Construction of a Hierarchical NiCo2S4@PPy CoreâShell

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Construction of Hierarchical NiCo2S4@PPy CoreShell Heterostructure Nanotubes Array on Ni Foam for High-Performance Asymmetric Supercapacitor Minglei Yan, Yadong Yao, Jiqiu Wen, Lu Long, Menglai Kong, Guanggao Zhang, Xiaoming Liao, Guangfu Yin, and Zhongbing Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05618 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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ACS Applied Materials & Interfaces

Construction

of

Hierarchical

NiCo2S4@PPy

Core-Shell

Heterostructure

Nanotubes Array on Ni Foam for High-Performance Asymmetric Supercapacitor Minglei Yan,a Yadong Yao,*a Jiqiu Wen,b Lu Long,a Menglai Kong,a Guanggao Zhang,a Xiaoming Liao,a Guangfu Yin,a and Zhongbing Huanga a

College of Materials Science and Engineering, Sichuan University, Chengdu,

Sichuan 610065, china b

Analytical and Test Center, Sichuan University, Chengdu 610065, china

ABSTRACT In this paper, hierarchical NiCo2S4@polypyrrole core-shell heterostructure nanotubes array on Ni foam (NiCo2S4@PPy/NF) was successfully developed as a bind-free electrode for supercapacitors. NiCo2S4@PPy-50/NF obtained under 50 s PPy electrodeposition shows a low charge-transfer resistance (0.31 Ω) and a high area specific capacitance of 9.781 F/cm2 at a current density of 5 mA/cm2, which is two times higher than that of pristine NiCo2S4/NF (4.255 F/cm2). Furthermore, an asymmetric supercapacitor was assembled using NiCo2S4@PPy-50/NF as positive electrode and activated carbon (AC) as negative electrode. The resulting NiCo2S4@PPy-50/NF//AC device exhibits a high energy density of 34.62 Wh/kg at a power density of 120.19 W/kg with good cycling performance (80.64% of the initial capacitance retention at 50 mA/cm2 over 2500 cycles). The superior electrochemical

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performance can be attributed to the combined contribution of both component and unique core-shell heterostructure. The results demonstrate that the NiCo2S4@PPy-50 core-shell heterostructure nanotubes array is promising as electrode material for supercapacitors in energy storage. KEYWORD: NiCo2S4; polypyrrole; NiCo2S4@polypyrrole nanotubes array; coreshell heterostructure; high area specific capacitance; asymmetric supercapacitor 1. INTRODUCTION In recent years, the ever increasing of energy consumption poses a serious challenge to humans. So, exploring the sustainable energy is urgent, such as solar energy, wind energy and nuclear energy. And the renewable energy requires highperformance, low cost and environmentally friendly energy storage. Supercapacitors, also called electrochemical capacitors, have been paid more and more attention due to their high power density, fast charge-discharge process and long lifetime.1 In general, supercapacitors can be divided into two types according to the charge storage mechanism: electrical double-layer capacitors (EDLCs) and pseudocapacitors.2 As for EDLCs, electrical energy is stored by the diffusion and accumulation of electrostatic charge at the interface between electrode and electrolyte. Carbon-based materials, such as activated carbon (AC), graphene and carbon nanotube, are usually used as electrode materials for EDLCs.3 In contrast, pseudocapacitors are principally

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dominated by Faradaic redox reactions, which possess much higher specific capacitance than that of EDLCs.4-5 Transition metal oxides are widely considered as the most potential candidates for pseudocapacitors owing to their low cost, low toxicity, environmental friendliness and multiple oxidation states. However, these transition metal oxides suffer from poor conductivity, which hinders the electron transport, leading to the gradual loss of capacitance.6-7 Therefore, in order to meet the requirements of supercapacitors, it is desirable to explore new electrode materials with high conductivity, enhanced capacitance and excellent chemical stability. Recently, transition metal sulfides such as CoS,8 Co9S8,9 NiS10 and NiCo2S411 have been investigated as novel electrode materials for supercapacitors because of their high electrical conductivity, large capacitance, excellent mechanical and thermal stability. Among these transition metal sulfides, ternary sulfide NiCo2S4 has been regarded as the most potential electrode material, which has become a new hotspot in the field of supercapacitors. The main reasons are as follows: firstly, NiCo2S4 was reported to possess much lower optical band gap energy with about 100 times as high as electric conductivity than that of corresponding metal oxide NiCo2O4;12 secondly, with both nickel and cobalt ions, the ternary sulfide NiCo2S4 can offer more redox reactions than those of single binary sulfides, resulting in better electrochemical performance.13-14 For example, Chen et al. prepared urchin-like NiCo2S4 with a high

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specific capacitance of 1149 F/g at 1 A/g and a capacitance retention of 91.4 % after 5000 cycles at 20 A/g.15 Xiao et al. has synthesized NiCo2S4 nanotube arrays grown on a flexible carbon fiber paper, which showed the highest area capacitance of 2.86 F/cm2 at 4 mA/cm2, good rate capability (still 2.41 F/cm2 at 20 mA/cm2) and excellent cycling stability (4 % loss after 2000 cycles at 10 mA/cm2 ).12 Li and his co-workers reported that Ni-Co sulfide nanowire arrays on Ni foam delivered an ultrahigh specific capacitance of 2415 F/g (6.0 F/cm2) at 2.5 mA/cm2 with a capacitance retention of 78.5 % after 3000 cycles at 15 mA/cm2.16 Mei et al. demonstrated that hierarchical mushroom-like CoNi2S4 arrays on Ni foam possessed a high area specific capacitance of 5.71 F/cm2 at 20 mA/cm2 and still had 4.62 F/cm2 after 3000 cycles at the same current density, which showed its good rate capability.17 In spite of the great progress made on the NiCo2S4 electrodes to improve their electrochemical performance, many previous reports demonstrated that most of the NiCo2S4 electrodes still could not meet the requirement of high capacitance. The structure, composition and morphology of electrode material have a striking effect on its electrochemical performance. It is difficult to achieve an ideal specific capacitance owing to the limitation of redox reaction sites and restricted contact between active materials and electrolyte.18 In this regard, reasonable design of hierarchical hybrid structure is an effective way to improve the electrochemical

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performance and area capacitance. Especially, core-shell heterostructure nanoarrays as novel electrode materials can provide many advantages such as enlarged contact surface area between electrode and electrolyte, a short path for ion diffusion, rich accessible redox reaction sites and synergistic effect resulting from core and shell materials.18-21 Recently, some researchers have successfully synthesized NiCo2S4based core-shell structure materials with superior electrochemical performance. For instance, Yang et.al prepared NiCo2S4@MnO2 heterostructure via a simple hydrothermal route coupled with carbonization treatment, which showed a high specific capacitance of 1337.8 F/g at 2 A/g with capacitance retention of 82 % after 2000 cycles.22 Fu et al. fabricated NiCo2S4@CoSx core-shell nanotubes grown on Ni foam with a high area specific capacitance of 4.74 F/cm2 at 5 mA/cm2 with 76.1 % capacitance retention after 1500 cycles at 50 mA/cm2.23 Niu and his co-workers prepared hierarchical NiCo2S4@Ni3V2O8 structure using a hydrothermal process combined with co-precipitation method and the results showed that as-prepared product possessed a high specific capacitance of 512 C/g at 1 A/g.24 More recently, Li et al. successfully fabricated NiCo2S4@Co(OH)2 nanotubes supported on Ni foam, which exhibited a relatively high area specific capacitance of 9.6 F/cm2 at 2 mA/cm2.18 In another work, Yang et al. reported a novel battery-type electrode of NiCo2S4@Ni(OH)2 core-shell hybrid nanosheet arrays supported on Ni foam for

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supercapacitors. The results demonstrated that NiCo2S4@Ni(OH)2 showed a high area capacity of 680 µAh/cm2 at 5 mA/cm2 with excellent rate performance (94.9% with current density increased to 100 mA/cm2).25 Obviously, the above reports suggest that NiCo2S4-based core-shell structure material is a great potential electrode candidate for supercapacitors. On the basis of the above ideas, we successfully designed and fabricated threedimensional (3D) NiCo2S4@polypyrrole nanotubes array supported on Ni foam (NiCo2S4@PPy/NF) via a facile two-step hydrothermal process coupled with electrodeposition method. Although NiCo2S4 and PPy have been widely studied individually, up to now, there is no research on electrochemical performance of integrated electrodes combining NiCo2S4 with PPy. On the one hand, the NiCo2S4 nanotube core offers high capacitance. On the other hand, the PPy shell not only provides the effective path for electron transport leading to enhanced reaction kinetics between electroactive center and current collector but provides additional pseudocapacitance.26-28 Owing to the unique core-shell structure with more electroactive

sites,

the

NiCo2S4@PPy-50/NF

obtained

under

50s

PPy

electrodeposition exhibits a high area capacitance of 9.781 F/cm2 at a current density of 5 mA/cm2, low charge-transfer resistance (0.31 Ω) and good cycling stability (72.19% capacitance retention after 3000 cycles at 50 mA/cm2 ). Furthermore, an

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asymmetric supercapacitor was assembled with NiCo2S4@PPy/NF as positive electrode and activated carbon (AC) as negative electrode, which shows a high energy density of 34.62 Wh/kg at a power density of 120.19 W/kg and good cycling performance (80.64% of the initial capacitance retention at 50 mA/cm2 over 2500 cycles). The above results imply that NiCo2S4@PPy-50 NTA is a promising electrode material in supercapacitors for practical application. 2. EXPERIMENTAL SECTIONS 2.1 Materials Cobalt chloride hydrate (CoCl2•6H2O), nickel chloride hydrate (NiCl2•6H2O) and urea were purchased from Chengdu Kelong Chemical Regent Co. Ltd (China). AC was bought from Fuzhou Yihuan Carbon Co. Ltd (China). All chemicals were analytical grade and used without further purification. If it is not specified, all solutions were prepared with deionized water. 2.2 Synthesis of NiCo2S4 /NF NiCo2S4/NF was prepared by two-step hydrothermal process.23 Prior to synthesis, a piece of Ni foam (2 cm × 4 cm) was treated with 3M HCl solution in an ultrasonic bath for 10 minutes to remove the oxide layer, and then ultrasonically washed with acetone, deionized water and ethanol for 30 minutes respectively. In a typical procedure, 4 mmol cobalt chloride hydrate (CoCl2•6H2O), 2 mmol nickel chloride

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hydrate (NiCl2•6H2O) and 12 mmol urea were dissolved in 35 ml deionized water and stirred for 15 minutes to form pink homogenous solution. Subsequently, the above solution and the treated Ni foam were transferred into 50 ml Teflon-lined stainlesssteel autoclave and heated 120 ℃ for 6 h. After being cooled to room temperature gradually, the Ni foam with pink product was carefully washed with deionized water and ethanol several times and then dried at 60℃ for 12 h. Finally, the Ni-Co precursor was obtained. In the next step, the Ni-Co precursor was immersed in the 0.2 M sodium sulfide (Na2S•9H2O) solution and then transferred into 50 ml Teflon-lined stainless-steel autoclave and heated 120 ℃ for 14 h. After being cooled to room temperature gradually, the Ni foam with black product was carefully washed with deionized water and ethanol several times and then dried in a vacuum oven at 60 ℃ for 6 h. The mass loading of NiCo2S4 was about 6.87 mg/cm2 through the weight difference before and after hydrothermal process. 2.3 Synthesis of NiCo2S4@PPy/NF An electrochemical workstation (CHI660E) was used to electrodeposit PPy film on the NiCo2S4 NTA. The electrodeposition was carried out on a three-electrode system by potentiostatic deposition.27 0.3355 g pyrrole, 0.4256 g LiClO4 and 1.6298 g sodium dodecyl sulfate (SDS) were dissolved into 50 ml deionized water to form

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electrolyte for electrodeposition of PPy. NiCo2S4 NTA/NF, Ag/AgCl and Pt foil were used as working electrode, reference electrode and counter electrode, respectively. The voltage was set to 0.8 V. In order to compare the effect of electrodeposition time on the performance, a series of NiCo2S4@PPy-n/NF with different electrodeposition time from 10 to 90 s were made (n: electrodeposition time). 2.4 Characterization The morphologies and structures of the samples were characterized by scanning electron microscope (SEM, Hitachi, S-4800) and transmission electron microscopy (TEM, Tecani G2 F20 S-TWIN, USA). The structure and phase of the products were characterized by X-ray diffraction (XRD) patterns on a DX-1000 diffractometer using a Cu Ka source in the 2θ range of 20-80°. The functional groups of the samples were measured by Fourier transform infrared (FTIR, Nicolet 6700) spectroscopy in the range of 400-4000 cm-1. 2.5 Electrochemical Measurements The Ni foams loaded with NiCo2S4 and NiCo2S4@PPy were used as bind-free positive electrodes without further treatment. All the electrochemical measurements of samples were carried out by using a three-electrode system on a CHI660E electrochemical workstation. The as-prepared samples, Hg/HgO and Pt foil were employed as working electrode, reference electrode and counter electrode respectively.

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All the electrochemical tests include cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS). The CV tests were performed in the potential window of 0-0.6V at the scan rate of 5, 10, 20, 30, 40, 50 and 100 mV/s. The GCD tests were conducted in the potential window of 0-0.55V at the current density of 5, 10, 20, 30, 40 and 50 mA/cm2. The EIS tests were collected using a frequency ranging from 100 KHz to 0.01 Hz. Finally, 3 M KOH was used as the electrolyte for all the electrochemical measurements. 2.6 Fabrication of Asymmetric Supercapacitor The asymmetric supercapacitor (ASC) was fabricated using NiCo2S4@PPy50/NF (1 cm × 1 cm) as positive electrode, AC as negative electrode and non-woven fabrics as separator. The negative electrode was prepared by the traditional slurry coating method. Specifically, the activated carbon, acetylene black and polyvinylidene fluoride (PVDF) were mixed in the mass ratio 80:10:10. The PVDF was dissolved in the N-Methylpyrrolidone. Then, the sample and acetylene black were dispersed in the above solution. The mixtures were ground adequately to form slurry. Next the slurry was pasted onto the treated Ni foam (1cm×1cm) and dried at 85 °C for 24 h. The mass loading of AC was determined by the formula S4. 3. Results and discussion 3.1 Morphological and Structural Characterization

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In this work, NiCo2S4@PPy was developed on Ni foam by the following three steps (Figure 1): (1) Ni-Co precursor nanowires array was hydrothermally grown on highly conductive macroporous Ni foam as current collector (step I);29 (2) Hydrothermal treatment of such Ni-Co precursor in the presence of Na2S leads to NiCo2S4 via an anion-exchange reaction (step II); (3) PPy shell was electrochemically deposited on NiCo2S4 (step III). Figure 2A shows the XRD pattern for NiCo2S4/NF. The diffraction peaks located at 31.6°, 38.3°, 50.5° and 55.3° can be indexed to the (311) , (400), (511) and (440) planes of the cubic phase NiCo2S4 (JCPDS 20-0782). The strong peaks at 44.6°, 52.0° and 76.6° can be assigned to Ni foam substrate. SEM observations suggest that Ni foam (Figure S1) was fully covered by cone-like NiCo2S4 nanowire array (Figure 2B and 2C). SEM (Figure 2D) and TEM (Figure 2E) characterizations for such 1D nanostructures separated from Ni foam by ultrasonication provide clear evidence to support the exclusive formation of hollow nanotubes with diameters in the range of 50-150 nm. The wall thickness for the NiCo2S4 nanotube was measured to be about 10 nm. NiCo2S4/NF was directly used as a working electrode for PPy electrodeposition. Figure 3A shows the FTIR spectrum for NiCo2S4@PPy-50. The peak at 1637 cm-1 is associated with the fundamental vibration of C=C bond in pyrrole ring.30 The peak at

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1095 cm-1 is attributed to stretching vibration of C-N bond.31 The peaks at around 2920 cm-1and 2852 cm-1 are assigned to N-H bond present in aromatic amines.32 In addition, peaks at 525 cm-1 and 1020 cm-1 characteristic of Ni-S and Co-S, respectively, are also observed.33-34 The peak at 3431 cm-1 can be attributed to the presence of hydroxyl groups. The peak at 1381 cm-1 is attributed to the presence of absorbed CO2.35 Figure 3B shows the corresponding SEM image, indicating the conformal coating of thin PPy layer on NiCo2S4 nanotubes. Energy-dispersive X-ray (EDX) spectrum (Figure 3C) demonstrates the existence of Ni, Co, S, C and N elements.

SEM

and

corresponding

EDX

elemental

mapping

images

for

NiCo2S4@PPy/NF confirm the uniform distribution of all elements throughout the film. TEM images clearly suggest that NiCo2S4 skeleton is coated by PPy film several nanometers in thickness to form a core-shell structure, as shown in Figure 3E and 3F. It is worthwhile mentioning that PPy thickness can be varied by using different electrodeposition time (Figure S2). XPS measurement was utilized to analyze the surface elemental composition and chemical valence states of different elements in the NiCo2S4@PPy. In order to avoid the effect of Ni foam substrate, NiCo2S4@PPy sample was separated from Ni foam by ultrasonication. The survey spectrum (Figure 4A) shows that NiCo2S4@PPy consists of Ni, Co, S, O, C and N without other impurities, and the O single may come from

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absorbed CO2. Figure 4B-F exhibit the typical fitted Ni 2p, Co 2p, S 2p, C 1s and N 2p of the NiCo2S4@PPy sample. The Ni 2p and Co 2p spectra of XPS can be well fitted with two spin-orbit doublets and two shakeup satellites (marked as “Sat.”). As shown in Figure 4C, the peaks at 855.0 eV and 872.6 eV correspond to Ni 2p3/2 and Ni 2p1/2 respectively. The difference of binding energy between Ni 2p3/2 and Ni 2p1/2 is 17.6 eV, indicating the coexistence of Ni2+ and Ni3+.36 The Figure 4D depicts the Co 2p XPS spectrum. The peaks at 780.8 eV and 795.9 eV are related to Co 2p3/2 and Co 2p1/2 respectively. The difference of binding energy between Co 2p3/2 and Co 2p1/2 is over 15 eV, suggesting the coexistence of Co2+ and Co3+.37-38 The S 2p XPS spectrum is shown in Figure 4E. The binding energies at 163.4 eV and 162.0 eV correspond to S 2p1/2 and S 2p3/2 respectively. According to the XPS analysis, the surface of NiCo2S4@PPy sample contains Ni2+, Ni3+, Co2+, Co3+ and S2- , which is in good agreement with other literature results for NiCo2S4.39-40 Figure 4F is the N 1s XPS spectrum. The intense peaks at 399.2 eV, 397.8 eV and 401.1 eV are assigned to pyrrole nitrogen (-NH-), the imine-like (=N-) structure and positively charged nitrogen (-NH+-) respectively.41 3.2 Electrochemical Performance of NiCo2S4 /NF and NiCo2S4@PPy/NF To explore the potential of NiCo2S4/NF and NiCo2S4@PPy/NF as electrodes for supercapacitors, CV, GCD and EIS measurements of as-synthesized electrodes were

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performed in a three-electrode system with a Pt plate as counter electrode, Hg/HgO as reference electrode and the obtained samples as working electrodes in a 3M KOH aqueous electrolyte. In order to obtain the optimum electrochemical performance of NiCo2S4@PPy electrode, the effects of different electrodeposition times were investigated. Figure 5A shows the CV curves of bare NF, NiCo2S4/NF and NiCo2S4@PPy-n/NF (n = 10, 30, 50, 70 and 90 s) at a scan rate of 30 mV/s with a potential window of 0-0.6 V. The areas of the CV curves represent their capacitances. Obviously, the area of CV curve of NF substrate is far less than those of the others. So, its contribution to overall capacitance of the electrode can be neglected. With the increase of the electrodeposition time, the area of CV curve is greatly augmented until the electrodeposition time is 50 s. When the electrodeposition time continues to extend, the thickness of PPy shell increases, meaning that excess PPy hinders the ion penetration to the NiCo2S4 core.42 It is clear that two pairs of redox peaks are observed in the CV curves of NiCo2S4 and NiCo2S4@PPy. The mechanism of the electrochemical reactions can be attributed to reversible Faradaic redox processes of Co2+/ Co3+/ Co4+and Ni2+/ Ni3+ based on the following equations:43-45 NiCo2S4 +OH－+H2O↔NiSOH+2CoSOH+2e-

（1）

CoSOH+OH－↔CoSO+ H2O+ e-

（2）

The area specific capacitance of the samples associated with CV curves according to

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the Formula S1. Figure 5B shows the area specific capacitance according to CV curves (Figure 5A). It is clear that NiCo2S4@PPy-50 has the highest area specific capacitance at the same scan rate, which is much higher than that of pristine NiCo2S4 electrode. This fact can also be verified by GCD measurement. Figure 5C exhibits GCD curves of the Ni foam, NiCo2S4 and NiCo2S4@PPy (10, 30, 50, 70 and 90 s) at a current density of 5 mA/cm2 with a potential window of 0-0.55 V. It is noticed that the discharge time of Ni foam is far shorter than those of other electrodes, indicating that the contribution of Ni foam to overall capacitance is almost negligible. Meanwhile, the NiCo2S4@PPy-50 electrode shows the longest discharge time at the same current density. The results of the CV and GCD measurements suggest that NiCo2S4@PPy-50 is the optimized electrode due to its excellent electrochemical performance. Figure 6A depicts that the typical CV curves of the optimized NiCo2S4@PPy-50 electrode at different scan rates with a potential range of 0-0.6V. The shapes of all the curves are similar. Remarkably, a couple of redox peaks are observed, implying the pseudocapacitive characteristic of the electrode. As the scan rate increases, the anodic peaks shift towards the higher potential and the cathodic peaks shift towards the lower potential, which is ascribed to the polarization effect of electrode.46 The GCD curves of NiCo2S4@PPy-50 and NiCo2S4 are presented in Figure 6B and Figure 6C respectively. The evident plateau region can be seen during the process, indicating the

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pseudocapacitive behavior of electrodes. It can be noticed that the curves have good symmetry, suggesting the superior reversible redox capacities. Apparently, the discharge time of NiCo2S4@PPy-50 is much longer than that of pristine NiCo2S4. The area specific capacitances of the samples associated with the GCD curves are calculated according to Formula S2. The results are shown in Figure 6D. The area specific capacitances of pristine NiCo2S4 are calculated to be 4.255, 3.939, 3.556, 3.267, 3.069 and 2.618 F/cm2 at current densities of 5, 10, 20, 30, 40 and 50 mA/cm2 respectively. Compared with the pristine NiCo2S4, the NiCo2S4@PPy-50 shows the excellent capacitive performance, as high as 9.781, 8.509, 6.062, 4.887, 4.116 and 3.818 F/cm2 at current densities of 5, 10, 20, 30, 40 and 50 mA/cm2 respectively. Keskinen et al. reported the use of PPy as electrode for supercapacitor with a specific capacitance of 200 F/g.47 Yang et al. synthesized PPy film for the application of supercapacitors with a capacitance of 261 F/g at 25 mV/s.48 The supercapacitors based on pure PPy generally exhibit a capacitance of 200-500 F/g.49-50 Therefore, the PPy shell contributes additional pseudocpapcitance in our present study. The high capacity of the hybrid could be reasonably explained as follows. (1) The macroporous nature for 3D Ni foam and the 1D nanoarray formation of this hybrid facilitate the diffusion of electrolyte ions. (2) The 1D nanowire array leads to the exposure of more electroactive sites for redox reaction. (3) The PPy shell with high conductivity builds

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the electrical conductive pathways for active materials, leading to enhanced conductivity and faster electronic transportation.26,51 (4) The additives-free feature for NiCo2S4@PPy enables a fast electrochemical reaction rate and low interfacial resistance.52 (5) The synergetic effect of NiCo2S4 and PPy also offers a positive effect on the capacitance. To further understand the ion diffusion of the electrodes, EIS measurement was conducted with a frequence range of 100 KHz-0.01 Hz. As shown in Figure 6E. The two impedence spectras are similar. Both of them are composed of one semicirle at high frequence and a linear at low frequence. The intercept with real axis at high frequency provides the value of a series resistance (Rs), which is the sum of the electrolyte resistance, the intrinsic resistance of the active electrode material, and the contact resistance at the interface of the active material and the current collector.53 The depressed semicircle of Nyquist curve corresponds to the charges transfer resistance (Rct), and the straight line in the low frequency region corresponds to a semi diffusion process.54 There are three differences in the two curves. Firstly, the Rs of NiCo2S4 is smaller than that of NiCo2S4@PPy-50. Secondly, the NiCo2S4 exhibits larger diameter of semicircle than that of NiCo2S4@PPy-50. The Rct of NiCo2S4@PPy-50 and NiCo2S4 is calculated as 0.31 Ω and 1.2 Ω respectively. Thirdly, compared with NiCo2S4, the NiCo2S4@PPy-50 has a more ideal straight line

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in the low frequency region, indicating a lower diffusion resistance.55 Cycling stability is an important factor in evaluating the performance of supercapacitors. Figure 6F depicts the cycling stability of the NiCo2S4@PPy-50 electrode over 3000 cycles between 0 and 0.55 V at a high current density of 50 mA/cm2. Although the area specific capacitance gradually decreases with the increase of cycle number, there is still 72.19% retention of the initial capacitance. The electrode exhibits relatively long cycle life. Besides, the morphology, synthesis method and electrochemical performance of as-obtained NiCo2S4@PPy-50 and other electrodes are provided for comparison as shown in Table S1. 3.3 Electrochemical performance of the NiCo2S4//AC and NiCo2S4@PPy-50//AC asymmetric supercapacitor devices To further value the NiCo2S4@PPy-50 electrode for practical application, an asymmetric supercapacitor (ASC) was fabricated using the optimized NiCo2S4@PPy50 as positive electrode and AC as negative electrode in 3 M KOH electrolyte with a piece of non-woven fabric as separator. Meanwhile, the NiCo2S4 //AC was assembled as the same method for comparasion. The schematic illustration of the assembled ASC configuration is shown in Figure 7A. The mass loading of AC is determined by balancing the charges stored in each electrode. The CV and GCD curves of AC are present in Figure S3. The specific capacitance of the NiCo2S4, NiCo2S4@PPy-50 and

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AC are calculated to be 4.255 F/cm2, 9.781 F/cm2 and 206 F/g respectively at the current density of 5 mA/cm2. The specific mass of AC is calculated based on the Formula S4. The mass loadings of AC in NiCo2S4 //AC and NiCo2S4@PPy-50//AC are 11.36 and 26.11 mg respectively. In order to explore the total voltage of the ASC device, the CV curves of NiCo2S4@PPy-50 and AC were measured at a scan rate of 30 mV/s in a three-electrode system. It can be seen that the voltage window of NiCo2S4@PPy-50 and AC is from 0 to 0.6 V and -1 to 0 V respectively. Therefore, it is expected that the voltage window of ASC device can be operated to 1.6 V. Figure 7C shows the CVcurves of the ASC device at different voltage windows, from which we can know that the stable operating voltage of the device can be extended up to 1.6 V as expected. This fact can also be demonstrated by GCD measurement. Figure 7D depicts the GCD curves of the NiCo2S4@PPy-50//AC device with different voltage windows at a curent density of 30 mA/cm2. Apparently, its maximum working voltage is 1.6 V. Figure 7E exhibits the CV curves of NiCo2S4 //AC and NiCo2S4@PPy50//AC device in 3M KOH electrolyte under 1.6 V at a scan rate of 5 mV/s. It is revealed that the NiCo2S4@PPy-50//AC device has a much larger area of CV curve than that of NiCo2S4 //AC, suggesting the NiCo2S4@PPy-50//AC has the much higher capacitance than that of NiCo2S4 //AC. The results of GCD mesaurtments also confirm this point. As shown in Figure 7F, the discharge time of NiCo2S4@PPy-

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50//AC is much longer than that of NiCo2S4 //AC at a current rate of 5 mA/cm2. Figure 8A presents the CV curves of NiCo2S4@PPy-50//AC in a voltage window of 0-1.6 V at different scan rates. It can be found that the CV curves do not display a distinct distortion with the increase of scan rate, indicating the ASC device has a instant current response and good capacitive behavior.56 It is noted that the CV curves shows a quasi-rectangular shape coupled with redox peaks, from which we can see that the whole capacitance of NiCo2S4@PPy-50//AC device is derived from the combined contribution of Faradaic pesudocapacitance and EDLC-type capacitance.57 Figure 8B depicts the GCD curves of the NiCo2S4@PPy-50//AC at the density of 5-50 mA/cm2 in the voltage window of 0-1.6 V, which is used to evaluate the electrochemical performance and calculate the specific capacitance. Both CV and GCD measurements were also conducted to value the electrochemical properties of NiCo2S4 //AC at various scan rates and current densities with a voltage window of 01.6 V, which are shown in Figure S4 A and B. The area specific capacitances and mass specific capacitances of the NiCo2S4@PPy-50//AC and NiCo2S4 //AC are calculated according to the corresponding discharge curves. As presented in Figure 8C, the specific capacitance of NiCo2S4@PPy-50//AC is much higher than that of NiCo2S4 //AC at different current densities. The area specific capacitances of NiCo2S4 //AC are calculated to be 0.985, 0.746, 0.57, 0.495, 0.455 and 0.416 F/cm2, and the

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corresponding mass specific capacitance are 54.03, 40.92, 31.27, 27.15, 24.96 and 22.82 F/g at current densities of 5, 10, 20, 30, 40 and 50 mA/cm2 respectively. Compared with the NiCo2S4 //AC, the NiCo2S4@PPy-50//AC delivers significantly enhanced area specific capacitance, as high as 3.24, 2.51, 2.29, 1.656, 1.46 and 1.288 F/cm2 , and the corresponding mass specific capacitance are 97.37, 75.44, 68.83, 49.75, 43.87 and 38.7 F/g at current densities of 5, 10, 20, 30, 40 and 50 mA/cm2 respectively. As usual, the specific capacitance decreases with the increase of discharge current density. As a crucial parameter to determine the supercapacitor for practical application, a long-term cycle stabiity of the NiCo2S4@PPy-50//AC device was valued by the GCD measurements at a high current density of 50 mA/cm2 for 2500 cycles. As displayed in Figure 8D, the NiCo2S4@PPy-50//AC device is able to maintain a capacitance retention of 80.64 % after 2500 cycles. The energy density (E) and power density (P) are two key factors to estimate the performance of supercapacitors for practical application, which are obtained according to the Formula S5 and Formula S6 respectively. The energy density and power density of NiCo2S4 //AC and NiCo2S4@PPy-50//AC at different current densities are displayed in Figure S5A and Figure S5B respectively. Notably, the energy density decreases with the increase of the curent density, while the power density enhances with the increase of the curent density. It is noting that

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NiCo2S4@PPy-50//AC delivers a maxmium energy density of 34.62 Wh/kg at a current density of 5 mA/cm2, while that of NiCo2S4 //AC has only 19.21 Wh/kg at the same current densty. Figure 8E shows the Ragone plot relating the energy density to the power density of ASC devices. The NiCo2S4@PPy-50//AC exhibits a high energy density of 34.62 Wh/kg at a power density of 120.19 W/kg. This value compares favorably to the behaviors of NiCo2S4 //C (22.8 Wh/kg at 160 W/kg),58 NiCo2S4 (nanoparticals)//AC (28.3 Wh/kg at 245 W/kg),59 AB-NiCo2S4 //AC (24.7 Wh/kg at 428 W/kg),60 NiSrGO//AC (18.7 Wh/kg at 124W/kg),61 NiCo2O4@NiO//AC (31.5 Wh/kg at 215.2W/kg),62 and β-Co(OH)2//AC (9.8 Wh/kg at 154 W/kg)63 while it is lower than that for Ni(OH)2/CNT-AC(50.6 Wh/kg at 95W/kg)64 and NCH//CHP(54 Wh/kg at 272W/kg).65 Furthermore, in order to demonstrate the practical application of our ASC device, two ASC devices in series can easily light five LED indicators (40 mW) after charing only 30 s at a current densty of 10 mA/cm2 (Figure 8F). The results indicate that NiCo2S4@PPy-50//AC device has great potential in practial application. 4. Conclusions In summary, a novel hierarchical NiCo2S4@PPy core-shell heterostructure nanotube arrays on Ni foam were successfully synthesized by two-step hydrothermal process coupled with electrodeposition method. The optimized NiCo2S4@PPy-50

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core-shell electrode exhibits high area specific capacitance of 9.781 F/cm2 at a current density of 5 mA/cm2, low charge-transfer resistance (0.31 Ω) and good long-term cycling stability for 3000 cycles even at a high current density of 50 mA/cm2. An asymmetric supercapacitor based on NiCo2S4@PPy-50/NF as positive electrode and AC as negative electrode has been assembled. The as-fabricated ASC can achieve a high specific capacitance of 97.37 F/g at a current density of 5 mA/cm2 with a stable voltage window of 1.6 V. Fuethermore, it can develier a maximum energy of 34.62 Wh/kg at a power density of 120.19 W/kg and good cycling stability (80.64 % after 2500 cycles). Therefore, the as-prepared ASC device is a promising candidate for high-performance energy storage devices. ASSOCIATED CONTENT Supporting Information Calculation formulas of single electrode and supercapacitor device; SEM images of Ni foam and NiCo2S4@PPy NTA/NF; CV and GCD curves of the AC and NiCo2S4//AC; the energy density and power density of NiCo2S4//AC and NiCo2S4@PPy-50//AC;

Comparison

of

synthesis

method,

morphology

and

electrochemical performance of different electrode materials. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *

Corresponding Author, E-mail: [email protected] (Y. Yao). Tel/fax: +86 28

85413003. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The support of Sichuan Province through a Key Technologies Research and Development Program Sichuan Province (2011gz0110) is acknowledged. REFERENCES (1) Winter, M.; Brodd, R. J. What Are Batteries, Fuel Cells, and Supercapacitors? Chem. Rev. 2004, 104, 4245-4270. (2) Lu, X.; Yu, M.; Wang, G.; Tong, Y.; Li, Y. Flexible Solid-State Supercapacitors: Design, Fabrication and Applications. Energy Environ. Sci. 2014, 7, 2160-2181. (3) Frackowiak, E. Carbon Materials for Supercapacitor Application. Phys. Chem. Chem. Phys. 2007, 9, 1774-1785. (4) Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. (5) Huang, L.; Chen, D.; Ding, Y.; Feng, S.; Wang, Z. L.; Liu, M. Nickel–Cobalt

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Three-Dimensional Nis-Reduced Graphene Oxide Hybrid Nanostructure for High Performance Supercapacitors. RSC Adv. 2015, 5, 23073-23079. (62) Liu, X.; Liu, J.; Sun, X. NiCo2O4@NiO Hybrid Arrays with Improved Electrochemical Performance for Pseudocapacitors. J. Mater. Chem. A 2015, 3, 13900-13905. (63) Senthilkumar, S.; Selvan, R. K. Fabrication and Performance Studies of a CableType Flexible Asymmetric Supercapacitor. Phys. Chem. Chem. Phys. 2014, 16, 15692-15698. (64) Tang, Z.; Tang, C. h.; Gong, H. A High Energy Density Asymmetric Supercapacitor from Nano‐Architectured Ni(OH)2/Carbon Nanotube Electrodes. Adv. Funct. Mater. 2012, 22, 1272-1278. (65) Zhang, L.; Gong, H. A Cheap and Non-Destructive Approach to Increase Coverage/Loading of Hydrophilic Hydroxide on Hydrophobic Carbon for Lightweight and High-Performance Supercapacitors. Sci. Rep .2015, 5, 18108-18118.

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Figure Captions Figure

1

Schematic

diagram

to

illustrate

the

preparation

process

for

NiCo2S4@PPy/NF. Figure 2 (A) XRD pattern for NiCo2S4/NF. (B, C) SEM images for NiCo2S4/NF. (D) NiCo2S4 nanotubes scratched down from Ni foam. (E, F) TEM images of NiCo2S4 nanotubes. Figure 3 (A) FTIR spectrum, (B) SEM image, and (C) EDX spectrum for NiCo2S4@PPy/NF. (D) SEM image and corresponding EDX elemental mapping of Ni, Co, S, C, and N for NiCo2S4@PPy/NF. (E, F) TEM images of NiCo2S4@PPy nanotubes. Figure 4 (A) XPS survey spectrum for NiCo2S4@PPy. XPS spectra in the (B) Ni 2p, (C) Co 2p, (D) S 2p, (E) C 1s and (F) N 1s regions for NiCo2S4@PPy. Figure 5 (A) Comparison of CV curves of bare Ni foam, NiCo2S4 and NiCo2S4@PPy at a scan rate of 30 mV/s. (B) Area specific capacitance corresponding to (A); (C) A comparison of GCD curves of the Ni foam, NiCo2S4 and NiCo2S4@PPy at a current density of 5 mA/cm2; (D) the area specific capacitance corresponding to (C). Figure 6 (A) CV curves of NiCo2S4@PPy-50 at different scan rates; (B) GCD curves of NiCo2S4@PPy-50 at different current densities; (C) GCD curves of pristine NiCo2S4 at different current densities; (D) Area specific capacitances of NiCo2S4 and

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NiCo2S4@PPy-50 at different current densities; (E) EIS spectra of NiCo2S4 and NiCo2S4@PPy-50; (F) Cycling performance of NiCo2S4@PPy-50 at a current density of 50 mA/cm2. Figure 7 (A) Schematic illustration of the assembled asymmetric supercapacitor configuration; (B) CV curves of NiCo2S4@PPy-50 and AC electrodes at a scan rate of 30 mV/s; (C) CV curves of NiCo2S4@PPy-50//AC asymmetric supercapacitor collected in different voltage windows at a scan rate of 30 mV/s; (D) GCD curves of NiCo2S4@PPy-50//AC asymmetric supercapacitor collected in different voltage windows at a current density of 30 mA/cm2; (E) CV curves of NiCo2S4@PPy-50//AC and NiCo2S4//AC at a scan rate of 5 mV/s; (F) GCD curves of NiCo2S4@PPy-50//AC and NiCo2S4//AC at a current density of 5 mA/cm2. Figure 8 (A) CV curves of NiCo2S4@PPy-50//AC at different scan rates; (B) GCD curves of NiCo2S4@PPy-50//AC at different current densities; (C) Area and mass specific capacitances of NiCo2S4//AC and NiCo2S4@PPy-50//AC at different current densities; (D) Cycling performance of NiCo2S4@PPy-50//AC at a current density of 50 mA/cm2; (E) Ragone plots of energy density and power density of NiCo2S4//AC. (F) A photograph of five red LED lighted up by two supercapacitors in series.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Table of Contents Graphic 230x168mm (150 x 150 DPI)


Construction of a Hierarchical NiCo2S4@PPy CoreâShell

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