Decoration NiCo2S4 nanoflakes onto Ppy nanotubes as core-shell heterostructure material for high-performance asymmetric supercapacitor

Accepted Manuscript Decoration NiCo2S4 nanoflakes onto Ppy nanotubes as core-shell heterostructure material for high-performance asymmetric supercapacitor Yayun Zheng, Jie Xu, Xiaoshan Yang, Yingjiu Zhang, Yuanyuan Shang, Xiaoyang Hu PII: DOI: Reference:

S1385-8947(17)31654-6 https://doi.org/10.1016/j.cej.2017.09.155 CEJ 17740

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

21 July 2017 20 September 2017 24 September 2017

Please cite this article as: Y. Zheng, J. Xu, X. Yang, Y. Zhang, Y. Shang, X. Hu, Decoration NiCo2S4 nanoflakes onto Ppy nanotubes as core-shell heterostructure material for high-performance asymmetric supercapacitor, Chemical Engineering Journal (2017), doi: https://doi.org/10.1016/j.cej.2017.09.155

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.

Decoration NiCo2S4 nanoflakes onto Ppy nanotubes as core-shell heterostructure material for high-performance asymmetric supercapacitor

Yayun Zhenga, Jie Xua*, Xiaoshan Yang a, Yingjiu Zhanga*, Yuanyuan Shanga, Xiaoyang Hub

a

School of Physical Engineering and Key Laboratory of Material Physics, Ministry of

Education, Zhengzhou University, NO. 75, Daxue Road, Zhengzhou 450052, China b

Collage of Science, Henan Institute of Engineering, NO. 1, Longhuxianghe Road,

Zhengzhou, 451191, China *

Corresponding author: Tel. +86 371 67766870, Fax. +86 371 67766629

E-mail address: [email protected]; [email protected]

Abstract In this work, a unique Ppy@NiCo 2S4 core-shell heterostructure material is prepared taking polypyrrole (Ppy) nanotubes as skeleton through a simple and environmentally friendly hydrothermal method. The heterostructured structure with Ppy nanotubes (NTs) as the core and intertwined flake-like NiCo2S4 as the outer shell, holds a large specific surface area (217 m2 g-1), convenient charge transport channel, high specific capacitance (908.1 F g-1 at a current density of 1 A g-1), and excellent cycling performance (87.7% of initial value is retained after 2000 cycles). What’s more, an asymmetric supercapacitor is fabricated by employing Ppy@NiCo2S4 core-shell heterostructured material as positive and nitrogen-doped carbon nanotubes (N-C NTs) as negative electrode. This Ppy@NiCo2S4//N-C asymmetric supercapacitor effectively provides a high energy density over 50.82 Wh kg-1 with a high power density of 160 W kg-1 and preeminent cyclic stability of 126.6% after 2000 cycles. Due to the introduction of one-dimensional flexible Ppy inner core, as-designed the core-shell heterostructure composite may be promising in the flexible energy storage devices. Keywords: Polypyrrole nanotubes; NiCo2S4; Core-shell heterostructure; Asymmetric supercapacitors

1. Introduction Global warming and fossil fuel depletion have forced researchers to develop sustainable and renewable energy sources [1,2]. Supercapacitor as an electrochemical energy storage device, occupies an important position by bridging the gap between conventional capacitors and batteries with their merits including superior power density, higher energy density, short charging time, long cycling life and environmental friendliness [3-5]. While the lower energy density of supercapacitors has hindered their further applications owing to the rapidly development of portable energy consumption. Among various supercapacitor electrode materials, the study has focused on the pseudocapacitive materials containing transition-metal oxides/sulfides (RuO2 [4], NiO [6], MnO2 [7], NiCo2O4 [8, 9], MoS2 [10], NixSy [11-13] etc.) and conducting polymers (polypyrrole [14], polyaniline [15] etc.), which store electric charge through highly reversible Faradaic redox reaction and display higher energy density than that of the carbon-based materials (carbon nanotube [16], graphene [17]). In the present study, these pseudocapacitive materials still suffer from low conductivity and large volume change during the charge-discharge processes, which is incapable for the requirements of new energy storage devices. Meanwhile, the inherent defect coming from the electrode materials is also an important influence factor for the regrettable electrochemical behavior. To further improve the electrochemical performance of pseudocapacitive materials, several approaches such as nanostructure or hybridization with carbon-based materials have been successfully developed [18]. It has been demonstrated that electrode materials with nanostructure could gain high specific surface area, short ion diffusion paths and increased active sites. And the hybridization can integrate the advantages of each component and mitigate their inherent shortages, provide the enlarged specific capacitance. Heterostructure composites with nanostructure fabricated by assembling different electrode

materials

together,

apparently

supply

enhanced

electrochemical

performance as desired due to the synergistic effects of different materials [19,20]. Finally, the increased specific capacitance of the heterostructured electrode could

improve the energy density of the supercapacitors based on the equation (E = 0.5CV2) [21]. The other effective way to increase the energy density is to extend the operational voltage window. While the organic/ionic liquid electrolytes with a higher operational voltage window could not be conducted in the practical application due to the higher cost and toxicity. Recently, the common approach of getting an extended operational voltage window is to configure an asymmetric supercapacitors using aqueous electrolyte, and suitable positive and negative electrodes with excellent capacitive behaviors. With the above analysis, we have prepared a Ppy@NiCo2S4 core-shell heterostructure material as the positive electrode in the asymmetric supercapacitors. Thereinto, NiCo 2S4 generally possesses richer redox reactions (Co 3+/Co2+ and Ni3+/Ni2+), higher conductivity and higher specific capacitance compared with single-component sulfides (NixSy or Co xSy), which make it receive extensive attention as the electrode material [15,22-24]. Unfortunately, the disadvantages of NiCo2S4 material such as inferior rate performance, poor cycling stability, rapid capacity decay and low utilization rate of active materials, still largely hamper further application in supercapacitors [25,26]. Notably, larger specific surface area and excellent electrical conductivity are propitious to increase the utilization rate of active materials and shorten the diffusion pathway of electron/ion, which results in excellent specific capacitance and high rate performance. Therefore, heterostructured NiCo2S4-based composites with enlarged active surface area, are expected to gain a desired electrochemical performance due to providing enhance ion-accessibility and more efficient charge transportation. Recently, Jia et al. prepared an asymmetric NiCo2S4@Fe2O3//MnO2 supercapacitor, and the hybrid NiCo2S4@Fe2O3 nanoneedle array anode material with the core-shell hierarchical nanoarchitecture displayed a large capacitance of 327 F g-1 at 5 mV s-1 [27]. Zhou et al. reported a process involving a simple hydrothermal route and a facile electrodeposition to prepare a hybrid with a rod-like NiCo 2S4 core and an urchin-like Ni(1−x)Co x(OH)2 shell. And the core-shell heterostructure showed an improved capacity of 3.54 C cm-2 at 1 mA cm-2 with 78% capacity retention after 4000 cycles [28]. According to these reports, the

NiCo2S4-based electrode material is expected to be a promising candidate in the supercapacitor field. Furthermore, polypyrrole (Ppy), as a common conducting polymer, offers high conductivity (1-104 S cm-1), low cost, flexible, nontoxicity and facile synthesis. In addition, Ppy core with hollow tubular structure served as a conductive and skeleton for the composite is beneficial for facilitating the electron collection and fast transport [29]. Considering synergistic effects between Ppy and NiCo2S4 materials, the Ppy@NiCo 2S4 core-shell heterostructure material are expected to realize improved electrochemistry performance. In this work, a core-shell heterostructure material composed of urchin-like NiCo 2S4 shell and one-dimensional Ppy NTs core, was developed by using a facile and effective approach. In this Ppy@NiCo2S4 composite, the Ppy NTs are employed as the skeleton proventing the aggregation of NiCo2S4, and served as the hollow electroactive component providing the enhanced conductivity and shortened ion diffusion pathway. Then, this core-shell heterostructure structure possesses accessible surface areas, rich electroactive sites, fast electrical pathways, stable structure and synergistic effects resulting from Ppy NTs core and NiCo 2S4 shell materials. Profiting from the features of structure and composition, the core-shell heterostructure electrode exhibits a remarkable specific capacitance of 908.1 F g-1 at a current density of 1 A g-1, and enhanced cycling performance with the capacity retention of 87.7% after 2000 cycles compared with the individual NiCo2S4. Moreover, a symmetric supercapacitor device based on the Ppy@NiCo2S4 core-shell heterostructure and the N-doped carbon nanotubes, was fabricated to further illustrate its excellent electrochemical performance. As expect, the assembled device holds a superior energy densities of 50.82 Wh kg-1, outstanding power density of 160 W kg-1 and good cycling stability of 126.6% retention after 2000 cycles.

2. Experimental section 2.1 Materials All the chemical reagents in this experiment were of analytical purity and directly used without any further purification.

2.2 Synthesis of polypyrrole nanotubes Polypyrrole nanotubes (Ppy NTs) taken as templates in this work were prepared in the similar method to the previous literature [30]. 2.6 g FeCl3·6H2O was dissolved in 320 mL of methyl orange solution (MO) (5×10-3 mol L-1) under magnetic stirring for 30 min, followed by the addition of 0.5 mL of pyrrole. With stirring for 4 hours at room temperature, the product was rinsed thoroughly with deionized (DI) water/ethanol several times. For further purification to remove the redundant iron ion, the Ppy NTs were dispersed in 1 mol L-1 HCl with 12 hours. After washing thoroughly, the black product was dried at 60 °C for 12 hours under vacuum. 2.3 Synthesis of Ppy@SiO2 NTs To improve the growth procedure of the shell material on the inner skeleton, the Ppy NTs were decorated with a layer of SiO2 before added into the following synthetic process. Briefly, 0.1 g Ppy NTs was evenly dispersed into 8.6 mL DI water and 40 mL isopropyl alcohol by ultrasound. Followed by the addition of 1 mL NH3· H2O, 0.3 g hexadecyl trimethyl ammonium bromide (CTAB) with stirring for 30 min under room temperature. Then, 0.16 mL ethylsilicate (TEOS) was slowly dropped into the above solution. After continually stirring for 3 hours, the product, noted as Ppy@SiO2, was obtained with rinsing thoroughly. 2.4 Synthesis of Ppy@NiCo2S4 core-shell heterostructure material The Ppy@NiCo2S4 core-shell material was prepared taking Ppy@SiO2 as accessory material through a two-step hydrothermal process. Firstly, 0.01 g of the as-prepared Ppy@SiO2 NTs was dispersed into 30 mL of DI water by ultrasonication. Then, 0.1 mmol Ni(NO3)2, 0.2 mmol Co(NO3)2 and 0.6 g urea were dissolved in 20 mL of ethylene glycol (EG) under magnetic stirring for 30 min. By adding the EG solution into the Ppy@SiO2 suspension, the mixture was transferred in a Teflon-lined autoclave after adequately stirring and heated under 120 °C for 12 hours. The precursor, signed as Ppy@NiCo-Pre, was collected after appropriately rinsing and drying. Finally, Ppy@NiCo2S4 was obtained by treating the precursor with the Na2S solution under the hydrothermal condition. Typically, 0.01 g of as-obtained

Ppy@NiCo-Pre was dispersed into 30 mL DI water with the addition of 0.05 g Na2S, then the mixture was sealed in a Teflon-lined autoclave and treated at 120 °C for 12 hours. After cooling down naturally, the black product was obtained by the rinse-centrifugation process and dried at 60 °C under vacuum for 12 hours. As compared to the core-shell composite, Ppy/NiCo 2S4 was developed from pure Ppy NTs without the coating of SiO2 under the same condition. Pure NiCo 2S4 were synthesized in a similar process with the absence of Ppy NTs. 2.5 Material characterizations The compositional and structural information of samples were acquired by X-ray power diffractometer (XRD, Netherlands Philip X’ Pert, Cu Κα, 10-80°), X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250XI), Fourier Transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iZ10) and Raman Spectrometer (Renishaw RM2000). The morphologies of as-prepared materials were evaluated by scanning electron microscope (SEM, JSM-6700F, JEOL) and transmission electron microscope (TEM, JEM-2100, 200 kV, JEOL). The surface area and pore size distribution of the samples were analyzed by Quantachrome NOVA 4200e system and estimated using the desorption isotherm based on the Barrett-Joyner-Halenda (BJH) method. The zeta potentials on the nanotubes were measured with a Malvern Zetasizer Nano ZSP. 2.6 Electrochemical measurements The working electrodes of supercapacitors were manufactured by mixing as-prepared active materials with acetylene black and polytetrafluoroethylene (PTEE) at a weight ratio of 80:15:5. After thorough mixing, the slurry was pressed onto fresh Ni foam (1.0 cm × 1.0 cm) that had been washed by HCl (3 mol L-1) and DI water/ethanol. And then working electrodes were dried under vacuum at 60 °C for 12 hours. The mass loading for working electrode is 80% of the weight difference before and after the preparation of working electrode. Herein, the loading mass of Ppy@NiCo2S4, NiCo2S4 and Ppy electrode is about 3.0, 3.4 and 2.7 mg cm-2 respectively. The electrochemical measurements were investigated by an electrochemical workstation (CorrTest, Wuhan, China) in a three-electrode system employing 2 mol L-1 KOH

aqueous solution served as electrolyte, Pt foil acted as the counter electrode and a standard calomel electrode (SCE) as the reference electrode, respectively. The electrochemical

tests

contain

cyclic

voltammetry

(CV),

galvanostatic

charge-discharge (GCD), electrochemical impedance spectroscopy (EIS) and cycle stability test by conducting GCD cycle. Notably, the specific capacitance (Cs) of the sample electrode can be calculated from GCD curves according to the following equation [30,31]: Cs = I × ∆t/(m × ∆V)

(1)

where Cs is the specific capacitance of the electrode (F g-1), I is the discharging current (A), ∆t is the discharge time (s), m is the mass of the active materials (g) and ∆V is the potential window (V). 2.7 Fabrication of the asymmetric supercapacitor The asymmetric supercapacitor (ASC) device using Ppy@NiCo2S4 taken as the positive and nitrogen-doped carbon nanotubes (N-C NTs) as negative electrode, a filter paper as separator and 2 mol L-1 KOH as electrolyte was tested in a two-electrode configuration. Therein, the N-C NTs were obtained by heating the Ppy NTs at 600 °C for 3 hours under Ar atmosphere with the heating rate of 3 °C min-1 [30,32]. Then the active materials of Ppy@NiCo2S4 and N-C NTs were coated on the Ni foam in the same preparation process to the working electrode in the three-electrode system. Especially, the mass of the positive and negative electrodes in the ASC device was determined by the balance theory of Q+ = Q- (Q = Csm∆V) to assure an efficient charge storage [33]. After measuring, the energy density and power density of ASC device are calculated according to the following equations [34]: Es = Cs∆V2/(2×3.6) P = 3600Es/∆t

(2) (3)

where Es is the specific energy density (Wh Kg-1), Cs refers to the specific capacitance (F g-1), ∆V is the voltage range (V), P represents the power density (W Kg-1), and ∆t is the discharge time (s).

3. Results and discussion 3.1 Structural and morphological characterization The schematic diagram for the Ppy@NiCo 2S4 core-shell heterostructure material is displayed in Fig. 1. Initially, the Ppy NTs were obtained by oxidation polymerization of pyrrole. Next, in order to improve the growth of outer shell material, the Ppy NTs were coated with a layer of SiO2 (Ppy@SiO2) to gain a negative potential on the surface. Therein, the Ppy NTs own the zeta potential of 0.447 mV, and the Ppy@SiO2 NTs share the zeta potential of -24.4 mV. Through the two-step hydrothermal treatment, the outer NiCo2S4 was successfully grown on the Ppy NTs surface in formation of the unique core-shell heterostructure material (Ppy@NiCo2S4). At the same time, the intermediate SiO2 layer was gradually removed at a mild alkaline environment during the hydrothermal treatment [35]. The typical XRD patterns of Ppy NTs, Ppy@SiO2 NTs, Ppy@NiCo-Pre, and Ppy@NiCo 2S4 composite are presented in Fig. 2. As shown in Fig. 2A and B, both Ppy NTs and Ppy@SiO2 NTs exhibit amorphous nature with a weak and broad peak situated at 2θ = 22-25°, which may be caused by the repeat unit of pyrrole ring in Ppy [36]. For the Ppy@NiCo-Pre (Fig. 2C), all the indexed peaks of the materials can be well

assigned

with

the

Co(CO3)0.5(OH)0.11·H2O

(JCPDS

48-0083)

and

Ni2(OH)2CO3·4H2O (JCPDS 38-0714). Especially, a series of diffraction peaks at 17.5°, 33.8°, 35.3°, 36.5°, 39.7°, 44.6°, 47.3°, 54.1°, 56.2°, 59.9° and 62.2° correspond well to (020), (221), (040), (301), (231), (050), (340), (060), (142), (412) and (450) reflections of Co(CO3)0.5(OH)0.11·H2O. The presence of carbonate in the Ppy@NiCo-Pre suggests that the SiO2 have no effect on the growth of NiCo2S4, except benefiting for the adsorption of metal ions (Ni2+/Co 2+) on the surface of Ppy NTs by providing negative potential [37,38]. For the case of Ppy@NiCo2S4 core-shell heterostructure material (Fig. 2D), a wide peak nearly 22.3° may come from the characteristic peak of Ppy, indicating the existence of Ppy NTs in the composite. The further analysis can be conducted thought the FTIR and Raman tests. Besides, the diffraction peaks of the Ppy@NiCo 2S4 at 31.6°, 38.3° and 55.4° are correspond to the (311), (400) and (440) planes of a cubic phase NiCo 2S4 (JCPDS 20-0782) [39]. These

results suggest the successful formation of Ppy NTs and NiCo 2S4 in the composite, which could be further confirmed by XPS measurement. FTIR and Raman tests were implemented to confirm the existence of Ppy NTs in the Ppy@NiCo 2S4 composite. As the FTIR spectra shown in Fig. 3A, the characteristic peaks at 1460 and 1545 cm-1 is caused by symmetric and antisymmetric vibration of pyrrole ring [34,40]. The band at 1168 cm-1 is assigned to the C-N stretching vibration between two pyrrole rings [40]. The bands at 1046 cm-1 and 1637 cm-1 are caused by N-H in-plane deformation vibration and C=C stretch vibration, respectively [40,41]. All these results matched well with the literatures have been reported, identifying the presence of the Ppy NTs in the Ppy@NiCo 2S4 composite. In the Raman spectrum of Ppy NTs (Fig. 3B), the characteristic peaks at 1386 (D-band) and 1590 cm-1 (G-band) are respectively attributed to C-N ring stretching mode and the C=C backbone stretching mode of Ppy [30]. The bond located at 1312 cm-1 is associated with N-H in-plane deformation [42]. In addition, the peaks at 975 and 1030 cm-1 can correspond to polaron on structure [30]. The Raman mode closed to 923 cm-1 is attributed to bipolaron symmetric C-H in-plane bending [42]. Notably, the growth of NiCo 2S4 on the surface of the Ppy NTs gradually weakens the D and G vibrational modes, indicating the presence of more defects and active sites on the sample surface [30,43]. In the Ppy@NiCo2S4 Raman spectra, the presence of the characteristic peaks of Ppy NTs observed at about 1335, 1581, 1053 and 983 cm-1 verifies the existence of Ppy NTs in the composite. Obviously, the band present at about 478 cm-1 in the spectra of Ppy@NiCo 2S4 is associated with the characteristic peak from NiCo2S4 spectra, which provide an evidence of the NiCo2S4 existing in the composite. In order to corroborating the chemical states of Ppy@NiCo2S4 composite, XPS measure was performed. The survey spectra shown in Fig. 4A indicates the existence of C, N, O, Ni, Co and S elements on the surface of the as-prepared core-shell composite. Thereinto, as-detected O signal may be due to the absorbed CO2 [44]. Notably, no observation of Si signal in the XPS spectrum indicates the completely remove of intermediate silica layer through tow-step hydrothermal treatment. In addition, the completely remove of SiO2 can be further confirmed by the Energy

Dispersion Spectrum (EDS) (Fig. S1). Fig. 4B illustrates the XPS spectrum of C 1s, it can be fitted by considering three different electronic states. Meanwhile, the binding energy at 284.1 and 287.7 eV corresponds to C-C and C=O bonding, respectively [44,45]. Besides, the characteristic peak of C-N bonding located at 285.8 eV corroborates the existence of Ppy NTs in the Ppy@NiCo2S4 materials [42,30]. As displayed in Fig. 4C, N 1s XPS spectrum can be divided into three peaks located at 397.8, 399.2 and 401.2 eV, which is respectively attributed to the characteristic peak of imine-like nitrogen (=N-), pyrrole nitrogen (-NH-) and positively charged nitrogen (-NH+-) from the Ppy nanotube [30,46,47]. The Ni 2p XPS spectrum as depicted in Fig. 4D, the difference of binding energy between Ni 2p3/2 and Ni 2p 1/2 respectively located at 855.3 and 873.1 eV is 17.8 eV, suggesting the coexistence of Ni2+ and Ni3+ [48]. In the Co 2p XPS spectrum (Fig. 4E), the difference of binding energy between the peak at 780.6 eV and 796.5 eV corresponds to Co 2p 3/2 and Co 2p 1/2 respectively is over 15 eV, indicating both Co 2+ and Co3+ can be found in the NiCo2S4 shell [44,48]. The S 2p XPS spectrum (Fig. 4F) can be divided into two spin-orbit doublets (S 2p1/2 and S 2p 3/2) and a shakeup satellite. The fitted peak at 163.4 eV comes from typical of metal-sulfur bonds, declaring the metal sulfide grown on the Ppy skeletons [44]. These results show that as-prepared composite contains Co 2+, Co3+, Ni2+, Ni3+ and S2−, further conforming that the formation of Ppy@NiCo2S4 core-shell hollow material [37,49]. To study the morphology evolution of Ppy@NiCo 2S4 core-shell composite, SEM images of the products obtained at various stages are shown in Fig. 5. Herein, Fig. 5A and B respectively display the morphology of NiCo-Pre and NiCo 2S4 prepared without the addition of Ppy substrate. It can be clearly observed that the NiCo-Pre has the spherical morphology with about 4 ~ 6 µm in diameter. And the spherical microstructures are built up of countless interconnected ultrathin nanoflakes (Fig. 5A). The same magnification SEM image of NiCo 2S4 (Fig. 5B) shows that the spherical morphology of the sample is perfectly retained after a further hydrothemal teatment. Moreover, the NiCo 2S4 microspheres share coarser surface compared with the initial precusor, which may support more active sites during the electrochemical process. Fig.

5C shows a large number of Ppy NTs have the well-developed nanoscale tubular structure with randomly spread. A closer look for the Ppy NTs shows that the nanotubes own smooth surface with nanoscale in diameter and several micrometers in length (Fig. 5D). Fig. 5E shows the SEM image of Ppy@SiO2 NTs. With the growth of a layer of SiO2 on the Ppy NTs surface, the slight increase in the diameter can be obseved. As shown in Fig. 5F, there are a lot of ultrathin NiCo-Pre nanoflakes on the surface of Ppy NTs, leading to a obvious increase in diameter of tubular structure. In the Fig. 5G-I, Ppy@NiCo2S4 composite obtained after vulcanization process still inherits the morphology of Ppy@NiCo-Pre (Fig. 5F). As observed, the micro-scale heterostructure owns a well-preserved tubular morphology after a series of treatment. Besides, the ultrathin NiCo 2S4 nanoflakes as the shell are interconnected with each other, and the Ppy NTs as the core are wrapped by the layer of NiCo 2S4 forming an unique core-shell tubular structure. The outside diameter of tubular-structure composite is in the range of 400 ~ 600 nm, which has obviously increased owing to the growth of NiCo2S4 nanoflakes on the Ppy NTs surface. It could be noticed that the unique core-shell heterostructure endows the high specific surface area, which is benift for the contact surface area between Ppy@NiCo2S4 and 2 mol L-1 KOH electrolyte. Besides, the desirable structure of Ppy@NiCo2S4 composite is expect to display the promising electrochemial performance. As compared to the core-shell composite, Ppy/NiCo 2S4 was developed from pure Ppy NTs with the absence of SiO2 under the same condition and its SEM image is shown in Fig. S2. The morphology of Ppy/NiCo 2S4 displays the independent growth of Ppy NTs and NiCo 2S4 without the coating of SiO2 on the Ppy NTs, which futher suggest that the presence of SiO2 on the surface of Ppy NTs is beneficial for the adsorption of metal ions (Ni2+/Co 2+) on the surface of Ppy NTs by providing negative potential. Furthermore, the little difference on the morphology between NiCo2S4 in Ppy/NiCo2S4 and pure NiCo2S4 in Fig. 5B, may be due to the presence of silica affects the acidity and basicity of the reaction systerm. In order to gain further morphology information of Ppy@NiCo 2S4 core-shell heterostruture, the TEM test was carried out with the rusults shown in Fig. 6. The

TEM images presented in Fig. 6A and the insert clearly illustrate the unidirectional growth of Ppy NTs in the process of oxidative polymerization of pyrrole monomer. And the Ppy substrates with the hollow structure possess the outer diameter of about 130 nm and uniform shell thickness of around 20 nm. With a slight increased in the outer diameter (135 ~ 180 nm) and the agglomeration due to the curvilinear deposition of the SiO2, a layer of SiO2 with thickness of around 5 ~ 50 nm can be clearly found on the Ppy NTs surface (Fig. 6B and C). The introduction of SiO2 layer possibly be benefit for the adsorption of metal ions (Ni2+/Co 2+) on the surface of Ppy NTs and would not effect NiCo2S4 growth. Fig. 6D-F present the TEM images of Ppy@NiCo 2S4 core-shell heterostructure. It can be seen that the tubular structure of composite is still well maintained after the growth of NiCo2S4 nanoflakes on the Ppy substrate surface. With a closer insight, the heterostruture employs Ppy NTs as the core and urchin-like NiCo 2S4 layer as the shell, which has completely covered the inner Ppy substrate (Fig. 6F). The results are in good agreement with the above SEM observations. The HRTEM inserted in Fig. 6F shows some clearly lattice fringes with an interplanar spacing of 0.23 nm, corresponding to the (400) planes of nanoflakes NiCo2S4 in the core-shell heterostructure. It is well know that the specific surface area and pore size distribution play important roles in textural properties analysis. As shown in Fig. 7A, the N2 adsorption/desorption isotherm of all the samples feature the type IV isotherms, confirming the existence of mesoporous structure [50]. The BET and BJH analyses reveal that the as-prepared Ppy@NiCo2S4 has a high specific surface area of 217 m2 g-1 and the average pore size is about 17.4 nm in diameter with a mesoporous characteristic. The values are superior to that of pure NiCo 2S4 (92.9 m2 g-1, 33.7 nm) and Ppy NTs (62 m2 g-1, 7.4 nm). As the above analysis, the large surface area and suitable mesoporous size of Ppy@NiCo2S4 may be due to that the existence of Ppy NTs can efficiently decrease the aggregation of NiCo 2S4 nanoflakes, providing a core-shell architecture with enlarged specific surface area. Notably, the core-shell architecture and mesoporous structure can provide enhanced ion-accessibility and more efficient charge transportation to the enlarged active surface area, and might

further improve the rate capability and specific capacitance. 3.2 Electrochemical performances of Ppy@NiCo2S4 core-shell heterostructure electrode The electrochemical performances of Ppy@NiCo 2S4 core-shell heterostructure material used as a working electrode for supercapacitors were tested in a three-electrode system. As shown in Fig. 8A, two pairs of well-defined redox peaks on the CV curves of Ppy@NiCo 2S4 electrode can be observed between the voltage window of 0-0.5 V, which is largely attributed to the Faradaic redox reactions of Ni2+/Ni3+ and Co 2+/Co3+ redox couples [51,52]. At the same scan rate (5 mV s-1), CV curve integral area of Ppy@NiCo2S4 electrode is apparently larger compared with NiCo2O4 electrode and Ppy NTs electrode, which indicates the superior capacitance of composite electrode [20]. Fig. 8B shows CV curves of the Ppy@NiCo2S4 electrode at various sweeping rates ranging from 5 to 50 mV s-1. With the increase of sweeping rates, the oxidation and reduction peaks respectively shift toward to higher and lower potential, which may be ascribed to the polarization effect of the electrodes [30]. Moreover, CV curves do not have a distinct distortion with sweeping rates increase, suggesting the Ppy@NiCo2S4 electrode has a low resistance and good reversible property [52]. GCD measurement of as-prepared electrodes was carried out within 0-0.5 V at various current densities ranging from 1 to 20 A g-1. As plotted in Fig. 8C, two visible and separated platforms are clearly found, corresponding to the dual redox processes of the Ppy@NiCo2S4 core-shell heterostructure electrode during charging/discharging process [49]. Notably, discharge time of the composite electrode material is increase with the current densities decrease, meaning the high utilization ratio of active material under low current density. Moreover, GCD curves own good symmetry implying high coulombic efficiency (above 90%) of the core-shell heterostructure electrode (Fig. S3A). Specific capacitance of as-prepared electrodes can be calculated according to equation (1) during the discharge process and the results are exhibited in Fig. 8D. Encouragingly, the Ppy@NiCo2S4 core-shell heterostructure electrode respectively exhibited excellent pseudocapacitances of 908.1, 879, 845.4, 796, 720, 584.9 and 494.9 F g-1 at current densities of 1, 2, 3, 5, 8, 15 and

20 A g-1, delivering that the specific capacitance of Ppy@NiCo2S4 core-shell heterostructure electrode are higher than that of NiCo2S4 microspheres and Ppy NTs electrode. As shown in Fig. 8E, the cycling curves of three electrode materials are conducted in the charge/discharge cycles up to 2000 cycles at a current density of 20 A g-1. During the cycling, the specific capacitance of the Ppy@NiCo2S4 core-shell heterostructure electrode are larger than NiCo2S4 electrode. After 500 cycles, the specific capacitance of Ppy@NiCo2S4 electrode is 461.9 F g-1 (only 5.5% fade). Even after 2000 cycles, the specific capacitance still is about 80.7% of the initial value, it is much higher than that of the NiCo2S4 electrode (about 50.3% capacitance retention), revealing the better cycling stability and high-performance capacitance of the composite electrode. The cycling performance of Ppy NTs shown in Fig. S4 displays its low capacitance but high cycling stabilities (about 88% capacitance retention), which suggests the excellent cycle stability and cycling life of Ppy@NiCo2S4 electrode are attributed to the Ppy substrate with high conductivity in the composite. To better understand the electrochemical behavior of the electrodes, EIS test was performed in the frequency range from 100 KHz to 0.01 Hz (Fig. 8F). In the low frequency region, the straight line parts of the Ppy@NiCo2S4 core-shell heterostructure electrode is more closer to the imaginary axis than that of NiCo2S4 microspheres and Ppy NTs, demonstrating its much lower diffusion resistance and faster ion diffusion of electrolyte, which is of huge benefits to the outstanding capacitive behavior of Ppy@NiCo2S4 electrodes [53]. In the high frequency region, the lower intercept of the impedance arc with the real axis reveals Ppy@NiCo2S4 electrode has the lower combined resistance (Rs), realizing the lower intrinsic resistance of electrode materials, ionic resistance of electrolyte and contact resistance between electrode and current collector [54]. The excellent electrochemical property of Ppy@NiCo2S4 electrode can be ascribed to the structural advantages. Specially, the unique core-shell heterostructure of Ppy@NiCo2S4 endows an enlarged surface area, ensuring efficient contact between the active materials and the KOH electrolyte. What’s more, the hollow tubular structure characteristic of Ppy@NiCo 2S4 composite provides convenient diffusion channels for electrolyte ion penetration into the interior

of the sample, increasing the utilization of active materials. Lastly, the ultrathin nanoflakes coating on the Ppy NTs can provide numerous electroactive sites during redox reactions. 3.3

Electrochemical

characterization

of

Ppy@NiCo 2S4//N-C

asymmetric

supercapacitor To evaluate the potential applications of Ppy@NiCo2S4 electrode, an asymmetric supercapacitor device was assembled based on the Ppy@NiCo2S4 core-shell heterostructure electrode and N-C NTs electrode. As shown in Fig. 9A, CV curves of Ppy@NiCo 2S4 core-shell heterostructure electrode and N-C NTs electrode are tested in a three-electrode system at voltage window of 0-0.5 V and -1-0 V, respectively. Hence it is possible to obtain a lowest working voltage of 1.5 V if these two electrodes are assembled to form an asymmetric supercapacitor device [55]. As shown in Fig. S5, N-C electrode reveals doule-layer characteristic with no obvious redox peaks in the CV curves. Besides, regularly triangular shapes of GCD curves suggest the ideal capacitance performance and reversible behaviors of N-C electrode [55]. From the discharge curve, the specific capacitance of the N-C electrode is calculated to be 119.9 F g-1 at the current densities of 1 A g-1. According the balance theory, the mass loading of N-C negative electrode is calculated to be 11.4 mg cm-2, and the optimal mass ratio between the two electrodes should be m+ (Ppy@NiCo2S4)/m- (N-C) = 0.26 in Ppy@NiCo2S4//N-C. Fig. 9B shows the CV curves of Ppy@NiCo2S4//N-C asymmetric supercapacitor at different voltage windows when scan rate is 10 mV s-1. A slight hump can be clearly observed when the potential window is higher than 1.6 V, which may be attributed to some irreversible reactions [56]. Therefore, 0-1.6 V is chosen as the optimal working potential window of this asymmetric supercapacitor for further investigation. Fig. 9C displays the CV curves of the Ppy@NiCo2S4//N-C asymmetric supercapacitor at various scan rates from 10 to 50 mV s-1. Clearly, there is no obvious distortion in the CV curves as the scan rate increased, suggesting the ASC device has an instant current response and good capacitive behavior [44]. Besides, a clear increase of current density and a little shift of oxidation peak are observed, which is similar to the behaviors occurred in the three-electrode configuration. A

redox peak is still obvious even at a high scan rate of 50 mV s-1, revealing Ppy@NiCo 2S4//N-C asymmetric supercapacitor has a good rate capability [57]. The GCD curves of the Ppy@NiCo 2S4//N-C asymmetric supercapacitor at current densities from 0.2 A g-1 to 5 A g-1 are illustrated in Fig. 9D. The nearly symmetric triangular shape demonstrates the device provides an increased coulombic efficiency from 60% to 99% (Fig. S3B). The decrease of discharge time in electrochemical process with increasing current density is attributed to limitation of charge transport diffusion coefficients at higher current density [30]. As shown in Fig. 9E, the calculated specific capacitance of Ppy@NiCo2S4//N-C asymmetric supercapacitor based on the total masses of the two electrodes is 141.8, 122.9, 94.68, 56.34, 41.27, 35.55, 31.23 and 26.56 F g-1 at current densities of 0.2, 0.3, 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 A g-1, respectively. The cycle performance of the Ppy@NiCo2S4//N-C asymmetric supercapacitor was carried by a 2000 cycle charge/discharge test at 5 A g-1, as shown in Fig. 9F. After 2000 cycles, the specific capacitance of the Ppy@NiCo2S4//N-C asymmetric supercapacitor is as high as 37.53 F g-1 with about 126.6% of the initial capacitance retained. The excellent cycling performance of device can be attributed to its stable structure and synergetic effects between the anode material (Ppy@NiCo2S4 composite) and cathode (N-C NTs). The Ragone plots of the Ppy@NiCo2S4//N-C ASC derive are obtained according to equation (2) and (3) in the charge-discharge measurements (Fig. 9G). As demonstrated, this asymmetric supercapacitor can provide a maximum energy density of 50.82 Wh kg-1 at a power density of 160 W kg-1, which is much higher than that of previously reported ASC devices, like NiCo2S4 nanoparticles//AC (28.3 Wh kg-1 at a power density of 245 W kg-1) [58], Co9S8 nanoflakes//AC (31.4 Wh kg-1 at a power density of 200 W Kg-1) [59], NiCo2S4 (nanotubes)//RGO (31.5 Wh kg-1 at 156.6 W kg-1) [51], NiCo2S4@NiMoO4 (29.1 W h kg-1 at a power density of 172 W kg-1) [60], NiCo 2O4-rGO/AC (23.3 Wh kg-1 at a power density of 324.9 W kg-1) [61].

4. Conclusion In general, we have conducted an efficient and environmental hydrothermal

method for a core-shell heterostructured electrode material in which employs NiCo2S4 nanoflakes as a shell and the Ppy NTs core. In view of this stable core-shell heterostructure, the Ppy@NiCo 2S4 electrode material displays an excellent performance for supercapacitors including a high specific capacitance of 908.1 F g-1 at a current density of 1 A g-1 and remarkable cycling stability (about 87.7% retention after 2000 cycles). For the Ppy@NiCo2S4//N-C asymmetric supercapacitor device, it also demonstrates an excellent electrochemical performance with the energy density as high as 50.82 Wh kg-1 at a power density of 160 W kg-1 and good cycle stability up to 126.6% retention after 2000 cycles, indicating their promising potential application in new fashioned high-performance supercapacitors.

Acknowledgements The present work is financially supported by the Natural Science Foundation under the grants of NSFC (51602289), the Outstanding Young Talent Research Fund of Zhengzhou University (32210461), the Startup Research Fund of Zhengzhou University (51090104).

Reference [1] X.H. Liu, Z.B. Wen, D.B. Wu, H.L. Wang, J.H. Yang, Q.G. Wang, Tough BMIMCl-based ionogels exhibiting excellent and adjustable performance in high-temperature supercapacitors, J. Mater. Chem. A 2 (2014) 11569-11573. [2] X.H. Xia, D.L. Chao, Z.X. Fan, C. Guan, X.H. Cao, H. Zhang, H.J. Fan, A new type of porous graphite foams and their integrated composites with oxide/polymer core/shell nanowires for supercapacitors: structural design, fabrication, and full supercapacitor demonstrations, Nano Lett. 14 (2014) 1651-1658. [3] J.R. Miller, P. Simon, Materials science-electrochemical capacitors for energy management, Science 321 (2008) 651-652. [4] Z.S. Wu, D.W. Wang, W.C. Ren, J.P. Zhao, G.M. Zhou, F. Li, H.M. Cheng, Anchoring hydrous RuO2 on graphene sheets for high-performance electrochemical capacitors, Adv. Funct. Mater. 20 (2010) 3595-3602.

[5] S.D. Perera, B. Patel, N. Nijem, K. Roodenko, O. Seitz, J.P. Ferraris, Y.J. Chabal, K.J. Balkus, Vanadium oxide nanowire-carbon nanotube binder-free flexible electrodes for supercapacitors, Adv. Energy Mater. 1 (2011) 936-945. [6] G.F. Cai, X. Wang, M.Q. Cui, P. Darmawan, J.X. Wang, A.L.S. Eh, P.S. Lee, Electrochromo-supercapacitor based on direct growth of NiO nanoparticles, Nano Energy 12 (2015) 258-267. [7] W.F. Wei, X.W. Cui, W.X. Chen, D.G. Ivey, Manganese oxide-based materials as electrochemical supercapacitor electrodes, Chem. Soc. Rev. 40 (2011) 1697-1721. [8] L.F. Hu, L.M. Wu, M.Y. Liao, X.S. Fang, High-performance NiCo 2O4 nanofilm photodetectors fabricated by an interfacial self-assembly strategy, Adv. Mater. 23 (2011) 1988-1992. [9] Y.R. Zhu, Z.B. Wu, M.J. Jing, W.X. Song, H.S. Hou, X.M. Yang, Q.Y. Chen, X.B. Ji, 3D network-like mesoporous NiCo2O4 nanostructures as advanced electrode material for supercapacitors, Electrochim. Acta 149 (2014) 144-151. [10] H. Hwang, H. Kim, J. Cho, MoS2 nanoplates consisting of disordered graphene-like layers for high rate lithium battery anode materials, Nano Lett. 11 (2011) 4826-4830. [11] Y.F. Zhang, L.Z. Zuo, L.S. Zhang, J.J. Yan, H.Y. Lu, W. Fan, T.X. Liu, Immobilization of NiS nanoparticles on N-doped carbon fiber aerogels as advanced electrode materials for supercapacitors, Nano Res. 9 (2016) 2747-2759. [12] K. Krishnamoorthy, G.K. Veerasubramani, S. Radhakrishnan, S.J. Kim, One pot hydrothermal growth of hierarchical nanostructured Ni3S2 on Ni foam for supercapacitor application, Chem. Eng. J. 251 (2014) 116-122. [13] S.J. Peng, L.L. Li, H.T. Tan, R. Cai, W.H. Shi, C.C. Li, S.G. Mhaisalkar, M. Srinivasan, S. Ramakrishna, Q.Y. Yan, MS2 (M = Co and Ni) hollow spheres with tunable interiors for high-performance supercapacitors and photovoltaics, Adv. Funct. Mater. 24 (2014) 2155-2162. [14] T.Y. Liu, L. Finn, M.H. Yu, H.Y. Wang, T. Zhai, X.H. Lu, Y.X. Tong, Y. Li, Polyaniline and polypyrrole pseudocapacitor electrodes with excellent cycling stability, Nano Lett. 14 (2014) 2522-2527.

[15] X.B. Liu, Z.P. Wu, Y.H. Yin, Hierarchical NiCo 2S4@PANI core/shell nanowires grown on carbon fiber with enhanced electrochemical performance for hybrid supercapacitors, Chem. Eng. J. 323 (2017) 330-339. [16] H.R. Byon, B.M. Gallant, S.W. Lee, S.H. Yang, Role of oxygen functional groups in carbon nanotube/graphene freestanding electrodes for high performance lithium batteries, Adv. Funct. Mater. 23 (2013) 1037-1045. [17] Z.J. Fan, J. Yan, T. Wei, L.J. Zhi, G.Q. Ning, T.Y. Li, F. Wei, Asymmetric supercapacitors based on graphene/MnO2 and activated carbon nanofiber electrodes with high power and energy density, Adv. Funct. Mater. 21 (2011) 2366-2375. [18] S.X. Liu, L.F. Hu, X.J. Xu, A.A. Al-Ghamdi, X.S. Fang, Nickel cobaltite nanostructures for photoelectric and catalytic applications, Small 11 (2015) 4267-4283. [19] M. Huang, Y.X. Zhang, F. Li, Z.C. Wang, Alamusi, N. Hu, Z.Y. Wen, Q. Liu, Merging of kirkendall growth and ostwald ripening: CuO@MnO2 core-shell architectures for asymmetric supercapacitors, Sci. Rep. 4 (2014) 4518. [20] J, Xu, S.L. Gai, F. He, N. Niu, P. Gao, Y.J. Chen, P.P. Yang, A sandwich-type three-dimensional layered double hydroxide nanosheet array/graphene composite: fabrication and high supercapacitor performance, J. Mater. Chem. A 2 (2014) 1022-1031. [21] P.P. Yu, Z.M. Zhang, L.X. Zheng, F. Teng, L.F. Hu, X.S. Fang, A novel sustainable flour derived hierarchical nitrogen-doped porous carbon/polyaniline electrode for advanced asymmetric supercapacitors, Adv. Energy Mater. 6 (2016) 1601111. [22] Q.Q. Hu, Z.X. Gu, X.T. Zheng, X.J. Zhang, Three-dimensional Co 3O4@NiO hierarchical nanowire arrays for solid-state symmetric supercapacitor with enhanced electrochemical performances, Chem. Eng. J. 304 (2016) 223-231. [23] J. Li, M. Wei, W. Chu, N. Wang, High-stable α-phase NiCo double hydroxide microspheres via microwave synthesis for supercapacitor electrode materials, Chem. Eng. J. 316 (2017) 277-287. [24] J.W. Xiao, L. Wan, S.H. Yang, F. Xiao, S. Wang, Design hierarchical electrodes

with highly conductive NiCo 2S4 nanotube arrays grown on carbon fiber paper for high-performance pseudocapacitors, Nano Lett. 14 (2014) 831-838. [25] R.J. Zou, Z.Y. Zhang, M.F. Yuen, J.Q. Hu, C.S. Lee, W.J. Zhang, Dendritic heterojunction nanowire arrays for high-performance supercapacitors, Sci. Rep. 5 (2015) 7862. [26] X.Y. Wu, S.M. Li, B. Wang, J.H. Liu, M. Yu, Graphene foam supported multilevel network-like NiCo2S4 nanoarchitectures for robust lithium storage and efficient ORR catalysis, New J. Chem. 41 (2017) 115-125. [27] R.Y. Jia, F. Zhu, S. Sun, T. Zhai, H. Xia, Dual support ensuring high-energy supercapacitors via high-performance NiCo 2S4@Fe2O3 anode and working potential enlarged MnO2 cathode, J. Power Sources 341 (2017) 427-434. [28] W.W. Zhou, K. Yu, D. Wang, J. Chu, J.Y. Li, L.M. Zhao, C.Y. Ding, Y. Du, X.T. Jia, H.T. Wang, G.W. Wen, Hierarchically constructed NiCo 2S4@Ni(1-x)Cox(OH)2 core/shell nanoarrays and their application in energy storage, Nanotechnology 27 (2016) 235402. [29] Y.B. He, Y.L. Bai, X.F. Yang, J.Y. Zhang, L.P. Kang, H. Xu, F. Shi, Z.B. Lei, Z.H. Liu, Holey graphene/polypyrrole nanoparticle hybrid aerogels with three-dimensional hierarchical porous structure for high performance supercapacitor, J. Power Sources 317 (2016) 10-18. [30] D.P. Dubal, N.R. Chodankar, Z. Caban-Huertas, F. Wolfart, M. Vidotti, R. Holze, C.D. Lokhande, P. Gomez-Romero, Synthetic approach from polypyrrole nanotubes to nitrogen doped pyrolyzed carbon nanotubes for asymmetric supercapacitors, J. Power Sources 308 (2016) 158-165. [31] T. Zhai, X.H. Lu, F.X. Wang, H. Xia, Y.X. Tong, MnO2 nanomaterials for flexible supercapacitors: performance enhancement via intrinsically and extrinsically modification, Nanoscale Horiz. 1 (2016) 109-124. [32] Q. Liu, Z.H. Pu, C. Tang, A.M. Asiri, A.H. Qusti, A.O. Al-Youbi, X.P. Sun, N-doped carbon nanotubes from functional tubular polypyrrole: A highly efficient electrocatalyst for oxygen reduction reaction, Electrochem. Commun. 36 (2013) 57-61.

[33] Y. Zhang, J. Xu, Y.Y. Zheng, Y.J. Zhang, X. Hu, T.T. Xu, NiCo2S4@NiMoO4 core-shell heterostructure nanotube arrays grown on Ni foam as a binder-free electrode displayed high electrochemical performance with high capacity, Nanoscale Res. Lett. 12 (2017) 412. [34] L.Q. Fan, G.J. Liu, J.H. Wu, L. Liu, J.M. Lin, Y.L. Wei, Asymmetric supercapacitor based on graphene oxide/polypyrrole composite and activated carbon electrodes, Electrochim. Acta 137 (2014) 26-33. [35] T. Zhu, H.B. Wu, Y.B. Wang, R. Xu, X.W. Lou, Formation of 1D hierarchical structures composed of Ni3S2 nanosheets on CNTs backbone for supercapacitors and photocatalytic H2 production, Adv. Energy Mater. 2 (2012) 1497-1502. [36] C. Bora, S.K. Dolui, Fabrication of polypyrrole/graphene oxide nanocomposites by liquid/liquid interfacial polymerization and evaluation of their optical, electrical and electrochemical properties, Polymer 53 (2012) 923-932. [37] B. Yang, L. Yu, H.J. Yan, Y.B. Sun, Q. Liu, J.Y. Liu, D.L. Song, S.X. Hu, Y. Yuan, L.H. Liu, J. Wang, Fabrication of urchin-like NiCo2(CO3)1.5(OH)3@NiCo 2S4 on Ni foam by an ion-exchange route and application to asymmetrical supercapacitors, J. Mater. Chem. A 3 (2015) 13308-13316. [38] W. Kong, C.C. Lu, W. Zhang, J. Pu, Z.H. Wang, Homogeneous core-shell NiCo2S4 nanostructure supported on nickel foam for supercapacitors, J. Mater. Chem. A 3 (2015) 12452-12460. [39] Y.R. Zhu, X.B. Ji, Z.B. Wu, Y. Liu, NiCo2S4 hollow microsphere decorated by acetylene black for high-performance asymmetric supercapacitor, Electrochim. Acta 186 (2015) 562-571. [40] J. Li, L. Cui, X.G. Zhang, Preparation and electrochemistry of one-dimensional nanostructured MnO2/PPy composite for electrochemical capacitor, Appl. Surf. Sci. 256 (2010) 4339-4343. [41] L. Ding, C. Hao, X.J. Zhang, H.X. Ju, Carbon nanofiber doped polypyrrole nanoscaffold for electrochemical monitoring of cell adhesion and proliferation, Electrochem. Commun. 11 (2009) 760-763. [42] A.P.P. Alves, R. Koizumi, A. Samanta, L.D. Machado, A.K. Singh, D.S. Galvao,

G.G. Silva, C.S. Tiwary, P.M. Ajayan, One-step electrodeposited 3D-ternary composite of zirconia nanoparticles, rGO and polypyrrole with enhanced supercapacitor performance, Nano Energy 31 (2017) 225-232. [43] P. Si, S.J. Ding, X.W. Lou, D.H. Kim, An electrochemically formed three-dimensional

structure

of

polypyrrole/graphene

nanoplatelets

for

high-performance supercapacitors, RSC Adv. 1 (2011) 1271-1278. [44] M.L. Yan, Y.D. Yao, J.Q. Wen, L. Long, M.L. Kong, G.G. Zhang, X.M. Liao, G.F. Yin, Z.B. Huang, Construction of a hierarchical NiCo 2S4@PPy core-shell heterostructure nanotube array on Ni foam for a high-performance asymmetric supercapacitor, ACS Appl. Mater. Interfaces 8 (2016) 24525-24535. [45] Y. Liu, H.H. Wang, J. Zhou, L.Y. Bian, E.W. Zhu, J.F. Hai, J. Tang, W.H. Tang, Graphene/polypyrrole intercalating nanocomposites as supercapacitors electrode, Electrochim. Acta 112 (2013) 44-52. [46] B. Wang, X.Y. He, H.P. Li, Q. Liu, J. Wang, L. Yu, H.J. Yan, Z.S. Li, P. Wang, Optimizing the charge transfer process by designing Co 3O4@PPy@MnO2 ternary core-shell composite, J. Mater. Chem. A 2 (2014) 12968-12973. [47] X. Jian, H.M. Yang, J.G. Li, E.H. Zhang, L.L. Cao, Z.H. Liang, Flexible all-solid-state

high-performance

supercapacitor

based

on

electrochemically

synthesized carbon quantum dots/polypyrrole composite electrode, Electrochim. Acta 228 (2017) 483-493. [48] J. Pu, F.L. Cui, S.B. Chu, T.T. Wang, E.H. Sheng, Z.H. Wang, Preparation and electrochemical characterization of hollow hexagonal NiCo 2S4 nanoplates as pseudocapacitor materials, ACS Sustainable Chem. Eng. 2 (2014) 809-815. [49] H.C. Chen, J.J. Jiang, L. Zhang, H.Z. Wan, T. Qi, D.D. Xia, Highly conductive NiCo2S4 urchin-like nanostructures for high-rate pseudocapacitors, Nanoscale 5 (2013) 8879-8883. [50] C. Li, J. Balamurugan, T.D. Thanh, N.H. Kim, J.H. Lee, 3D hierarchical CoO@MnO2 core-shell nanohybrid for high-energy solid state asymmetric supercapacitors, J. Mater. Chem. A 5 (2016) 397-408. [51] H.C. Chen, J.J. Jiang, L. Zhang, D.D. Xia, Y.D. Zhao, D.Q. Guo, T. Qi, H.Z. Wan,

In situ growth of NiCo2S4 nanotube arrays on Ni foam for supercapacitors: Maximizing utilization efficiency at high mass loading to achieve ultrahigh areal pseudocapacitance, J. Power Sources 254 (2014) 249-257. [52] T. Zhai, L.M. Wan, S. Sun, Q. Chen, J. Sun, Q.Y. Xia, H. Xia, Phosphate ion functionalized Co3O4 ultrathin nanosheets with greatly improved surface reactivity for high performance pseudocapacitors, Adv. Mater. 29 (2017) 1604167. [53] Y.R. Zhu, X.B. Ji, Z.P. Wu, W.X. Song, H.S. Hou, Z.B. Wu, X. He, Q.Y. Chen, C.E. Banks, Spinel NiCo 2O4 for use as a high-performance supercapacitor electrode material: understanding of its electrochemical properties, J. Power Sources 267 (2014) 888-900. [54] D.L. Li, Y.N. Gong, C.X. Pan, Facile synthesis of hybrid CNTs/NiCo2S4 composite for high performance supercapacitors, Sci. Rep. 6 (2016) 29788. [55] W.D. Yu, W.R. Lin, X.F. Shao, Z.X. Hu, R.C. Li, D.S. Yuan, High performance supercapacitor based on Ni3S2/carbon nanofibers and carbon nanofibers electrodes derived from bacterial cellulose, J. Power Sources 272 (2014) 137-143. [56] X. Wang, C.Y. Yan, A. Sumboja, P.S. Lee, High performance porous nickel cobalt oxide nanowires for asymmetric supercapacitor, Nano Energy 3 (2014) 119-126. [57] Z.X. Yin, Y.J. Chen, Y. Zhao, C.Y. Li, C.L. Zhu, X.T. Zhang, Hierarchical nanosheet-based CoMoO4-NiMoO4 nanotubes for applications in asymmetric supercapacitors and the oxygen evolution reaction, J. Mater. Chem. A 3 (2015) 22750-22758. [58] Y.R. Zhu, Z.B. Wu, M.J. Jing, X.M. Yang, W.X. Song, X.B. Ji, Mesoporous NiCo2S4 nanoparticles as high-performance electrode materials for supercapacitors, J. Power Sources 273 (2015) 584-590. [59] R.B. Rakhi, N.A. Alhebshi, D.H. Anjum, H.N. Alshareef, Nanostructured cobalt sulfide-on-fiber with tunable morphology as electrodes for asymmetric hybrid supercapacitor, J. Mater. Chem. A 2 (2014) 16190-16198. [60] M.M. Yao, Z.H. Hu, Y.F. Liu, P.P. Liu, Design and synthesis of hierarchical NiCo2S4@NiMoO4 core/shell nanospheres for high-performance supercapacitors, New J. Chem. 39 (2015) 8430-8438.

[61] X. Wang, P.S. Lee, Dodecyl sulfate induced fast faradic process in nickel cobalt oxide/reduced graphite oxide composite material and its application for asymmetric supercapacitor device, J. Mater. Chem. 22 (2012) 23114-23119.

Figure captions: Fig. 1 Schematic diagram to illustrate the preparation of Ppy@NiCo2S4 core-shell heterostructure material. Fig. 2 XRD patterns of Ppy NTs (A), Ppy@SiO2 NTs (B), Ppy@NiCo-Pre (C), and Ppy@NiCo 2S4 core-shell composite (D). Fig. 3 FTIR spectra (A) and Raman spectra (B) for the Ppy, Ppy@NiCo2S4 and NiCo2S4. Fig. 4 XPS spectrum of as-prepared Ppy@NiCo 2S4 composite: survey (A), C 1s (B), N 1s (C), Ni 2p (D), Co 2p (E) and S 2p (F). Fig. 5 SEM images for NiCo-Pre microsphere (A), NiCo2S4 microsphere (B), Ppy NTs (C and D), Ppy@SiO2 NTs (E), Ppy@NiCo-Pre (F) and Ppy@NiCo2S4 core-shell composite (G, H and I). Fig. 6 TEM images for Ppy NTs (A), Ppy@SiO2 NTs (B and C), Ppy@NiCo2S4 core-shell composite (D, E and F). Fig. 7 The N2 adsorption/desorption isotherms (A) and pore size distributions curves (B) of Ppy@NiCo 2S4 composite, NiCo 2S4 microsphere and Ppy NTs. Fig. 8 CV curves of the Ppy NTs, NiCo2S4 microspheres and Ppy@NiCo2S4 core-shell composite electrodes at the sweeping rates of 5 mV s-1 (A); CV curves of the Ppy@NiCo 2S4 electrode at various sweeping rates (B); GCD curves of the Ppy@NiCo 2S4 composite electrode at various current densities (C); Specific capacitance of as-prepared electrodes at different current densities (D); Comparison in the cycling performance of the Ppy@NiCo 2S4 electrode and NiCo 2S4 microspheres electrode (E); Nyquist plots of Ppy@NiCo2S4, NiCo 2S4 and Ppy electrodes (F). Fig. 9 CV curves of Ppy@NiCo2S4 electrode and N-C NTs electrode at a scan rate of 5 mV s-1 (A). CV curves of Ppy@NiCo2S4//N-C asymmetric supercapacitor at different voltage window (B) and at various sweeping rates (C); GCD curves of

Ppy@NiCo 2S4//N-C asymmetric supercapacitor at various current densities (D); Calculated specific capacitance of the device at various current densities (E); Cycling performance of Ppy@NiCo2S4//N-C at a current density of 5 A g-1 (F); Ragone plots of energy density and power density of Ppy@NiCo2S4//N-C asymmetric supercapacitor (G).

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Graphical Abstract

Core-shell

heterostructure

Ppy@NiCo2S4

material

was

prepared

taking

polypyrrole nanotubes as skeleton through a facile and effective approach. The Ppy@NiCo 2S4//N-C asymmetric supercapacitor device exhibits high electrochemical performance with excellent cycling stability.

1.

Core-shell heterostructure Ppy@NiCo2S4 composite was fabricated.

2.

Ppy@NiCo2S4//N-C asymmetric supercapacitor (ASC) was further assembled.

3.

This ASC delivers a high energy density of 50.82 Wh kg-1 at 160 W kg-1.

4.

This ASC owns prominent cyclic stability of 126.6% after 2000 cycles.