C QDs-CNTs-rGO can reach 241 mAh g
1, which is about 1.5 times of the NiS2@CNTs-rGO (154 mAh g
1) and Ni7S6@CNTs-rGO (167 mAh g
1). Even at high current densities of 20 and 40 A g
1, the specific capacity of the NiS@C QDs-CNTs-rGO can still reach 149 and 120 mAh g
1, while the NiS2@CNTs-rGO only presents 52 and 36 mAh g
1, and the Ni7S6@CNTs-rGO presents 124 and 115 mAh g
1, respectively. The specific capacity of the NiS@C QDs-CNTs-rGO at 20 A g
1 is much higher (149 mAh g
1) than most reported nickel sulfide electrodes at the same current density, such

as Ni7S6/rGO (97 mAh g
1) [14]. NiS/N-doped carbon (75 mAh g
1), and Ni3S2eNiS nanowires (123 mAh g
1) [41]. Even it is better than some other nickel-based compounds, such as NiSe2 porous nanosheets (55 mAh g
1) [42], ZnS/NiCo2S4/Co9S8 nanotubes (102 mAh g
1) [43], NiP2/Ni hollow spheres (104 mAh g
1 at 10 A g
1) [44], Ndoped NiS hollow shells (103 mAh g
1) [45], and rGO/CoNiSx/Ndoped carbon (140 mAh g
1 at 10 A g
1) [46]. These details were summarized in Table S1(Supporting Information). To demonstrate the capacity contribution from the carbon material of the NiS@C QDs-CNTs-rGO, we performed CV and GCD tests on rGO-CNT without NiS treated at the same conditions (Figs. S6a and b). It could be seen that the specific capacity is only 6.7 mAh g
1 at a current density of 1 A g
1, which is similar to the specific capacity of carbon materials reported by others [47,48]. When the current density increases to 10 A g
1, it's hard to find discharge plateau. The remarkable difference reveals that the capacity of the composite mainly comes from nickel sulfide NPs. At the same time, we also carried out electrochemical tests on the pure NiS2. The cyclic voltammetry (CV) curves at different scan rate of 2, 5, 10, 20 and 40 mV s
1 in a potential window of 0e0.55 V are presented in Fig. S6c (Supporting Information). The redox peaks related to reversible Faradaic reactions can be clearly observed in the CV profiles, demonstrating its battery-type characteristics. The NiS2 delivers a capacity of 127, 95, 12 mAh g
1 at current densities of 1, 2, 5 A g
1, respectively. It can be seen that the addition of carbon material greatly enhances its stability. The weight ratio of NiS and carbon materials was further estimated by TG analysis (Fig. S7, supporting information). It can be calculated that the NiS@C QDsCNTs-rGO composite contains 42.5% of nickel sulfide by weight. The addition of a suitable amount of carbon material balances its performance and stability very well.

Fig. 5. (a) CV curves at various scan rates and (b) Galvanostatic charge-discharge curves at different current densities for the ACS device using the NiS@C QDs-CNTs-rGO as cathode and graphene hydrogels as anode. (c) Ragone plots of the ACS device and other reported works. (d) Long-term cycling performance of the ACS device at 1 A g
1.

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Fig. 4e shows the relationship of specific capacity, current density and IR drop of the three samples. It is obvious that the specific capacity of the NiS@C QDs-CNTs-rGO is higher than that of NiS2@CNTs-rGO and Ni7S6@CNTs-rGO at various current densities. However, at a large current density of 40 A g
1, the specific capacity of Ni7S6@CNTs-rGO is close to that of NiS@C QDs-CNTs-rGO with similar structure, which can be attributed to the stable structure modified by CNTs and graphene after annealing at high temperature. As shown in the electrochemical impedance spectrums (EIS) in Fig. 4f, the diameter of the semicircle represents the charge transfer resistance (Rct) at the electrode material/electrolyte interface, and the slope of the straight line corresponds to lower diffusion resistance (Zw) at low frequency region [11]. The equivalent circuit used to fit the impedance spectra of the composite is shown in the inset of Fig. 4f. For NiS@C QDs-CNTs-rGO (Rct ¼ 0.16), which is smaller than Ni7S6@CNTs-rGO (Rct ¼ 0.22) and NiS2@CNTs-rGO (Rct ¼ 0.35). It indicates efficient ions diffusion of electrolyte during

89

redox reaction using the NiS@C QDs-CNTs-rGO electrode. Besides, the intercept on the real axis at high frequency inserted shows equivalent series resistance (RESR) of the electrode [41]. The Ni7S6 and NiS NPs contain more metal elements and make them more conductive compared with NiS2, which leads to smaller RESR values of the Ni7S6@CNTs-rGO (RESR¼1.27 U) and NiS@C QDs-CNTs-rGO (RESR¼1.41 U) than that of NiS2@CNTs-rGO (RESR¼1.51 U). The excellent conductivity of the NiS@C QDs-CNTs-rGO could be attributed to the synergistic effect of suitable NiS phase and multidimensional carbon materials providing 3D conductive networks. To further estimate the practicability of the NiS@C QDs-CNTsrGO electrode, it was used as cathode combined with graphene hydrogel (GH) (more information is shown in the Supporting Information of Fig. S8) as anode to form asymmetric supercapacitor (ASC). The mass ratio of the positive and negative electrodes is approximately calculated as 1:3 to ensure charge balance (Qþ ¼ Q
)

Fig. 6. (a) TEM image of the NiS@C QDs-CNTs-rGO electrode material after 5000 cycles for the ACS device. (b) CV curves of the NiS@C QDs-CNTs-rGO electrode before and after 5000 cycles. (c) Schematic illustration of the structure of the composite, displaying the synergetic effect of 0D C QDs, 1D CNTs, 2D graphene, and S-C bonds between NiS NPs and carbon matrix.

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[21]. The CV curves of the ACS at various scan rates in a potential range of 0e1.6 V are shown in Fig. 5a. It comes from the combined contribution of electrical double layer capacity and battery-type behavior. Furthermore, the similar symmetric GCD curves measured at different current densities with a potential window of 0e1.6 V indicate a high electrochemical reversibility (Fig. 5b). The specific capacity is calculated to be 9.35 mAh g
1 at 1 A g
1. The corresponding energy and power densities were also presented in the Ragone plot in Fig. 5c. The ASC device has an energy density of 21 Wh kg
1 at a power density of 811 W kg
1, which maintain at 12 Wh kg
1 at a high power density of 9343 W kg
1. The energy and power densities are better than some other reported results, such as the nickel sulfide system (NiS/carbon aerogels//carbon aerogels, NiS2//rGO) [46,49], other sulfide system (CoS2/rGO//active carbon, NiCo2S4//C) [50,51], and graphene hydrogel-based symmetric supercapacitors (GH/polyaniline//GH/polyaniline) [52]. Fig. 5d shows the cycling performance of the ASC device at 1 A g
1, which indicates excellent long-term cycling stability with capacity retention of about 82% after 5000 cycles. The superior electrochemical performance was further disclosed by examining the active material before and after 5000 cycles. The active materials were separated and collected from electrolyte and binders by washing and centrifugation. The TEM image of the NiS@C QDs-CNTs-rGO after 5000 cycles is shown in Fig. 6a. Although NiS NPs were slightly broken (marked by dot circle) after long-term charging and discharging, they were still well dispersed in the carbon matrix, keeping a 3D structure. It might be ascribed to the tight binding of NiS NPs and carbon matrix. Besides, the CV curves show only a little change of their peak intensity and integral area before and after 5000 cycles (Fig. 6b). In addition, the NiS@C QDs-CNTs-rGO//GH after 5000 cycles has a smaller RESR values (1.7 U) than that at initial (RESR ¼ 2.15 U), which might be caused by activation process. The charge transfer resistance (Rct ¼ 0.8 U) at the electrode material/electrolyte interface has no obvious change before and after cycles (Fig. S9, Supporting Information). These demonstrate that the NiS@C QDs-CNTs-rGO electrode material has excellent cycling stability and reversibility. As illustrated in Fig. 6c, the combined 0D QDs, 1D CNTs, and 2D graphene provide 3D networks for electron transfer. These carbon materials also wrap NiS NPs to protect them from pulverization during long-term cycling. Meanwhile, the strong SeC bonds between NiS NPs and carbon matrix improve the structural stability of the composite. The enhanced electronic conductivity and structural stability of the NiS@C QDs-CNTs-rGO electrode provide highefficient electrochemical performance. 4. Conclusion In summary, the hexagonal NiS octahedrons co-modified by 0D C QDs, 1D CNTs, and 2D rGO sheets were synthesized by a simple freeze-drying and heat-treatment method. The multiple dimensional carbon materials fabricated 3D spatial structures, which also served as a reduction agent for the phase-transition synthesis of nickle sulfide. The as-prepared NiS@C QDs-CNTs-rGO electrode exhibits a high specific capacity of 241 mAh g
1 at a current density of 1 A g
1, which maintain at 149 mAh g
1 at 20 A g
1 and 120 mAh g
1 at 40 A g
1, while the NiS2@CNTs-rGO only present 53 and 36 mAh g
1, and the Ni7S6@CNTs-rGO present 124 and 115 mAh g
1 accordingly. In addition, the corresponding asymmetric supercapacitor, using the NiS@C QDs-CNTs-rGO as cathode electrode and graphene as anode electrode, delivers an excellent cycling stability with ~82% capacity retention at 1 A g
1 after 5000 cycles. XPS results confirm strong CeS bonds between and carbon matrix and NiS NPs, which remarkably strengthens the structural stability of the NiS@C QDs-CNTs-rGO. The NiS NPs are uniformly wrapped by

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