shell hybrid nanostructure for high performance supercapacitor

Diamond & Related Materials 73 (2017) 80–86

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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond

MWCNT/NiCo2S4 as core/shell hybrid nanostructure for high performance supercapacitor Lemu Girma Beka a, Xin Li a,⁎, Xianjun Xia b, Weihua Liu a a b

Department of Microelectronics, School of Electronics and Information Engineering, Xi'an Jiaotong University, Shaanxi 710049, China School of Science, Xi'an Jiaotong University, Shaanxi 710049, China

a r t i c l e

i n f o

Article history: Received 28 June 2016 Received in revised form 11 January 2017 Accepted 11 January 2017 Available online 21 January 2017 Keywords: Supercapacitor MWCNT NiCo2S4 Core/shell

a b s t r a c t MWCNT/NiCo2S4 nanocomposite is designed as core/shell hybrid nanostructure to be used as electrode material for supercapacitor application. NiCo2S4 is grown on the surface of acid functionalized MWCNT by two step hydrothermal process. The MWCNT serves as an effective support for NiCo2S4 due to their open mesoporous tubular readily accessible surface areas. Using the excellent electrical conductivity and mechanical stability of MWCNT and excellent pseudocapacitive property of NiCo2S4 the as-synthesized MWCNT/NiCo2S4 exhibits excellent supercapacitive performances in 6 M KOH, including high specific capacitance of 1423.7 F/g at 2.5 A/g, excellent rate capability of 70.9% as current density increase from 2.5 to 25 A/g and excellent cycle stability of 81.4% after 4000 cycles of charge discharge by applying a constant current density of 10 A/g. This impressive results show great promise of ternary metal sulfide and MWCNT core/shell nanohybrid in future energy storage applications. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The fast development of portable electronic devices has been severely triggering the need for advanced energy storage devices. Supercapacitors (SCs) become promising device for energy storage due to their higher power density, long cycling life span, short charging time, and low maintenance cost [1–3]. There are two kinds of SCs according to their energy storing principles, the first one is electrical double-layer capacitors (EDLCs) in which energy storage is dominated by electrostatic charge accumulation at the interface between the electrode and electrolytes and the second one is pseudocapacitors in which energy storage is by fast and reversible Faradic reactions that occur at the electrode surface [4]. Different materials have long been intensively studied as promising materials for SCs, for example, carbon based materials [5–7], transition metal oxides and hydroxides [8–11] and conducting polymers [12]. However, the low capacitance value of carbon based materials; the low conductivity of transition metal oxides and the low cycle stability of conducting polymers limit their practical applications. Transition metal sulfide based composites are becoming promising material for SCs application because of their good conductivity and good chemical activity [13,14]. Particularly, ternary nickel cobalt sulfides have exhibited a higher capacitive performance than the corresponding binary nickel sulfides and cobalt sulfides due to their multiple oxidation participation in redox reaction and good electrical ⁎ Corresponding author. E-mail address: [email protected] (X. Li).

http://dx.doi.org/10.1016/j.diamond.2017.01.008 0925-9635/© 2017 Elsevier B.V. All rights reserved.

conductivity. Different researchers reported ternary nickel cobalt sulfides as a good candidate of electrode materials for SCs; for example, Wan et al. [15] has reported NiCo2S4 porous nanotubes synthesized via sacrificial templates as high-performance electrode materials of supercapacitors and has shown good electrochemical performance of 933 F/g at 1 A/g; Pu et al. [16] has reported Preparation and electrochemical characterization of hollow hexagonal NiCo2S4 nanoplates as pseudocapacitor materials with highest capacitance of 437 F/g at current density of 1 A/g, Chen et al. [17] reported urchin-like NiCo2S4 nanostructure using a facile precursor transformation method with capacitance value of 1149 F/g at 1 A/g and others [18]. Even though ternary metal sulfides showed promising performance, practical application of ternary metal sulfides is limited because of two main challenges, namely, low rate performance at high current densities and short life cycle. To address these problems reducing the device size to nanoscale to increase its active area and mechanical stability is a key concept [19], moreover, hybridizing of nanostructrured pseudocapacitive materials as a shell with highly conductive materials as a core are expected to give improved performance for energy storage devices [20]. So far, considerable effort has been devoted to synthesize core/shell nanostructures for high-performance SCs. Especially, the excellent electrical conductivity and regular tubular framework for anchoring pseudocapacitive materials makes CNTs an ideal material for core/shell structure design. For example, M. Mao et al. [21] reported facile synthesis of cobalt sulfide/carbon nanotube shell/core composites for high performance supercapacitors with improved cyclic properties which show about 13% decay in available specific capacity after 2000 cycles; Y. G. Zhu et al. [22] reported CoO nanoflowers woven by CNT network for

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high energy density flexible micro-supercapacitor and obtained improved cycling stability; and Liu et al. [23] reported a three dimensional vertically aligned multiwall carbon nanotube/NiCo2O4 core/shell structure for novel high-performance supercapacitors with improved capacitance performance. Even though great improvement of capacitance is reported for CNT/binary metal sulfides and CNT/binary metal oxides by introducing core/shell structures, the core/shell structure of CNT/ternary metal sulfides is less exploited. Herein, we proposed MWCNT/NiCo2S4 as core/shell nanostructure hybrid material as high performance SCs. Initially, CNT is grown by using CVD process. Then, NiCo2S4 is hydrothermally grown on MWCNT by using two step hydrothermal processes. The as-synthesized core/shell nanostructure hybrid composite showed excellent electrochemical performance. Such core/shell hybrid electrodes are promising materials for future energy storage applications. 2. Experimental methods 2.1. Preparation The preparation of our hybrid composite involves CVD growth followed by two step hydrothermal processes. Firstly, 0.1 g of MWCNT is grown by using CVD processes and functionalized by treating in mixture of nitric and sulfuric acids in the volume ratio of (1:3, in 80 ml of DI water) and ultrasonicated for 2 h at a temperature of 50 °C. Then, after cooling down to room temperature the acid treated MWCNT was washed with DI water using vacuum filtration until it becomes neutral and is dried at temperature of 60 °C in a vacuum. Secondly, 0.5 g of CoCl2, 0.25 g of NiCl2 and 1.2 g of urea are treated in solution of 6, 12 and 6 ml of ethanol, DI water and ethylene glycol, respectively and magnetically stirred until a homogeneous pink solution is obtained. Then, 0.05 g of acid functionalized MWCNT sample and the pre-prepared homogenous pink solution is mixed by magnetic string and sonicated for 30 min and transferred to stainless steel auto clave and hydrothermally treated at temperature of 180 °C for 8 h, and naturally cooled down and washed by ethanol and DI water many times and dried at 60 °C in a vacuum and MWCNT/NiCo2S4 precursor is obtained (Fig. 1, step I). Finally, the obtained precursor is treated hydrothermally in a homogenous solution of 0.51 g of Na2S·9H2O and 20 ml of DI water for 8 h at temperature of 180 °C and cooled down to normal temperature and after washing and drying MWCNT/NiCo2S4 core/shell structure is obtained (Fig. 1, step II).

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Rigaku Max-2200 X-ray diffractometer with monochromatized CuKα radiation (λ = 0.1542 nm). 2.3. Electrochemical measurements To study the electrochemical properties of MWCNT/NiCo2S4 core/ shell hybrid electrode we conducted cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS) and cycling tests. All the electrochemical measurements were carried out in a three-electrode cell (CS 2350 Corrtest, Wuhan, China) electrochemical workstation. The composite electrode served as the working electrode, while a Pt plate and Ag/AgCl with saturated KCl were used as the counter and a reference electrode, respectively. The working electrode is prepared by mixing as-synthesized powder MWCNT/NiCo2S4, acetylene black and poly(tetrafluoroethylene) in a weight ratio of 80:10:10 and more wetted by adding little ethanol and DI water and coated on 1 × 1 cm2 area of nickel foam current collector. After the solvent is evaporated, the resulting paste was pressed at 10 MPa to form a thin nickel foil. The weight of active materials loaded on nickel foam current collector was around 5 mg/cm2. A 6.0 M KOH aqueous electrolyte was used for all the measurements. The specific capacitance of the composite is calculated from CV using the following relation [24]: Cs ¼

1 vc ∫ iðvÞdv 2vmðΔV Þ va

ð1Þ

where Cs (F/g) is the specific capacitance, i (v) is the instantaneous current in (A), v (mV/s) is the scan rate, Δv(V) is the applied potential window (va to vc), and m (g) is the weight of the active material. On the other hand, to calculate the specific capacitance from charge discharge curve we have used the following relation: CS ¼

I  Δt m  ΔV

ð2Þ

where, C (F/g), I (A), t (S), m (g) and V (volt) are correspondingly specific capacitance, discharge current, discharge time, mass of active material and potential window [25]. 3. Results and discussion 3.1. Material characterization and electrochemical properties

2.2. Structural characterization Scanning electron microscopy (SEM) and EDAX analysis were conducted by Hitachi S-4800 microscope to study the morphology and elemental distribution of as-synthesized composite. The crystal structure of the composite was examined using X-ray diffraction (XRD) patterns

Fig. 2 shows typical SEM images of as-synthesized samples. Fig. 2(a) shows the MWCNT/NiCo2S4 precursor grown after the first hydrothermal process. Clearly, the surface of the functionalized MWCNT is covered by NiCo2S4 precursor. Fig. 2(b) show SEM image of MWCNT/ NiCo2S4 after the second hydrothermal process, obviously, very thin

Fig. 1. Schematic illustration of growth processes.

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Fig. 2. SEM images of as synthesized samples. (a) SEM image of MWCNT/NiCo2S4 precursor after first hydrothermal process, (b) SEM image of MWCNT/NiCo2S4 core/shell structure after second hydrothermal process, (c) high resolution SEM images of pristine MWCNT and (d) high resolution SEM image of MWCNT/NiCo2S4 core/shell after second hydrothermal process (the inset image shows the high resolution SEM image of core/shell structure to show NiCo2S4 on MWCNT).

(ultra thin) NiCo2S4 nanosheets warped on and between MWCNT are observed. Especially, the high resolution magnified image of pristine MWCNT and MWCNT/NiCo2S4 are shown in Fig. 2(c) and (d) to clearly compare the surface of MWCNT before and after the growth of NiCo2S4 active material on MWCNT. Clearly, compared to the surface of pristine MWCNT of Fig. 2(c) the surface of MWCNTs in inset Fig. 2(d) are covered by ultra thin nanosheets and form core/shell structure and on the other hand, the separately grown nanosheets are also interwoven by MWCNT showing successful transformation of MWCNT to MWCNT/ NiCo2S4 hybrid structure. In this hybrid structure, the ultra thin and rough NiCo2S4 nanosheet structure could be used as excellent pseoudocapacitive active area while the MWCNTs serve as the mechanical support and high electron transport channel, moreover, the entangled three dimensional MWCNT allow direct and easy contact of the active materials with the electrolyte ions by decreasing the distance of electrolyte ion diffusion. The growth mechanism of MWCT/NiCo2S4 core/shell hybrid structure is illustrated as follows. At the beginning, oxygen-containing functional groups were introduced on the surface of MWCNT by treating in strong acid by using sonication process [26]. In particular, the CVD grown MWCNTs sonicated within a mixture of strong acids such as HNO3 and H2SO4 to generate oxygenated functional groups (carboxylic (− COOH), carbonyl (\\C_ O) and hydroxyl (\\COH) groups) on MWCNT [27]. Ebbesen et al. [28] has experimentally showed the reaction between carboxylic groups on the nanotube surface (NT-COOH) with a metal ion (M+) using Eq. (3). NT−COOH þ Mþ X− →NT−COO− Mþ þ HX

ð3Þ

In our case the chloride salts are solvated to Ni2+ and Co+2 cations and electrostatically attracted to the carboxyl groups on the MWCNT and forms nucleation point (NT-COONi or NT-COOCo) as of Eq. (3).

Moreover, during the hydrothermal treatment urea is hydrolysis into hydroxyl (OH−) and carbonate (CO−2 3 ) ions according to Eqs. (4) and (5) [29]. Then, these anions react with nucleation site metal cations and commence the growth of NiCo2S4 precursor (bimetallic carbonate hydroxide precursors) according to Eq. (6) [29,30]. 2− þ 2OH− COðNH2 Þ2 þ 3H2 O→2NHþ 4 þ CO

ð4Þ

CO2 þ 2OH− →CO2− 3 þ H2 O

ð5Þ

xNi2þ þ 2xCo2þ þ 6xOH− þ 0:5xCO2− 3 →Nix Co2x ðCO3 Þ0:5x ðOHÞ6x

ð6Þ

DI water, ethanol and ethyl glycol are used as dispersing and solvent during our growth process. Moreover, the ethyl glycol serves as reducing agent for metal cations [31]. On the other hand, the excessive Ni2+ and Co+2 cations are reduced in the solution to form separate NiCo2S4 nanosheets in the solution form the loosely packed composite powder as can be seen from the SEM images. After the growth of MWCNT/ NiCo2S4 precursor (Fig. 1 step I) the next step was transforming our composite precursor to MWCNT/NiCo2S4 (Fig. 1 step II). At this step the pre-grown MWCNT/NiCo2S4 precursors was transformed into the MWCNT/NiCo2S4 counterpart by reacting with Na2S·9H2O by the process known as hydrothermal anion exchange reaction. In this particular case, NiCo2S4 is obtained by anion exchange reaction between S−2 in the NaS2·9H2O solution and hydroxyl (OH−) and carbonate (CO23 −) ions in the bimetallic carbonate hydroxide precursor at hydrothermal and OH− anions react with hydrogen environment. The replaced CO2− 3 cations in the solution to produce CO2 and H2O [32]. The bubbling of CO2 gas resulted during anion exchange resulted in porous and rough texture of NiCo2S4 sheet Fig. 2(d) after the second hydrothermal process.

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To further confirm the transformation of MWCNT to MWCNT/ NiCo2S4 we conducted EDAX analysis and studied distribution of elements in the composite before and after growth of core/shell structure (Fig. 3). In Fig. 3(a) and (b) the rectangle marked places show the area where EDAX analysis is conducted for MWCNT and MWCNT/NiCo2S4, respectively, while Fig. 3(c) and (d) shows the corresponding intensity count ratio of elements in the composite for MWCNT and MWCNT/ NiCo2S4, respectively. Clearly, from Fig. 3(c) the count ration intensity shows as almost all of the area is covered by carbon element showing excellent purity of our MWCNT. On the other hand, in Fig. 3(d) the existence of C, Ni, Co and S elements in the sample signifies the successful conversion of MWCNT into its MWCNT/NiCo2S4 hybrid counterpart. To know about the crystal structure of as synthesized composite, the crystal phase of MWCNT/NiCo2S4 core/shell was investigated using XRD patterns as shown in Fig. 3(e). Clearly, it shows distinct peaks at 26.8°, 31.6°, 38.3°, 50.5°, 55.3° and 69.36° which corresponds to (220), (311), (400), (511), and (440) diffraction planes, respectively, which can be indexed to the cubic phase of NiCo2S4 (JCPDS 43-1477). The weak intensity peaks shows amorphous or low crystalline nature of the as synthesized composite [33]. The electrochemical properties of MWCNT/NiCo2S4 core/shell structure were explored by using a three electrode configuration in 6 M of aqueous KOH. Fig. 4(a) shows the typical CV curves at various scan rates between 10 and 50 mV/s within the potential window of − 0.4 to 0.4 V. Obviously, the CV curves show a pair of redox peaks during the cathodic and anodic sweeps, demonstrating the pseudocapacitive behavior of the composite. Fig. 4(b) shows the GCD plot of MWCNT/

83

NiCo2S4 core/shell structure at various current densities within a potential widow of −0.4 to 0.4 V, clearly, it shows plateaus shape indicating pseudocapacitve behavior of as-synthesized sample which is in agreement with the CV redox peaks. The specific capacitance of as-synthesized hybrid composite calculated from CV graph by using Eq. (1) are 1500, 1425.6, 1350 and 1224.5 F/g at scan rates of 10, 20, 30 and 50 mV/s, respectively. On the other hand, the specific capacitance calculated from GCD curve using Eq. (2) are 1423.7, 1338.6, 1221.6, 1162.7, 1099.5 and 1005.94 F/g at corresponding current densities of 2.5, 5, 10, 15, 20 and 25 A/g. Fig. 4(c) shows the plot of specific capacitance calculated from GCD curve as a function of current density for MWCNT/ NiCo2S4 core/shell, pristine NiCo2S4 and pristine MWCNT. Clearly, the as-synthesized core/shell hybrid electrode exhibit an outstanding specific capacitance as high as 1423.7 F/g at a current density of 2.5 A/g while pristine NiCo2S4 and pristine MWCNT shows 933 F/g and 78 F/g at current density of 2.5 A/g. Moreover, the core/shell composite shows 70.9% capacitance retention even after the current density is increased from 2.5 to 25 A/g which is less than 96% retention of pristine MWCNT and much better than that of 53% of pristine NiCo2S4, implying the important role of MWCNT in improving the rate capability of ternary metal sulfides by hybridizing in the form of core/shell structure. Fig. 4(d) shows the comparative cycle stability performance of MWCNT, MWCNT/NiCo2S4 and pristine NiCo2S4; as we can see clearly, MWCNT/ NiCo2S4 core/shell structure shows 81.4% of the initial capacitance after 4000 cycles of charge discharge by applying a constant current density of 10 A/g which is much better than the pristine NiCo2S4 which shows 48% of capacitance retention after 4000 of cycles under

Fig. 3. EDAX and XRD patterns (a) area where EDAX analysis was conducted for MWCNT, (b) area where EDAX analysis was conducted for MWCNT/NiCo2S4 core/shell hybrid composite, (c) EDAX analysis results of MWCNT, (d) EDAX analysis results of MWCNT/NiCo2S4 and (e) XRD pattern of MWCNT/NiCo2S4 core/shell hybrid composite.

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Fig. 4. Electrochemical performance. (a) CV plots of MWCNT/NiCo2S4 core/shell hybrid at various scan rates, (b) GCD plots of MWCNT/NiCo2S4 core/shell hybrid at various current densities, (c) plot of specific capacitance as a function of current densities for MWCNT/NiCo2S4 core/shell hybrid, pristine NiCo2S4 and pristine MWCNT, (d) plot of capacitance retention as a function of cycle numbers for MWCNT/NiCo2S4 core/shell hybrid, pristine NiCo2S4 and pristine MWCNT, and (e and f) EIS plot of MWCNT/NiCo2S4 core/shell hybrid, pristine NiCo2S4 and pristine MWCNT.

similar charge discharge current density. The main drawbacks of NiCo2S4 supercapacitor in cycling stability is because of typical shrinkage, breaking, and cracks appearing in subsequent cycles because of volume changes. On the other hand, CNTs have excellent mechanical flexibility [34] which makes it to adapt to the volume changes and resulted in excellent cycling stability for supercapacitors application. MWCNTs/NiCo2S4 composite has shown improved cycling performance because of the following reasons; first, the size of NiCo2S4 prepared during this hybrid is very thin which resulted in better mechanical strength because the lattice stress can be largely alleviated due to their small size. Second, the excellent mechanical flexibility of CNTs can serve as a buffer to accommodate the volume expansion of NiCo2S4 during continues charge discharge. Third, the separately grown NiCo2S4 nanosheet grown are also interwoven by MWCNT network which makes this composite to adapt to the volume change using the free spaces between the networked MWCNT and mechanical flexibility of MWCNT as back bone and resulted in improved cycling performance [35]. To study the conductivity of as-synthesized composite we conducted comparative EIS test for all the three samples (MWCNT, MWCNT/ NiCo2S4 and NiCo2S4). Fig. 4(e) and (f) describes the impedance responses of MWCNT, MWCNT/NiCo2S4 and NiCo2S4 electrodes; which were measured at the open-circuit potential in the frequency ranging

from 100 KHz to 0.01 Hz with AC perturbation voltage of 5 mV. In the EIS plots, the Z′ intercept is related to the equivalent series resistance (RESR), the high frequency semicircle diameter corresponds to the charge transfer resistance (Rct) and the more slope line in low frequency region is related to good capacitance behavior [36,37]. So, to make clear analysis of high frequency region, the magnified scale and the approximate full semicircle of Fig. 4(f) is shown in inset Fig. 4(f). The full semicircle of inset Fig. 4(f) is manually approximated by adding doted lines. The approximate Rct values from inset Fig. 4(f) are 0.1213, 0.132 and 0.217 Ω for MWCN, MWCNT/NiCo2S4 and NiCo2S4, respectively. Similarly, the approximate RESR values are 0.435, 0.467, and 0.468 Ω for MWCN, MWCNT/NiCo2S4 and NiCo2S4, respectively [38,39]. It could be inferred from EIS plots that MWCNT/NiCo2S4 core/shell structure shows improved conductance compared to pristine NiCo2S4 demonstrating the advantage of MWCNT in improving conductance of MWCNT/NiCo2S4 electrode. The lower Rct of MWCNT/NiCo2S4 could be attributed to the presence of excellent conductive MWCNTs which served as super high way channel for electron transport in the composite. To show the advantage of our core/shell structure we compared electrochemical performance of our MWCNT/NiCo2S4 core/shell electrode with some of recently reported NiCo2S4 electrodes as shown in Table 1. Clearly, our composite showed superior electrochemical

Table 1 Comparison of electrochemical performance of our MWCNT/NiCo2S4 core/shell with pervious works. Sample

Capacitance value

Cycle performance (%)

Rate capability (%)

Ref.

Porous NiCo2S4 nanotube NiCo2S4 hollow hexagonal plates Ball-in-ball hollow spheres NixCo3 − xS4 hollow prisms Tube-like NiCo2S4 MWCNT/NiCo2S4 core/shell

933 F/g (1 A/g) 437 F/g (1 A/g) 1036 F/g (1 A/g) 895 F/g (1 A/g) 1048 F/g, (3 A/g) 1423.7 F/g, (2.5 A/g)

63 (1000 cycles, 1 A/g) 81 (1000 cycles, 2 A/g) 87 (2000 cycles, 5 A/g) – 75.9 (5000 cycles, 10 A/g) 81.4 (4000, 10 A/g)

50.3 (5 A/g) 53.2 (20 A/g) 65.08 (20 A/g) 63.3 (20 A/g) 50.09 (3–10 A/g) 70.9 (2.5–25 A/g)

[15] [16] [40] [41] [42] This work

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Fig. 5. Electrochemical performance of symmetric electrode device. (a) CV plots of MWCNT/NiCo2S4 core/shell hybrid at various scan rates and (b) GCD plots of MWCNT/NiCo2S4 core/shell hybrid at various current densities.

performance due to the synergistic advantage of NiCo2S4 and MWCNT in the following aspects. First, ultra thin NiCo2S4 nanosheet structures grown on the MWCNT provide excellent pseudocapacitive active area which can easily contacted with electrolyte ion and leading to improved capacitance performance. Second, the superior rate capability can be ascribed to the highly conductive pathway provided by the MWCNT core materials. Third, the ultra thin nature of NiCo2S4 nanosheet results in less volume changes during subsequent charge discharges and more importantly, the excellent flexibility property of MWCNT to volume changes resulted in good stability during fast reversible redox reactions. These excellent electrochemical properties show the promising application of MWCNT/NiCo2S4 core/shell structure as excellent candidates for supercapacitor electrodes. To study the practical application of our MWCNT/NiCo2S4 core/shell structure as electrode material we assembled MWCNT/NiCo2S4 symmetric electrode device. Fig. 4(a) shows the CV plots of as-synthesized symmetric device at various scan rates ranging from 10 to 50 mV/s in the potential window of 0 to 1 V. Fig. 5(b) illustrates the charge discharge curves of as-synthesized sample at different current densities within a potential window of 0 to 1 V. The calculated capacitances of as fabricated electrode using Eq. (2) are 317, 288, 276 and 268 F/g at corresponding current densities of 1, 1.5, 2, and 3 A/g. Energy density and power density are the two important parameters to test the practical performance of electrodes. The energy and power densities are calculated using the Eqs. (7) and (8), respectively:

4. Conclusion A high performance MWCNT/NiCo2S4 core/shell structure was fabricated by CVD growth followed by two step hydrothermal process. The MWCNT/NiCo2S4 core/shell based structure exhibited excellent electrochemical performance by incorporating the synergistic advantage of ultrathin NiCo2S4 shell anchored on MWCNT as high pseudocapacitive active material and excellent conductivity and mechanical stable MWCNT as core material. Using this advantages the as-synthesized composite exhibits high capacitance of 1423.7 F/g at current density of 2.5; excellent rate performance of 70.9% as current increase from 2.5 to 25 A/g and excellent cycling stability of 81.4% after 4000 cycles of charge discharge using 10 A/g of current density. Thus, MWCNT/ NiCo2S4 core/shell structure shows promising performance in future energy storage applications. Acknowledgement This work was financially supported by grants from the National Natural Science Foundation of China (Nos. 91123018, 61172040, 61172041), Shanxi Province Natural Science Foundation (2014JM7277), and the Fundamental Research Funds for the Central Universities. Some SEM work was done at International Center for Dielectric Research (ICDR), Xi”'an Jiaotong University, Xi”'an, China. The authors also thank Ms. Dai and Mr. Yang for their help in using SEM. References

E¼

CV2 2  3600

ð7Þ

P¼

E  3600 t

ð8Þ

where E (wh/kg), P (w/kg), C (F/kg), V (V) and t (S) are energy density, power density, specific capacitance, charging potential widow and discharge time, respectively [23]. Interestingly, the assembled symmetric device shows excellent energy and power densities of 37.2 Wh/kg and 1425.53 W/kg at current density of 3 A/g. Our hybrid MWCNTs/ NiCo2S4 composite resulted in high power density and enhanced energy density by using the synergistic advantage of both MWCNT and NiCo2S4. First, the MWCNT network provides a high path for electrical conduction and the three dimensionally entangled network makes easy diffusion of electrolytes and resulted in reduced resistance and consequently increase the power density; moreover, its excellent surface area serve as backbone for supporting NiCo2S4 nanosheets which is a crucial part to enhance the capacitive performance. Second, the porous NiCo2S4 nanosheets resulted in excellent pseudocapacitive performance. Thus, the as synthesized electrode material is interesting for future SCs electrode application.

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