shell nanowires grown on carbon fiber with enhanced electrochemical performance for hybrid supercapacitors

Accepted Manuscript Hierarchical NiCo2S4@PANI core/shell nanowires grown on carbon fiber with enhanced electrochemical performance for hybrid supercapacitors Xianbin Liu, Ziping Wu, Yanhong Yin PII: DOI: Reference:

S1385-8947(17)30659-9 http://dx.doi.org/10.1016/j.cej.2017.04.115 CEJ 16869

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

19 January 2017 25 April 2017 25 April 2017

Please cite this article as: X. Liu, Z. Wu, Y. Yin, Hierarchical NiCo2S4@PANI core/shell nanowires grown on carbon fiber with enhanced electrochemical performance for hybrid supercapacitors, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.04.115

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Hierarchical NiCo2S4@PANI core/shell nanowires grown on carbon fiber with enhanced electrochemical performance for hybrid supercapacitors Xianbin Liu*, Ziping Wu, Yanhong Yin School of Materials Science and Engineering, Jiangxi University of Science and Technology, 86 Hong Qi Road, Ganzhou 341000, P. R. China * Corresponding author. E-mail address: [email protected]

Abstract Rational assembly involving hetero-growth of hybrid structures consisting of multiple components with distinctive features is a promising strategy to develop materials with enhanced electrochemical performance for supercapacitors. Herein, hierarchical polyaniline-coated NiCo2S4 nanowires grown on carbon fiber (NiCo2S4@PANI/CF) were fabricated through hydrothermal method and potentiostatic deposition. The core/shell heterostructure endowed the NiCo2S4@PANI/CF composite materials with high electron diffusion efficiency and abundant accessible electroactive sites. The PANI shell improved the structural stability of the core NiCo2S4 nanowires. When employed as a free-standing electrode, the NiCo 2S4@PANI/CF exhibited impressive electrochemical performances with a high specific areal capacitance of 4.74 F/cm2 (1823 F/g) at 2 mA/cm2 and an excellent cycling stability with capacitance retention of 86.2 % after 5 000 cycles. Furthermore, an asymmetric supercapacitor device was assembled using NiCo2S4@PANI/CF as positive electrode and graphene/CF as negative electrode. The resultant device delivers a high energy density of 64.92 Wh/kg at a power density of 276.23 W/kg, as well as considerable flexibility. The core/shell heterostructure design is expected to realize high-performance flexible supercapacitor for future portable and wearable electronic devices.

Keywords: NiCo2S4 nanowires; polyaniline; core/shell structure; high energy density; excellent cycling stability; flexible supercapacitor

1

Introduction With the growing developments of backup power sources, portable electronics devices, renewable energy power plants, and electric vehicles, further improvement of energy storage and conversion technology is imperative.1-4 Supercapacitors have attracted considerable attention as high-power energy storage devices for various applications, owing to the favorable properties such as fast charge/discharge rate, high power density, low maintenance cost, and long service life.5-9 According to their energy storage mechanism, supercapacitors can be divided into two categories: electrical double-layer capacitors (EDLCs) and pesudocapacitors.10 EDLCs store energy through charge separation at the electrode/electrolyte interface, while pesudocapacitors store energy through redox reactions that occur at or near the interface of electrode (with approximately 5 nm).11, 12 By contrast, pesudocapacitors can deliver almost 10 times the capacitance and energy density of EDLCs. Therefore, it is extremely significant to develop new electrode materials with pesudocapacitance for constructing asymmetric supercapacitors with high energy desnity. Pseudocapacitive materials mainly consist of metal oxides/hydroxides/suldes and conductive polymers.7,

13-15

The ternary transition metal sulfides offer higher

electrochemical activity than metal oxides/hydroxides and conductive polymers and thus have been investigated extensively for use in supercapacitor electrodes.16-18 NiCo2S4 is a current research focus on account of its remarkable theoretical capacitance, cost efficiency, and simple preparation process.18-20 However, NiCo2S4 suffers from low electrical conductivity and large volume change in its charge/discharge processes, leading to poor rate capability and cycling stability of its electrodes. As a result, the practical application of pure NiCo 2S4 in supercapacitors has been restricted.18 In recent years, considerable research has been performed to overcome the limitations of NiCo 2S4 without sacrificing its advantages. One competitive strategy to enhance the performance of NiCo2S4 is to build free-standing electrodes by growing NiCo2S4 on conductive substrate materials,21, 22 such as nickel foam23 and carbon cloth24. Construction of heterostructures by introducing other components

is

another

effective

approach 2

to

improve

supercapacitive

performance.25-27 For example, hybrid NiCo2S4@MnO2 heterostructures were investigated

for

high-performance

supercapacitor,

hybrid

NiCo2S4@MnO2

heterostructured electrodes possessed a remarkable specific capacitance of 1337.8 F/g at a current density of 2.0 A/g and excellent cycling stability (82 % retention after 2000 cycles) because of the synergistic effects of NiCo 2S4 and MnO2.28 Moreover, Cobalt sulfide (CoSx) nanosheets were coated on NiCo 2S4 nanotube arrays as electrode

materials

for

high-performance

supercapacitors.29

However,

the

conductivity and structural stability of the shells reported to date are not sufficient to meet the requirements for high-performance supercapacitors. Polyaniline (PANI) is promising to combine with other materials because of its high theoretical pseudocapacitance, excellent chemical stability, high conductivity, low cost and easy preparation.14,

30, 31

By combining PANI with NiCo2S4, many

complementary characteristics such as abundant accessible electroactive sites and high electron diffusion efficiency may be realized. In addition, construction of a core/shell structure is efficient to improve cycling stability.32, 33 However, covering NiCo2S4 with PANI layers to fabricate supercapacitor electrodes has not been reported. Herein, we use a PANI layer as a shell coating on core NiCo 2S4 nanowires grown on carbon fiber cloth (denoted as NiCo2S4@PANI/CF). Benefiting from the core/shell heterostructure, the fabricated NiCo 2S4@PANI/CF shows excellent electrochemical performance, especially in terms of superior cycling life. Furthermore, the NiCo2S4@PANI/CF electrode as positive electrode was assembled with graphene/CF as negative electrode to fabricate an asymmetric supercapacitor. The device delivered a high energy density of 64.92 Wh/kg and a power density of 276.23 W/kg, while displaying considerable flexibility at wider bending angles.

3

Figure 1 Schematic illustration of the synthetic procedure for NiCo2S4@PANI/CF composites

Experimental detail Materials:

NiCl2·6H2O,

CoCl2·6H2O,

CO(NH2)2,

Na2S·9H2O,

sodium

p-toluenesulfonate (p-TSS, C7H7SO3Na) and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd., of analytical grade and were used as received without any further purification. Carbon fiber cloth (W0S1002) was supplied by Taiwan CeTech Co., Ltd. Deionized (DI) water was used throughout. Synthesis of NiCo2S4/CF: NiCo2S4 nanowires grown on carbon fiber cloth were prepared via two-step hydrothermal method. Before hydrothermal growth, carbon fiber cloth (4 cm×2 cm) were pretreated in concentrated nitric acid to improve their hydrophilia. Afterwards, NiCl2·6H2O (4 mM), CoCl2·6H2O (8 mM), CO(NH2)2 (20 mM) were dissolved in 80 mL deionized water to form claret-red solution, and the solution with pretreated carbon fiber cloth was transferred to 100 mL Teflon-lined stainless-steel autoclave and kept in an oven at 120℃ for 6 h. The obtained sample was washed with ethanol and deionized water. Secondly, the as-obtained sample was further hydrothermal treated with Na2S·9H2O at 160℃ for 8 h. After cooling down to room temperature, the sample was washed, then dried at 60℃ for 24 h. And NiCo2S4/CF was obtained. Graphene/CF was prepared by via hydrothermal method. The CF was dipped in 2 M graphene oxide solution, and then kept in an oven at 180℃ 4

for 2 h. Synthesis of NiCo2S4@PANI/CF: PANI was coated on the surfaces of NiCo2S4 nanowires through potentiostatic deposition. The electrodeposition was performed at a constant voltage of 0.8 V in a three-electrode system. The Ag/AgCl was used as a reference electrode; the Pt foil was as counter electrode and the NiCo2S4/CF as the working electrode. An aqueous solution of 0.1 M aniline monomer dispersed in 1 M p-TSS was used as the electrolyte. In order to achieve the best performance, PANI with electrodeposition time of 5 min was deposited on NiCo2S4/CF. Materials characterizations The morphologies and microstructures of obtained samples were investigated by field-emission transmission electron microscope (FE-TEM, FEI Tecnai G2 F20, USA) and field-emission scanning electron microscopy (FE-SEM, ZEISS SUPRA 55, German) equipped with an energy dispersive X-ray spectrometer (EDS). The structural and componential analyses were detected by X-ray diffractometer (XRD, Rigaku Ultima III, Japan), Fourier Transform Infrared Spectrometer (FTIR, Thermo Nicolet 5700 system, USA) and X-ray photoelectron spectrometer (XPS, Thermal Scientific ESCALAB 250, USA). Electrochemical measurements The electrochemical measurements were operated on the electrochemical workstation CHI 660D in a standard three-electrode system with 6 M KOH solution as electrolyte. The counter electrode and reference electrode was platinum foil and Hg/HgO electrode, respectively. The core/shell structural NiCo 2S4@PANI grown on carbon fiber was used directly as self-standing working electrode. Cyclic voltammogrammic (CV) and galvanostatic charge-discharge behaviors were tested within a potential range of 0-0.6 V. And the Electrochemical impedance spectroscopy (EIS) measurement was conducted in the frequency range of 10 −2 to 105 Hz with an amplitude of 10 mV. The specific capacitance of NiCo2S4@PANI/CF electrode was calculated by the following equation:
=

 × ∆  × ∆ 5

Where I (A) is the charge/discharge current, ∆ (s) is the discharge time, S (cm2) is the area of electrode, ∆ (V) is the potential window in the discharge curve. The energy density (E, Wh/kg) and power density (P, W/kg) were calculated from the discharge curves based on asymmetric supercapacitor by the following equations; 1
∆ 7.2  = 3600 ∆

=

Where C (F/g) is the specific capacitance of asymmetric supercapacitor, and ∆ (s) is the discharge time, ∆ (V) is the potential window. And the asymmetric supercapacitor was assembled with NiCo2S4@PANI/CF electrode and graphene/CF, as positive and negative electrode. The positive and negative electrodes were separated by a piece of cellulose paper separator that has been immersed in 6 M KOH solution. The electrochemical performance of asymmetric supercapacitor were tested through CV and galvanostatic charge-discharge.

Results and discussion

a

b

c

10 µm

10 µm

500 nm

d

e

f

500 nm

50 nm

5 µm

6

h

g

i

5 µm Figure 2 (a) FE-SEM image of bare CF; (b) low-magnification and (c) high-magnification FE-SEM images NiCo2 S4/CF composites; (d) low-magnification and (e, f) high-magnification FE-SEM images NiCo2S4@PANI/CF composites; (g, h, i) EDS element mappings

The growth of NiCo 2S4@PANI core/shell structure on CF was traced by FE-SEM. The morphology of each sample is shown in Figure 2. The bare CF is rough and the diameter is nearly 10 µm (Figure 2(a)), which is conducive to the growth of NiCo2S4 nanowires. Through the hydrothermal reaction, a great many of NiCo 2S4 nanowires were grown on the CF (Figure 2(b)). The high-magnification image in figure 2(c) reveals that NiCo 2S4 grew vertically and divergently. After the further electrochemical deposition, the integration of PANI growth shell into the NiCo 2S4 nanowires does not change the original structure as illustrated in Figure 2(d). Figure 2(e) and (f) show high-magnification FE-SEM images of the NiCo 2S4@PANI/CF composite. The surface of NiCo 2S4 nanowires was uniformly coated with a PANI shell. And the optical images (Figure S1) show that NiCo2S4@PANI was successfully grown on CF. The composite contained abundant interspaces. Therefore, this heterostructure should possess both a large surface area to contain numerous active sites and high structural integrity to improve cycling stability. Figure 2(g–i) display the elemental mapping images obtained for NiCo2S4@PANI/CF. C, Ni, Co, and S were evenly and continuously distributed in NiCo2S4@PANI/CF. In addition, the measured Ni/Co/S atom ratio is about 0.98:1.96:3.97, which is very close to the original stoichiometric ratio of 1:2:4. The microstructures of NiCo2S4 nanowires and NiCo 2S4@PANI core/shell nanowires were further observed by FE-TEM. Figure 3(a) shows that the scraped NiCo2S4 is wire-like in shape with a diameter of 20-40 nm. Furthermore, The 7

nanowires possess a rough surface, which indicates that they are composed of many tiny nanoparticles (~5 nm in diameter). The nanoparticles were characterized by the selected area electronic diffraction (SAED). The SAED pattern of NiCo 2S4 in Figure 3(b) shows several well-defined diffraction rings, suggesting that NiCo 2S4 is polycrystalline. The diffraction rings were consistent with the (111), (220), (311), (511), and (440) planes of the cubic phase NiCo 2S4.34 Figure 3(c) displays a typical image of NiCo 2S4@PANI core/shell nanowires. It is obviously that the NiCo 2S4 is uniformly covered by the amorphous PANI.35 The thickness of PANI shell is 10 nm according to Figure 3(d).

And the coating layer should increase the structural

stability of NiCo2S4 nanowires. The crystal structure and phase of the composite were characterized by XRD. Figure S2 compares of XRD pattern of the NiCo2S4/CF and NiCo 2S4@PANI/CF. Both samples displayed diffraction peaks at 2θ values of 19.3°, 22.6°, 38.3°, 50.5°, and 55.3° that could be indexed to the (111), (220), (311), (511), and (440) planes of the cubic phase of NiCo 2S4 (JCPDS 20-0782),29, 34 which is associated with the SAED result. The strong peaks at 24.6° were assigned to the (110) plane of the CF substrate. PANI is amorphous so it did not exhibit any diffraction peaks. The PANI shell did not obviously change the crystallinity of NiCo2S4, as demonstrated by the composite material NiCo 2S4@PANI/CF retaining the crystalline characteristics of NiCo2S4/CF.

a

b

5 1/nm

50 nm

8

c

d

PANI 5 nm

50 nm

Figure 3 (a) FE-TEM image and (b) SAED pattern of NiCo2S4 nanowires; (c) low-magnification and (d) high-magnification FE-TEM images of individual NiCo2S4@PANI with core/shell structure

Figure 4 FTIR spectra of bare CF, NiCo2S4/CF and NiCo2 S4@PANI/CF

FTIR spectroscopy was performed to investigate the molecular characteristics of obtained samples. As shown in Figure 4, the bare CF only exhibited a few inconspicuous peaks. NiCo2S4/CF displayed the same peaks as the bare CF and new peaks appeared at 525 and 1029 cm−1, which were characteristic of Ni-S and Co-S.34 This proved that NiCo 2S4 was successfully grown on CF. In the spectrum of 9

NiCo2S4@PANI/CF, the peaks at 1305, 1482 and 1566 cm-1 were associated with the C-H bending of quinoid ring, C-N+ stretching, C-C stretching of benzene ring and C=C

stretching,

confirming

the

successful

incorporation

of

PANI

with

NiCo2S4/CF.36-38 The small peaks at around 2920 and 2852 cm−1 are assigned to the N-H bond of the aromatic amine moieties of PANI.39, 40 XPS measurement was utilized to analyze the surface elemental composition and chemical

valence

states

of

different

elements

in

the

NiCo2S4/CF

and

NiCo2S4@PANI/CF. As illustrated in Figure 5(a), both of the obtained samples consist of Ni, Co, S and C; in addition, NiCo2S4@PANI/CF contained N originating from PANI. Figure 5(b), (c), (d), and (e) display high-resolution N 1s, Ni 2p, Co 2p and S 2p, fitted by using a Gaussian method. As shown in Figure 5(b), the N 1s peak can be split into three peaks at 399.3 eV, 399.8 eV and 400.9 eV which are assigned to quinonoid amine (=N-), benzenoid amine (-NH-) and nitrogen cationic amine (-N+-) structure, resspectively.41 The Ni 2p core-level spectrum (Figure 5(c)) was well fitted with two spin-orbit doublets characteristic of Ni2+ and Ni3+ and two shake-up satellites (labeled Sat.). And in the Figure 5(d), the Co 2p spectrum, similarly, was fitted two spin-orbit doublets, characteristic of Co2+ and Co3+, and two shake-up satellites. Figure 5(e) shows the spectrum of the S 2p region, in which two resolved peaks of binding energies at approximately 162.0 and 163.1 eV attributed to S 2p1/2 and S 2p 3/2. The binding energy at 163.1 eV is ascribed to the metal-sulfur bonds.42, 43 These

results

agree

well

with

previous

reports

NiCo2S4@PANI/CF contain N, Ni2+, Ni3+, Co 2+, Co3+ and S2-.

10

and

demonstrate

the

a

c

b

d

e

Figure 5 (a) full-scan XPS spectra of NiCo2S4/CF and NiCo2S4 @PANI/CF; high-resolution spectra for (b) N 1s, (c) Ni 2p, (d) Co 2p and (e) S 2p

a

b

c

d

Figure 6 (a) CV curves of CF, NiCo2S4/CF and NiCo2S4@PANI/CF at 100 mV/s; (b) CV curves of NiCo2S4 @PANI/CF at different scan rates; (c) GCD curves of CF, NiCo2S4 /CF and NiCo2S4@PANI/CF at 2 mA/cm2; and (d) GCD curves of NiCo2S4@PANI/CF at different current densities 11

To

explore

the

superiority

of

electrochemical

performances

of

NiCo2S4@PANI/CF compared with NiCo2S4/CF, CV and GCD measurements were performed in three-system electrode. Figure 6(a) displays the CV curves of CF, NiCo2S4/CF and NiCo2S4@PANI/CF at a scan rate of 100 mV/s. The CV curve of CF is a narrow rectangle, which is the characteristic of double electric layer capacitance. Comparison of the CV curve of CF with those of the composite electrodes reveals the contribution from the CF substrate was negligible. The CV curves of NiCo2S4/CF and NiCo2S4@PANI/CF contain peaks that are assigned to the following reversible faradaic reaction: NiS + OH− ↔ NiSOH + e− CoS + OH− ↔ CoSOH + e− CoSOH + OH− ↔ CoSO + H2O + e− The CV behavior of NiCo2S4@PANI/CF at various scan rates was investigated (Figure 6(b)). The shape of the CV curves remained basically the same and the peak current densities increase nearly linearly with the scan rates, indicating the rapid redox reaction of NiCo 2S4@PANI/CF. Figure 6(c) compares of GCD curves of CF, NiCo2S4/CF, and NiCo2S4@PANI/CF at a current density of 2 mA/cm2. It is easily observed that the NiCo2S4@PANI/CF electrode displays the longest charge/discharge time, demonstrating the highest specific capacitance. The NiCo2S4@PANI/CF electrode was further investigated at a wide range of current densities as shown in Figure 6(d). The symmetric shapes of GCD curves at higher current density indicate that its redox reactions are highly reversible. The detailed specific capacitance (C, F/cm2) is calculated by the following formula, C = (I·∆t)/(S·∆V), where I is the current density (A), ∆t is the discharge time (s), S is the area of electrode (cm2), and ∆V is the voltage range. The calculated specific capacitance of obtained samples based on the discharge curves as a function of the current density is plotted in Figure 7(a). The NiCo 2S4@PANI/CF electrode exhibits the highest specific capacitance of 4.74 F/cm2 at 2 mA/cm2. The mass loading was 2.6 mg/cm2, therefore, the mass specific capacitance was 1823 F/g. With the current density increasing to 50 mA/cm2, the rate capability of the 12

NiCo2S4@PANI/CF was 73.8 % retention (3.5 F/cm2, 1346 F/g), which was better than that of NiCo2S4/CF (56.9

%). The

superior rate performance of

NiCo2S4@PANI/CF is ascribed to the high conductivity of the PANI shell.

a

b

Figure 7 (a) Rate capability and (b) Nyquist plots of CF, NiCo2S4/CF and NiCo2S4@PANI/CF To further evaluate the electrochemical performances of the samples, we conducted electrochemical impedance spectroscopy (EIS) in the frequency range of 10 -2 Hz to 105 Hz. As shown in Figure 7(b), the Nyquist plots contained a depressed arc in the high-frequency range and, typically, a straight line in the low-frequency range. The diameter of the arc reflects the charge-transfer resistance (Rct) of the redox reaction at the electrode/electrolyte interface, which is related to the porous structure of the composite material, and the slope of the line corresponds to the capacitive behavior of the material. The Rct of NiCo 2S4@PANI/CF electrode is 2.1 Ω，which is smaller

than

that

(3.8

Ω)

of

NiCo 2S4/CF

electrode.

Meanwhile,

the

NiCo2S4@PANI/CF electrode shows a more vertical line, indicating faster ion diffusion than NiCo2S4/CF. These favorable properties may be explained by the core/shell structure of the NiCo2S4@PANI/CF electrode endowing it with faster electronic transmission and greater contact area than those of the NiCo 2S4/CF one. To verify the effect of PANI shell on the structural stability of NiCo 2S4 nanowires, a cycling test was performed at 20 mA/cm2 with 5000 cycles. Figure 8(a) compare of areal capacitance change of NiCo 2S4/CF and NiCo 2S4@PANI/CF electrode. After 5000 cycles, the NiCo 2S4@PANI/CF retained 86.2 % of its initial 13

areal capacitance, which is higher than that of NiCo2S4/CF. The SEM images in Figure 8(b) and (c) show that the pure NiCo2S4 nanowires were fractured and disordered

after 5000

cycles.

In contrast,

the PANI

shell allowed the

NiCo2S4@PANI/CF electrode to retain its original appearance after 5000 cycles, as depicted in Figure 8(d) and (e). These data indicate that the PANI shell induced the excellent cycling stability of NiCo2S4@PANI/CF.

a

b

c

50 nm

50 nm

d

e

50 nm

50 nm

Figure 8 (a) A comparison of cycling performances of NiCo2 S4/CF and NiCo2S4@PANI/CF; SEM images of pristine (b) NiCo2S4/CF and (d) NiCo2S4@PANI/CF; and (c), (e) corresponding SEM images of NiCo2S4/CF and NiCo2S4 @PANI/CF after 5000 charge/discharge cycles

14

To explore the possibility of practical application, we employed the NiCo2S4@PANI/CF electrode as positive electrode and graphene/CF (shown in Figure S3) as negative electrode with 6 M KOH as electrolyte to fabricate an asymmetric supercapacitor as shown in 9(a). It is well known that graphene is an ideal supercapacitor electrode material because of its high specific surface area, good conductivity and excellent electrochemical stability.44-46 The mass loading of graphene/CF and NiCo2S4@PANI/CF was matched according to the principle of charge balance. The CV curves of graphene/CF and NiCo 2S4@PANI/CF obtained at 100 mV/s are compared in Figure 9(b). According to the equation of charge balance m+q +=m-q, the mass loading of graphene and NiCo2S4@PANI are about 8.9 mg/cm2 and 2.6 mg/cm2, respectively. The graphene/CF electrode behaved as an ideal EDLC with a potential window from −1.0 to 0 V. The potential window of NiCo2S4@PANI/CF was from 0 to 0.6 V. Thus, the working voltage window of the supercapacitor was 1.6 V. Figure 9(c) displays the CV curves of the full device over the voltage range from 0 to 1.6 V at different scan rates. All the CV curves possess rectangular-like shapes, demonstrating that the device exhibited nearly ideal capacitive behavior.

15

b

a

c

d

e

f

Figure 9 (a) the structural illustration of the asymmetric supercapacitor based on the NiCo2S4@PANI/CF and graphene/CF; (b) CV curves of NiCo2S4@PANI/CF and graphene/CF at a scan rate of 100 mV/s; (c) CV curves, (d) galvanostatic charge/discharge curves, (e) rate capability and (f) Ragone plots of the NiCo2S4@PANI/CF//graphene/CF asymmetric supercapacitor

The superior electrochemical performance of the full supercapacitor was further confirmed by galvanostatic charge/discharge measurement. As illustrated in Figure 9(d), the triangular-shaped GCD curves at wider range of current densities again demonstrated its capacitive characteristics. The detailed specific capacitance of the supercapacitor was calculated and is plotted in Figure 9(e). The areal capacitance of the full supercapacitor reached 2.10 F/cm2 at a current density of 5 mA/cm2. With the current density increasing to 100 mA/cm2, the areal capacitance still maintained 1.67 16

F/cm2, which is 79.5 % of that at 5 mA/cm2. And the corresponding specific capacitances were 182.6 F/g and 145.2 F/g, respectively. The energy density and power density were calculated from galvanostatic discharge curves. Figure 9(f) list the Ragone plots NiCo 2S4@PANI/CF//graphene/CF asymmetric supercapacitor. Notably, the supercapacitor achieved a maximum energy density of 64.92 Wh/kg at a power density of 276.23 W/kg, which is much higher than those of the latest reported NiCo2S4-based asymmetric and symmetric supercapacitors (see table S1). This high energy density may be ascribed to the core/shell heterostructure of NiCo 2S4@PANI/CF and the good match of NiCo 2S4 with graphene.

Figure 10 CV curves of the NiCo2S4@PANI/CF//graphene/CF asymmetric supercapacitor under different bending angles at 50 mV/s

To further evaluate the potential of the NiCo 2S4@PANI/CF//graphene/CF supercapacitor for flexible energy storage under real conditions, a device was placed under bending angles and its CV curves were obtained. Figure 10 shows the CV curves of the device bent at different angles measured at a scan rate of 50 mV/s. The shape of the CV curves basically remained the same, which indicated that bending had little influence on the capacitive behavior of the device. Such stable performance can be attributed to the high mechanical flexibility of CF and strong connections between NiCo2S4@PANI and CF.

17

Conclusions In summary, a core/shell heterostructural NiCo2S4@PANI/CF composite was fabricated by a hydrothermal method and potentiostatic deposition, and then employed as supercapacitor electrode. Benefiting from the unique heterostructure of PANI shell cladding on core NiCo2S4 nanowires, the NiCo 2S4@PANI/CF electrode shows more electrochemical activity sites and faster ionic diffusion, then exhibited enhanced electrochemical performance compared with NiCo2S4/CF: an areal capacitance of 4.74 F/cm2 (1823 F/g) at 2 mA/cm2, a rate capability of 73.8 % with the current density increasing to 50 mA/cm2 and a capacitive retention of 86.2 % after 5000 cycles. Moreover, the NiCo2S4@PANI/CF was served as positive electrode to assemble a supercapacitor device. The device delivered a remarkable energy density of 64.92 Wh/kg at a power density of 276.23 W/kg with a wide potential window of 0−1.6 V. The device also possessed good flexibility. This study indicates the superiority of core/shell heterostructure for composite electrodes and paves new avenues for the design and fabrication of high-performance flexible supercapacitor.

Acknowledgements The authors greatly acknowledge the financial supports by the Department of Science & Technology of Jiangxi Province (Grant No. 20153BCB23011).

Supporting Information The optical image, XRD and SEM of obtained materials, and comparison of synthesis method, morphology, and electrochemical performance of different electrode materials.

References 1. Chu, S.; Majumdar, A., Opportunities and challenges for a sustainable energy future. Nature 2012, 488 (7411), 294-303. 2. Zhou, Z. B.; Benbouzid, M.; Charpentier, J. F.; Scuiller, F.; Tang, T. H., A review of energy storage technologies for marine current energy systems. Renew Sust. Energ Rev. 2013, 18, 18

390-400. 3. Dubal, D. P.; Ayyad, O.; Ruiz, V.; Gomez-Romero, P., Hybrid energy storage: the merging of battery and supercapacitor chemistries. Chem. Soc. Rev. 2015, 44 (7), 1777-1790. 4. Veeramani, V.; Madhu, R.; Chen, S. M.; Sivakumar, M., Flower-like nickel-cobalt oxide decorated dopamine-derived carbon nanocomposite for high performance supercapacitor applications. ACS Sustain. Chem. Eng. 2016, 4 (9), 5013-5020. 5. Conway, B., Electrochemical supercapacitor. Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York 1999. 6. Simon, P.; Gogotsi, Y., Materials for electrochemical capacitors. Nat. Mater. 2008, 7 (11), 845-854. 7. Wang, G. P.; Zhang, L.; Zhang, J. J., A review of electrode materials for electrochemical supercapacitors. Chem. Soc. Rev. 2012, 41 (2), 797-828. 8. Liu, X. B.; Wen, N.; Wang, X. L.; Zheng, Y. Y., A high-performance hierarchical graphene@polyaniline@graphene sandwich containing hollow structures for supercapacitor electrodes. ACS Sustain. Chem. Eng. 2015, 3 (3), 475-482. 9. Lin, X. X.; Tan, B.; Peng, L.; Wu, Z. F.; Xie, Z. L., Ionothermal synthesis of microporous and mesoporous carbon aerogels from fructose as electrode materials for supercapacitors. J. Mater. Chem. A 2016, 4 (12), 4497-4505. 10. Simon, P.; Gogotsi, Y.; Dunn, B., Where do batteries end and supercapacitors begin? Science 2014, 343 (6176), 1210-1211. 11. Sk, M. M.; Yue, C. Y.; Ghosh, K.; Jena, R. K., Review on advances in porous nanostructured nickel oxides and their composite electrodes for high-performance supercapacitors. J. Power Sources 2016, 308, 121-140. 12. Yuan, C. Z.; Yang, L.; Hou, L. R.; Shen, L. F.; Zhang, X. G.; Lou, X. W., Growth of ultrathin mesoporous Co3O4 nanosheet arrays on Ni foam for high-performance electrochemical capacitors. Energ. Environ. Sci. 2012, 5 (7), 7883-7887. 13. Zhang, C. Q.; Chen, Q. D.; Zhan, H. B., Supercapacitors based on reduced graphene oxide nanofibers supported Ni(OH)2 nanoplates with enhanced electrochemical performance. ACS Appl. Mater. Inter. 2016, 8 (35), 22977-22987. 14. Snook, G. A.; Kao, P.; Best, A. S., Conducting-polymer-based supercapacitor devices and electrodes. J. Power Sources 2011, 196 (1), 1-12. 15. Lokhande, C. D.; Dubal, D. P.; Joo, O. S., Metal oxide thin film based supercapacitors. Curr. Appl. Phys. 2011, 11 (3), 255-270. 16. Yu, X. Y.; Yu, L.; Lou, X. W., Metal sulfide hollow nanostructures for electrochemical energy storage. Adv. Energy Mater. 2016, 6 (3), 1501333. 17. Rui, X. H.; Tan, H. T.; Yan, Q. Y., Nanostructured metal sulfides for energy storage. Nanoscale 2014, 6 (17), 9889-9924. 18. Chen, H. C.; Jiang, J. J.; Zhang, L.; Wan, H. Z.; Qi, T.; Xia, D. D., Highly conductive NiCo2S4 urchin-like nanostructures for high-rate pseudocapacitors. Nanoscale 2013, 5 (19), 8879-8883. 19. Peng, S. J.; Li, L. L.; Li, C. C.; Tan, H. T.; Cai, R.; Yu, H.; Mhaisalkar, S.; Srinivasan, M.; Ramakrishna, S.; Yan, Q. Y., In situ growth of NiCo2 S4 nanosheets on graphene for high-performance supercapacitors. Chem. Commun. 2013, 49 (86), 10178-10180. 20. Yang, J.; Yu, C.; Fan, X. M.; Liang, S. X.; Li, S. F.; Huang, H. W.; Ling, Z.; Hao, C.; Qiu, J. 19

S., Electroactive edge site-enriched nickel-cobalt sulfide into graphene frameworks for high-performance asymmetric supercapacitors. Energ Environ. Sci. 2016, 9 (4), 1299-1307. 21. Krishnamoorthy, K.; Veerasubramani, G. K.; Radhakrishnan, S.; Kim, S. J., One pot hydrothermal growth of hierarchical nanostructured Ni3S2 on Ni foam for supercapacitor application. Chem. Eng. J. 2015, 266, 386-386. 22. Huang, L.; Chen, D. C.; Ding, Y.; Feng, S.; Wang, Z. L.; Liu, M. L., Nickel-cobalt hydroxide nanosheets coated on NiCo2O4 nanowires grown on carbon fiber paper for high-performance pseudocapacitors. Nano Lett. 2013, 13 (7), 3135-3139. 23. Chen, H. C.; Jiang, J. J.; Zhang, L.; Xia, D. D.; Zhao, Y. D.; Guo, D. Q.; Qi, T.; Wan, H. Z., 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 2014, 254, 249-257. 24. Xiao, J. W.; Wan, L.; Yang, S. H.; Xiao, F.; Wang, S., Design hierarchical electrodes with highly conductive NiCo2S4 nanotube arrays grown on carbon fiber paper for high-performance pseudocapacitors. Nano Lett. 2014, 14 (2), 831-838. 25. Xia, X. H.; Chao, D. L.; Fan, Z. X.; Guan, C.; Cao, X. H.; Zhang, H.; Fan, H. J., 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. 2014, 14 (3), 1651-1658. 26. Xia, X. H.; Zhang, Y. Q.; Fan, Z. X.; Chao, D. L.; Xiong, Q. Q.; Tu, J. P.; Zhang, H.; Fan, H. J., Novel metal@carbon spheres core-shell arrays by controlled self-assembly of carbon nanospheres: a stable and flexible supercapacitor electrode. Adv. Energy Mater. 2015, 5 (6). 27. Zhou, C.; Zhang, Y. W.; Li, Y. Y.; Liu, J. P., Construction of high-capacitance 3D CoO@polypyrrole nanowire array electrode for aqueous asymmetric supercapacitor. Nano Lett. 2013, 13 (5), 2078-2085. 28. Yang, J.; Ma, M. Z.; Sun, C. C.; Zhang, Y. F.; Huang, W.; Dong, X. C., Hybrid NiCo2S4@MnO2 heterostructures for high-performance supercapacitor electrodes. J. Mater. Chem. A 2015, 3 (3), 1258-1264. 29. Fu, W. B.; Zhao, C. H.; Han, W. H.; Liu, Y.; Zhao, H.; Ma, Y. F.; Xie, E. Q., Cobalt sulfide nanosheets coated on NiCo2S4 nanotube arrays as electrode materials for high-performance supercapacitors. J. Mater. Chem. A 2015, 3 (19), 10492-10497. 30. Wang, K.; Wu, H. P.; Meng, Y. N.; Wei, Z. X., Conducting polymer nanowire arrays for high performance supercapacitors. Small 2014, 10 (1), 14-31. 31. Grover, S.; Goel, S.; Marichi, R. B.; Sahu, V.; Singh, G.; Sharma, R. K., Polyaniline all solid-state pseudocapacitor: role of morphological variations in performance evolution. Electrochim. Acta 2016, 196, 131-139. 32. Xu, W. W.; Zhao, K. N.; Niu, C. J.; Zhang, L.; Cai, Z. Y.; Han, C. H.; He, L.; Shen, T.; Yan, M. Y.; Qu, L. B.; Mai, L. Q., Heterogeneous branched core-shell SnO2-PANI nanorod arrays with mechanical integrity and three dimentional electron transport for lithium batteries. Nano Energy 2014, 8, 196-204. 33. Jabeen, N.; Xia, Q. Y.; Yang, M.; Xia, H., Unique core-shell nanorod arrays with polyaniline deposited into mesoporous NiCo2O4 support for high-performance supercapacitor electrodes. ACS Appl. Mater. Inter. 2016, 8 (9), 6093-6100. 34. Zhu, T.; Zhang, G. X.; Hu, T.; He, Z. N.; Lu, Y. S.; Wang, G. Q.; Guo, H. B.; Luo, J.; Lin, C.; 20

Chen, Y. G., Synthesis of NiCo2 S4-based nanostructured electrodes supported on nickel foams with superior electrochemical performance. J. Mater. Sci. 2016, 51 (4), 1903-1913. 35. Yang, Y.; Wan, M., Chiral nanotubes of polyaniline synthesized by a template-free method. J. Mater. Chem. 2002, 12 (4), 897-901. 36. Huang, F.; Vanhaecke, E.; Chen, D., In situ polymerization and characterizations of polyaniline on MWCNT powders and aligned MWCNT films. Catal. Today 2010, 150 (1-2), 71-76. 37. Cong, H. P.; Ren, X. C.; Wang, P.; Yu, S. H., Flexible graphene-polyaniline composite paper for high-performance supercapacitor. Energ Environ. Sci. 2013, 6 (4), 1185-1191. 38. Dhibar, S.; Bhattacharya, P.; Hatui, G.; Sahoo, S.; Das, C. K., Transition metal-doped polyaniline/single-walled carbon nanotubes nanocomposites: efficient electrode material for high performance supercapacitors. ACS Sustain. Chem. Eng. 2014, 2 (5), 1114-1127. 39. Fan, H. S.; Wang, H.; Zhao, N.; Zhang, X. L.; Xu, J., Hierarchical nanocomposite of polyaniline nanorods grown on the surface of carbon nanotubes for high-performance supercapacitor electrode. J. Mater. Chem. 2012, 22 (6), 2774-2780. 40. Tran, V. C.; Nguyen, V. H.; Nguyen, T. T.; Lee, J. H.; Huynh, D. C.; Shim, J. J., Polyaniline and multi-walled carbon nanotube-intercalated graphene aerogel and its electrochemical properties. Synth. Met. 2016, 215, 150-157. 41. Guo, F. J.; Mi, H. Y.; Zhou, J. P.; Zhao, Z. B.; Qiu, J. S., Hybrid pseudocapacitor materials from polyaniline@multi-walled carbon nanotube with ultrafine nanofiber-assembled network shell. Carbon 2015, 95, 323-329. 42. Shen, L. F.; Wang, J.; Xu, G. Y.; Li, H. S.; Dou, H.; Zhang, X. G., NiCo2S4 nanosheets grown on nitrogen-doped carbon foams as an advanced electrode for supercapacitors. Adv. Energy Mater. 2015, 5 (3), 1400977. 43. Pu, J.; Wang, T. T.; Wang, H. Y.; Tong, Y.; Lu, C. C.; Kong, W.; Wang, Z. H., Direct growth of NiCo2S4 nanotube arrays on nickel foam as high-performance binder-free electrodes for supercapacitors. Chempluschem 2014, 79 (4), 577-583. 44. Allen, M. J.; Tung, V. C.; Kaner, R. B., Honeycomb Carbon: A review of graphene. Chem. Rev. 2010, 110 (1), 132-145. 45. Choi, H. J.; Jung, S. M.; Seo, J. M.; Chang, D. W.; Dai, L. M.; Baek, J. B., Graphene for energy conversion and storage in fuel cells and supercapacitors. Nano Energy 2012, 1 (4), 534-551. 46. Choi, B. G.; Yang, M.; Hong, W. H.; Choi, J. W.; Huh, Y. S., 3D Macroporous graphene frameworks for supercapacitors with high energy and power densities. ACS Nano 2012, 6 (5), 4020-4028.

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Highlights:

* Hierarchical NiCo2S4@PANI/CF composite was fabricated; * The core/shell heterostructure endows composite with abundant electroactive sites; * The free-standing electrode exhibits high capacitance and excellent cycling stability; * The assembled supercapacitor delivers high energy density and excellent flexibility.

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