Ni(OH)2 core-shell heterostructured nanotube arrays on carbon-fabric as high-performance pseudocapacitor electrodes

Accepted Manuscript Title: NiCo2 S4 /Ni(OH)2 core-shell heterostructured nanotube arrays on carbon-fabric as high-performance pseudocapacitor electrodes Author: J. Zhang H. Gao M.Y. Zhang Q. Yang H.X. Chuo PII: DOI: Reference:

S0169-4332(15)01203-9 http://dx.doi.org/doi:10.1016/j.apsusc.2015.05.084 APSUSC 30410

To appear in:

APSUSC

Received date: Revised date: Accepted date:

28-12-2014 13-5-2015 14-5-2015

Please cite this article as: J. Zhang, H. Gao(, M.Y. Zhang, Q. Yang, H.X. Chuo, NiCo2 S4 /Ni(OH)2 core-shell heterostructured nanotube arrays on carbon-fabric as high-performance pseudocapacitor electrodes, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.05.084 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

NiCo2S4/Ni(OH)2 core-shell heterostructured nanotube arrays on carbon-fabric as high-performance pseudocapacitor electrodes 

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J. Zhang, H. Gao , M. Y. Zhang, Q. Yang, H. X. Chuo Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of

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Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, P. R. China



corresponding author：E-mail：[email protected]

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Highlights

1. Carbon fiber/NiCo2S4/Ni(OH)2 core-shell heterostructure arrays are rationally designed.

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2. The heterostructure arrays exhibit a specific capacitance of 2700 F/g at 1 mA/cm2.

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3. There are some advantages for NiCo2S4 NTAs in the heterostructure arrays.

Abstract

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High-performance supercapacitor electrodes with a core-shell heterostructure of carbon-fabric/NiCo2S4/Ni(OH)2 are rationally designed. The NiCo2S4 nanotube core

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synthesized by two step hydrothermal methods has a diameter of about 120 nm and a thickness of about 25 nm. The Ni(OH)2 shell prepared by electrochemical deposition

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method is made up by the small Ni(OH)2 ultrathin sheets. With this design and method, high specific capacitance of 2700 F/g at a current density of 1mA/cm2 is found from

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NiCo2S4/Ni(OH)2 heterostructured arrays. These samples show energy density of 120

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Wh/kg and power density of 0.4 kW/kg at a current density of 1 mA/cm2, and good cycle stability with 78% capacitance retention after 2000 circles.

Keywords: pseudocapacitance, Ni(OH)2, NiCo2S4, nanotube, high electrical

conductivity.

1 Introduction: Various emerging energy storage technologies have been designed to meet the requirements of rapid development of portable electronic devices and electrical vehicles. Safe and powerful energy storage devices are becoming increasingly important, in which batteries and supercapacitors have attracted great attentions.

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Supercapacitors can provide longer charge-discharge time and higher power density than batteries. Supercapacitors can be divided into electrical double-layer capacitors (EDLCs) and pseudocapacitors (PCs), according to the charge storage mechanisms [1]. In particular, PCs, which make use of reversible faradaic reactions that occur at the

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electrode surface, offer much higher specific capacitance than EDLCs [2-5]. Transition metal oxides (RuO2 [6], MnO2 [7], NiOx [8], and CoOx [9]), hydroxides

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(Ni(OH)2 [10], Co(OH)2 [11]) and their compounds are being widely explored for

high-performance PCs because of their low cost, low toxicity, and great flexibility in

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structure and morphology. Among them, Ni(OH)2 is an especially attractive transition hydroxide for electrodes due to its high theoretical specific capacitance, well-defined

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electrochemical redox activity [12-14]. However, such high theoretical capacitance has not been obtained in experiment because of the poor electrical conductivity of

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Ni(OH)2. In addition, when the loading of Ni(OH)2 is high in the electrode, Ni(OH)2 is densely packed and thus has only very limited accessible surface area for

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participating in the electrochemical charge storage process, which remarkably increases the contact resistance and in turn decreases the specific capacitance.

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Therefore, it is still a great challenge to boost the electrochemical utilization of

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Ni(OH)2 materials by rationally designing electrodes with their microstructures. An emerging attractive concept is to directly grow smart integrated array architectures with the combination of two types of materials and nanostructures on conducting substrates as binder-free electrodes for supercapacitors [15-17]. In this way, many competitive advantages such as rich accessible electroactive sites, short ion transport pathways, superior electron collection efficiency, desirable cycle life and excellent rate performance are obtained.

Particularly, NiCo2S4 exhibits a higher electric conductivity than that of Ni(OH)2 materials. In this sense, nanostructured NiCo2S4 is considered to be an excellent skeleton for docking electroactive materials. In addition, the ternary NiCo2S4 can offer rich redox reactions owing to the contributions from both nickel and cobalt ions with different valence states [18]. However, up to now, there is no study on electrochemical capacitance of integrated electrodes combining merits of NiCo2S4 and

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Ni(OH)2, although the individual capacitive property of both has been extensively investigated [19, 20]. In this paper, we develop a simple strategy to design and fabricate NiCo2S4/Ni(OH)2

core-shell

heterostructured

nanotube

arrays

(NTAs)

on

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carbon-fabric (CF) as a binder-free electrode for high-performance supercapacitors,

where the porous NiCo2S4 NTAs are the “core’’ and ultrathin Ni(OH)2 nanosheets are

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the ‘‘shell’’ layer. The thickness of the pseudocapacitive materials Ni(OH)2 has high

specific surface, which can ensure the nearly complete utilization of the

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pseudocapacitive materials for energy storage. In addition, the NiCo2S4 NTAs have excellent conductivity and porous structure which provides convenient channels for

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the fast transport of electrons and electrolyte ions. The electrochemical characterizations of CF/NiCo2S4/Ni(OH)2 show that it can be applied as an excellent

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binder-free supercapacitor electrode material. The excellent electrochemical performances, such as high specific capacitance, high energy density, high power

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2. Experimental

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electrodes are achieved.

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density, and good long-term cycling stability of the NiCo2S4/Ni(OH)2 composite

All of chemical reagents are analytically pure and used without further purification.

2.1.1 Synthesis of CF/NiCo2S4 NTAs

In brief, NiCo2S4 NTAs were synthesized on CF substrates by two step hydrothermal methods in the mixed solution of 0.84 g Co(NO3)2·6H2O, 0.42 g Ni(NO3)2·6H2O, and

0.28 g urea in 100 mL deionized (DI) water at 120℃ in an electric oven for 16 h. The Co-Ni precursor was obtained. NiCo2S4 NTAs were synthesized in a solution of 0.2 g thioacetamide (TAA) in 40 mL DI water together with the Co-Ni precursor at 160℃ for 14 h. After washed with DI water for several times, the NiCo2S4 NTAs were obtained. The mass of NiCo2S4 NTAs was 0.36 mg/cm2.

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2.1.2 Synthesis of CF/NiCo2S4/Ni(OH)2 core-shell NTAs

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Ni(OH)2 nanosheets supported by NiCo2S4 NTA skeletons were grown through a simple electrochemical deposition method. The above as-prepared NiCo2S4 NTA electrode was used as the working electrode, a Pt foil was used as the counter electrode

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and a saturated calomel electrode as the reference electrode. The Ni(OH)2 nanosheet

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shell was deposited by the potential static with -1.0 V for 7 mins in the solution of dissolving 0.02 g Ni(NO3)2·6H2O in 100 mL DI water. The composite electrode was taken off and rinsed with DI water and ethanol several times, then dried in air. The

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2.2. Materials characterization

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mass of Ni(OH)2 nanosheets was 0.4 mg/cm2.

The morphology of the as-synthesized composites is characterized by scanning

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electron microscopy (SEM, Hitachi SU 70). Their structure is determined by x-ray

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diffraction (XRD, RigakuD/max-2600PC) using the Cu Kα radiation (λ=1.5406 Å), transmission electron microscope (TEM, FEI Tecnai F20).

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2.3. Electrochemical performance measurements

The electrochemical properties are measured on a biologic VMP3 electrochemical workstation using a three-electrode configuration in 1 M KOH, where the CF/NiCo2S4/Ni(OH)2 NTW composite electrode serves as working electrode, a Pt foil is used as the counter electrode and a Hg/HgO electrode as the reference electrode in order to precisely control electrochemical potentials. Cycling voltammetry (CV), galvanostatic charge/discharge (GCD), electrochemical impedance spectroscopy (EIS), and galvanostatic cycling are performed. CV curves are tested at different scan rates of 1, 2, 5, 10, and 20 mV/s. GCD was measured at 1, 2, 5, 10 and 20 mA/cm2. EIS is

conducted in the frequency range between 100 kHz and 100 mHz with a perturbation

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amplitude of 5 mV versus the open-circuit potential. Galvanostatic cycling is tested at 10 mA/cm2. All the measurements are carried out at room temperature.

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3. Results and discussion

Fig. 1 shows typical SEM images of the as-synthesized NiCo2S4 NTAs and

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NiCo2S4/Ni(OH)2 core-shell NTAs. Fig. 1a demonstrates a typical SEM image of the NiCo2S4 NTAs, showing that the NiCo2S4 NTAs are grown radially on CFs and dense

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NiCo2S4 NTAs uniformly cover on the CF substrates. The insert of Fig. 1a shows a SEM of pure CFs. Fig. 1b is the high-magnification SEM image of a local area in the Fig. 1a. It can be seen that the NiCo2S4 nanotubes have a typical length of 2~3 μm and

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an average diameter of about 120 nm, which is a rod-like nanostructure with some nanoparticles on it. The insert of Fig. 1b is the SEM of Co-Ni precursor. Fig. 1c and

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1d demonstrate the representative SEM image of the NiCo2S4/Ni(OH)2 NTAs. After

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electrochemical deposition, the NiCo2S4 NTAs are decorated by the Ni(OH)2 films which are made up by the small Ni(OH)2 ultrathin sheets, forming a typical core-shell

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heterostructure (Fig. 1d). The crystal phase of NiCo2S4/Ni(OH)2 NTAs are examined

by XRD shown in Fig. 2a. It can be seen clearly that all the diffraction peaks of samples can be indexed to cubic phase of NiCo2S4 (JCPDS card No. 20-0782). The peaks at 31.6°, 38.4°, 50.5°, and 55.3° correspond to the respective (311), (400), (511) and (440) planes of NiCo2S4, respectively. The mean crystallite size of NiCo2S4 is about 12 nm which is calculated by applying Scherrer formula to the full width at half maximum of the most intensive diffraction peaks. However, the Ni(OH)2 phase is not

obvious in the XRD spectrum, which can be due to there only being a tiny amount possibly. The presence of Ni(OH)2 can be due verified by the HRTEM image. Transmission electron microscope (TEM) provides more insight into the details of the NiCo2S4 and NiCo2S4/Ni(OH)2 nanotubes. Fig. 3a shows a TEM image of a

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single NiCo2S4 nanotube. The NiCo2S4 nanotube is highly porous which enhances the surface area and shortens the electrons and ions diffusion path [21]. Fig. 3b shows the typical HRTEM image of NiCo2S4. The interplanar spacing is calculated to be 0.24 nm and 0.28 nm, respectively, corresponding to the (400) and (311) lattice planes. The Fig.

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3c shows a TEM image of a single NiCo2S4/Ni(OH)2 core-shell nanotube. It is evidently observed that the porous NiCo2S4 nanotube is totally covered by Ni(OH)2

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nanosheets, forming a typical core-shell heterostructure, which is consistent with the

SEM observations. The Ni(OH)2 shell has a thickness of several nanometers. The

corresponding to the (002) planes of Ni(OH)2.

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HRTEM examination shown in Fig. 3d reveals an interplanar spacing of 0.23 nm,

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The pseudocapacitive performances of the CF/NiCo2S4/Ni(OH)2 core-shell composite electrode are investigated to highlight the benefits of the unique architecture

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in a three-electrode configuration with 1 M KOH as the electrolyte. Fig. 4a shows the CV curves of the NiCo2S4/Ni(OH)2 composite electrode supported on CF. A pair of

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redox peaks with an anodic peak at around 0.5 V and a cathodic peak at about 0.25 V when the scan rate is 10 mV/s shows a typical characteristic of faradaic

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pseudocapacitance. With the increase of scan rate, the value of the peak increases,

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demonstrating that the kinetics of interfacial faradic redox reactions and the rate of electronic and ionic transport are rapid enough [22, 23]. The cathodic and anodic peaks shift towards lower and higher potential with the increase of the rate, respectively. The mechanism of electrochemical reactions may be explained by the diffusion of OHions into the composite electrodes. The Eq. (1), (2), (3), and (4) of the major faradaic reactions of composite electrodes may occur during the electrochemical reactions [24, 25]. NiS

+

OH-



NiSOH

+

e-

(1) CoS + OH-  CoSOH + e-

(2)

CoSOH + OH-  CoSO+ H2O + e-

(3)

Ni(OH)2

＋

OH-



NiOOH

＋

H2O

＋

e-

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(4) The NiCo2S4/Ni(OH)2 composite electrodes are further performed by the galvanostatic charge-discharge (GCD) measurements, as shown in Fig. 4b. The GCD curves are highly symmetrical at low current densities, revealing excellent electrochemical

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reversibility. Two separated plateaus in the charge-discharge process resulting from

redox reactions indicate the pseudocapacitive behavior, which is consistent with the

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above mentioned CV results.

In order to demonstrate the electrochemical performance benefits of the

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NiCo2S4/Ni(OH)2 core-shell composite electrodes, CV tests are carried out on the respective NiCo2S4, Ni(OH)2 and NiCo2S4/Ni(OH)2 electrodes at a scan rate of 10

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mV/s, as shown in Fig. 5a. The composite electrode exhibits substantially bigger area of CV curves compared to NiCo2S4 and Ni(OH)2 electrodes, indicating the superior

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electrochemical performances of the composite electrode and the NiCo2S4 NTAs are good skeletons for Ni(OH)2. The advantages of NiCo2S4 nanotube skeletons maybe lie

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in follows: (1) The excellent conductivity promotes the fast transport of electrons and electrolyte ions. (2) A conducting 3D skeleton effectively enhance the specific surface

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area of active Ni(OH)2 sheets. (3) The porous structure provides convenient channels

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for the transport of electrons and electrolyte ions. As shown in Fig. 5b, GCD measurements of NiCo2S4, Ni(OH)2 and NiCo2S4/Ni(OH)2 electrodes are further

performed at a current density of 1 mA/cm2. The results indicate that NiCo2S4/Ni(OH)2 composite electrode exhibits the longest discharge time among these electrodes. The discharge specific capacitance (CS) performance is calculated from the GCD

according to Eq. (5):

Cs 

It mV

(5)

where I is the discharge current, ∆t is the discharge time, m is the loading mass of active material, and ∆V is the potential window during the discharge process. The CS of NiCo2S4/Ni(OH)2 NTA electrode is much higher than that of NiCo2S4 and Ni(OH)2

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electrodes, as shown in Fig. 5c. The NiCo2S4/Ni(OH)2 NTA electrodes exhibit the discharge Cs of 2700 F/g at 1 mA/cm2, and still 1600 F/g at 20 mA/cm2, whereas the Ni(OH)2 electrodes only show the discharge Cs of 2100 and 470 F/g at 1 and 20 mA/cm2, respectively. It is thus clear that the NiCo2S4 NTAs play a vital role in the

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composite electrode. In addition, the specific capacitance value reported here (1900

F/g at 10 mA/cm2) is superior to most other previously reported core/shell

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nanoarchitectures such as Co3O4/NiMoO4 with 1230 F/g at 10 mA/cm2 [26], and

NiCo2O4/CoMoO4 with 1347.3 F/g at 10 mA/cm2 [27]. The energy density (E) and the

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power density (P) are important parameters to characterize the electrochemical performance of electrodes. In this study, these quantities are calculated by Eq. (6) and

P

E t

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C s V 2 2

(6)

(7)

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E

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Eq. (7):

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Fig. 5d shows the Ragone plots for the NiCo2S4, Ni(OH)2 and NiCo2S4/Ni(OH)2

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composite electrodes at different current densities. At a current density of 1 mA/cm2, the calculated energy density is 120 Wh/kg and the power density is 0.4 kW/kg. At a current density of 20 mA/cm2, the calculated energy density is 56 Wh/kg and the power density is 0.7 kW/kg, suggesting that the NiCo2S4/Ni(OH)2 composite electrode

is very promising for next generation high-performance supercapacitors. Cycle stability is another key parameter in relation to the electrochemical

performance of supercapacitors and is investigated at a charge-discharge current density of 10 mA/cm2 in the potential range of 0 to 0.45 V for 2000 repetitive cycles. Fig. 6a shows the capacitance retention of the NiCo2S4, Ni(OH)2 and NiCo2S4/Ni(OH)2 as a function of cycle numbers. For NiCo2S4 NTAs, a high capacitance retention of 94% can be obtained. The Ni(OH)2 electrode exhibits poor cycling life with 60% capacitance retention. The NiCo2S4/Ni(OH)2 composite electrode displays 78%

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capacitance retention after 2000 circles which is superior to the Ni(OH)2. Fig. 6b shows the impedance Nyquist plots of the NiCo2S4, Ni(OH)2 and NiCo2S4/Ni(OH)2 composite electrodes. According to the insert of Nyquist plots, the internal resistances (Rb) at the high-frequency intercept of the real axis are measured to be 0.65, 0.68, and

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0.75 Ω for NiCo2S4, NiCo2S4/Ni(OH)2, and Ni(OH)2, respectively. The slight increase of NiCo2S4/Ni(OH)2 for the internal resistances compared to NiCo2S4 is probably

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attributed to the low electrical conductivity of the Ni(OH)2 [28]. The NiCo2S4/Ni(OH)2

exhibits a slightly smaller semicircle than the Ni(OH)2 at the medium-frequency and a

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higher slope at the low-frequency compared to the Ni(OH)2. Those once again prove that the NiCo2S4 NTAs are the excellent skeletons which enhance the conductivity of

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Ni(OH)2 and thus improve the electrochemical performances of the composite

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electrodes.

Conclusion:

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In summary, NiCo2S4/Ni(OH)2 composite electrodes are successfully synthesized.

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There are some advantages for NiCo2S4 NTAs in such a composite electrode: (1) the excellent conductivity of NiCo2S4 NTAs contributes to improve the electrochemical

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performances of the composite electrodes; (2) the porous NiCo2S4 NTAs serve as the

skeleton to effectively transport electrolytes and shorten ion diffusion path; and (3) they serve as a conducting skeleton to support active Ni(OH)2 materials to effectively

enhance the surface area, which enables fast and reversible redox reaction to improve the specific capacitance. The excellent electrochemical performances, such as high specific capacitance, high energy density, high power density, and good long-term cycling stability of the NiCo2S4/Ni(OH)2 composite electrodes are achieved. These results might contribute to the fundamental researches and technologies of the high-performance energy storage devices.

Acknowledgments

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Project supported by the National Natural Science Foundation of China (No. 5117205 8 and 51402076), The Youth Science Foundation of Heilongjiang Province (QC2014C056).

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Figure captions:

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Fig. 1 (a) SEM of CF/NiCo2S4 NTAs and the insert is SEM image of pure CFs. (b) High-magnification SEM image of CF/NiCo2S4 NTAs and the insert is SEM image of Co-Ni

precursor.

(c)

SEM

(d)

high-magnification

SEM

image of

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CF/NiCo2S4/Ni(OH)2 NTAs.

and

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Fig. 2 XRD of the NiCo2S4/Ni(OH)2 NTA composite electrode.

Fig. 3 (a) TEM and (b) HRTEM image of the NiCo2S4 nanotube. (c) TEM and (d)

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HRTEM image of NiCo2S4/Ni(OH)2 nanotube.

Fig. 4 (a) CV curves of CF/NiCo2S4/Ni(OH)2 composite electrodes at different scan

rates. (b) GCD of different current densities of CF/NiCo2S4/Ni(OH)2 composite

electrodes.

Fig. 5 (a) CV curves of the NiCo2S4, Ni(OH)2 and NiCo2S4/Ni(OH)2 at the scan rate of 10 mV/s. (b) GCD curves of the three electrodes at the current density of 1 mA/cm2. (c) Specific capacitance of the three electrodes. (d) Ragone plots of the three electrodes.

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Fig. 6 (a) Nyquist plots of the NiCo2S4, Ni(OH)2 and NiCo2S4/Ni(OH)2 electrodes and the insert is the details of high frequency. (b) Cycle stabilities of the NiCo2S4,

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Ni(OH)2 and NiCo2S4/Ni(OH)2 electrodes.

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Fig. 1 by J. Zhang, et al.

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Graphical Abstract (for review)

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The SEM of CF/NiCo2S4/Ni(OH)2 core-shell heterostructured arrays on carbon-fabric and the electrochemical properties of the heterostructured arrays.

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Fig. 1 by J. Zhang, et al.

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Fig. 2 by J. Zhang, et al.

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Fig. 3 by J. Zhang, et al.

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Fig. 4 by J. Zhang, et al.

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Fig. 5 by J. Zhang, et al.

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Fig. 6 by J. Zhang, et al.

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