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Shape-controlled synthesis of NiCo2S4 and their charge storage characteristics in supercapacitors Yufei Zhang,ab Mingze Ma,b Jun Yang,b Chencheng Sun,b Haiquan Su,a Wei Huang*bc and Xiaochen Dong*bc In this work, a facile hydrothermal approach for the shape-controlled synthesis of NiCo2S4 architectures is reported. Four different morphologies, urchin-, tube-, flower-, and cubic-like NiCo2S4 microstructures, have been successfully synthesized by employing various solvents. The obtained precursors and products have been characterized by X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy. It is revealed that the supersaturation of nucleation and crystal growth is determined by the solvent polarity and solubility, which can precisely control the morphology of NiCo2S4 microstructures. The detailed electrochemical performances of the various NiCo2S4
Received 23rd May 2014 Accepted 19th June 2014
microstructures are investigated by cyclic voltammetry and galvanostatic charge–discharge measurements. The results indicate that the tube-like NiCo2S4 exhibits promising capacitive properties with high capacitance and excellent retention. Its specific capacitance can reach 1048 F g1 at the
DOI: 10.1039/c4nr02833c
current density of 3.0 A g1 and 75.9% of its initial capacitance is maintained at the current density of
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10.0 A g1 after 5000 charge–discharge cycles.
1. Introduction With high power density, long cycle lifetime and fast charge– discharge capability, electrochemical supercapacitors show great potential in future energy storage devices and have attracted much attention in the past decades.1–3 Generally, supercapacitors can be classied into two categories: the electrical double-layer capacitors (EDLCs) and the Faradaic redox reaction pseudocapacitors.4 Among the two types, the pseudocapacitors, usually including conductive polymers and metal oxides, can provide much higher specic capacitance and higher energy density than that of double-layer capacitors and attract widespread attention among researchers.5,6 Traditionally, the most notable pseudocapacitive material is RuO2.7,8 However, the high cost and rareness of the Ru element greatly hinders its application on a large scale. Since transition metal oxides possess multiple oxidation states that enable rich redox reactions for pseudocapacitance, they have been investigated as alternative materials for RuO2 in next-generation supercapacitors.9–11 Recently, a few active materials with excellent capacitance have undergone substantial progress, such as
a
School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, Inner Mongolia, China
b
Jiangsu-Singapore Joint Research Center for Organic/Bio-Electronics & Information Displays and Institute of Advanced Materials (IAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China. E-mail:
[email protected];
[email protected]
c Key Laboratory for Organic Electronics & Information Displays (KLOEID), Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China
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Co3O4,12 NiO,13 ZnO (ref. 14) MnO2 (ref. 15) and Mn3O4.16 Especially, binary metal oxides, such as nickel cobaltite (NiCo2O4), have drawn great research interest because of their intriguing advantages as redox supercapacitors. For example, the electronic conductivity of NiCo2O4 is at least two orders of magnitude higher than nickel and cobalt oxides due to its cooperative contributions from both nickel and cobalt ions in the oxide structure.17,18 As a result, binary metal oxide NiCo2O4 possesses multiple oxidation states and high theoretical specic capacitances.19 Motivated by the excellent electrochemical performance of NiCo2O4, some research has been done with nickel cobalt sulphide (NiCo2S4) as an electrode material for supercapacitors. Furthermore, NiCo2S4 achieves richer redox ability than nickel sulphide (NiS) and cobalt sulphide (Co9S8) and exhibits higher conductivity over NiCo2O4.20 However, the morphology that was synthesized in previous research was unitary.20,21 In addition, the inuence of morphology on the electrochemical performance of the electrode has not been investigated in detail. Therefore, to improve the electrochemical performance of NiCo2S4 and investigate the inuence of morphology on the electrochemical performance, shapecontrolled synthesis of NiCo2S4 via a simple route is becoming an increasingly important issue. In this work, we reported a facile hydrothermal approach for the shape-controlled synthesis of NiCo2S4 architecture and studied its electrochemical performance. The NiCo2S4 microstructures with urchin-, tube-, ower-, and cubic-like morphologies were synthesized via hydrothermal method by tailoring the components of the mixed solvent. Electrochemical
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measurements indicated that the tube-like NiCo2S4 microstructures exhibited the best capacitance and good cycling performance among the four samples. The outstanding performance indicates that NiCo2S4 presents promising applications in supercapacitors.
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2.
Experiment
Materials preparation The shape-controlled synthesis of NiCo2S4 involves two-step hydrothermal routes. First, 2.0 mmol CoCl2 and 1.0 mmol NiCl2 were added into 30 ml ethanol (Et) or ethylene glycol (Eg) and deionized (DI) water mixed solvent with vigorous stirring at room temperature to form a transparent pink solution. The morphology of the resulting NiCo2S4 was controlled by the ratio of solvents. For urchin-like NiCo2S4, only 30 ml DI water was used. For the synthesis of ower-, tube-, and cubic-like NiCo2S4, 27 ml Et/3 ml H2O, 12 ml Et/12 ml H2O/6 ml Eg, and 12 ml Et/6 ml H2O/12 ml Eg were used, respectively. Then, 20.0 mmol of urea was dissolved into the as-obtained solution and transferred to a 50 ml Teon-lined stainless steel autoclave. Aer heated 6 h at 180 C, the resulting precursors were ushed with DI water and ethanol for several times and stayed at 80 C overnight. Aer that, 0.2 g precursor and 0.6 g Na2S$9H2O were mixed into 20 ml DI water with vigorous stirring, and the homogeneous solution was transferred into a 25 ml Teon-lined stainless steel autoclave. And then, it was heated to 180 C and maintained for 12 h. Aer cooling to room temperature, the precipitate was collected and washed with DI water and ethanol several times, and dried at 80 C. Characterization The morphologies of the samples were characterized by eld emission scanning electron microscopy (FESEM, Hitachi, S-4800, Japan) and transmission electron microscopy (TEM, Hitachi, HT7700). The structure of the sample were examined by X-ray diffraction (XRD, Bruker D8 Advance) with Cu-Ka ˚ operating at 40 kV, 100 mA. The radiation (l ¼ 1.5418 A) nitrogen adsorption–desorption isotherm was conducted using Brunauer–Emmett–Teller (BET) theory by a Micromeritics ASAP 2020 surface area and porosity analyzer.
charge–discharge tests, and electrochemical impedance spectroscopy (EIS) measurements (1–100 000 Hz).
3.
Results and discussion
Fig. 1 shows the SEM images of the NiCo2S4 precursors synthesized with different solvents. It can be observed that the NiCo2S4 precursors obtained only with DI water present a uniform urchin-like structure with a diameter of about 3 mm (Fig. 1a). The urchin-like structure is made of numerous ordered nanorods growing pointing toward the center of the sphere. Fig. 1b–d depict the structures of the precursors obtained using different solvents. The sample synthesized with 27 ml Et and 3 ml DI water shows a ower-like structure with multiple thin slices (Fig. 1b). Nevertheless, the sample synthesized with Et–H2O–Eg mixed solution (Et–H2O–Eg ¼ 1/2/2) shows uniform scattered nanorods with a diameter of about 70– 100 nm (Fig. 1c). With the reduction of DI water and the increase of Et to Et–H2O–Eg ¼ 2/1/2, the morphology of the NiCo2S4 precursors changed to cubic-like structures with a diameter of around 4–5 mm (Fig. 1d). To investigate the effect of morphology on the structure of the NiCo2S4 precursors, X-ray diffraction (XRD) patterns were obtained and are shown in Fig. 2. It is apparent that the precursors are composites of Co(CO3)0.5OH (JCPDS card no. 48-0083) and CoCO3 (JCPDS card no. 11-0692) owing to the formation of Co2+ or Ni2+ carbonate hydroxide in a hydrothermal reaction under basic conditions.22 When heated at high temperature, the released OH anions and CO32 ions from urea decomposition combine with metal ions. As the atomic radii of Co and Ni are quite similar, the partial substitution of Co ions by Ni ions does not change the crystal structure of Co(CO3)0.5OH and CoCO3 except several lattice parameters. Therefore, Ni composites cannot be directly seen from the XRD patterns. Aer the formation of NiCo2S4, the morphologies were investigated with SEM. Fig. 3a shows the SEM image of NiCo2S4
Electrochemical measurements All the electrochemical measurements were carried out using an electrochemical workstation (CHI 760D, CH instrument Inc, China). A conventional three-electrode system was employed with NiCo2S4 as a working electrode, saturated Ag/AgCl electrode as a reference, and a platinum plate as a counter electrode. 6.0 M KOH was used as electrolyte for all the measurements. The working electrodes were prepared by coating the active NiCo2S4 material, a conductive agent (acetylene black) and polyvinylidene uoride (PVDF) in a weight ratio of 7 : 2 : 1 on the Ni foils. The mass loading of the active material was 4 mg cm2. The electrochemical performance was characterized by cyclic voltammetry (CV), galvanostatic
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Fig. 1 SEM images of the NiCo2S4 precursors with (a) urchin-, (b) flower-, (c) tube-, and (d) cubic-like morphology.
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Fig. 2 XRD patterns of NiCo2S4 precursors with urchin-, flower-, tube-, and cubic-like morphology.
Fig. 3 FESEM images of NiCo2S4 with different morphologies: (a) urchin-, (b) flower-, (c) tube-, and (d) cubic-like NiCo2S4.
synthesized with DI water. It presents a uniform urchin-like structure, which is similar to its precursor except the structures with open ended nanotubes instead of nanorods (the insert of Fig. 3a). These hollow nanotubes may result from the Kirkendall effect of the sacricial template.23–25 When using water–ethanol mixed solution as hydrothermal solvent (Et–H2O ¼ 9/1), the NiCo2S4 presents ower-like morphology, as shown in Fig. 3b. The ower-like structures are composed of symmetrical NiCo2S4 sheets with a diameter of about 4–5 mm. Fig. 3c illustrates the scattered nanotube-shaped NiCo2S4 micro-particles prepared with ethanol, H2O and ethylene glycol mixed solutions (Et– H2O–Eg ¼ 1/2/2). The diameter of the scattered nanotubes present uniform size distribution just as the precursors shown in Fig. 3c. Still owing to the S2 ion exchange reaction, the nanorods are changed to hollow nanotube structures by the Kirkendall effect. The tube-like structures are uniformly scattered as the insert of the Fig. 3c displays and further proves that the tube-like NiCo2S4 are not decomposed from the urchin-like structure. Interestingly, the morphology of the NiCo2S4 microparticles prepared by increasing ethanol and reducing H2O
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(Et–H2O–Eg ¼ 2/1/2) were not changed from the precursors. They also exhibit the cubic-like structure with a diameter of 3– 4 mm (Fig. 3d). This may have resulted from the limited specic area compared to other structured precursors. As shown, the morphology of NiCo2S4 structures is largely determined by the precursors. The NiCo2S4 precursors are distinctively different with the change of solvent volume ratio, suggesting the volume ratio of the solvents with different polarity and saturated vapor pressure plays a crucial role for the formation of different morphologies of the NiCo2S4 precursors. As reported by literature, different solvents affect the morphology by making unique adjustments to the homogenization of the reactants in the reaction medium. Additionally, the amount of individual nucleus formation, the amalgamation, and the direction preference of the growing nucleus are also largely affected by the solvents.26 In this research, the precise control of the ratios of the solvents provides different degrees of supersaturation that directly determines the crystal growth mode of NiCo2S4 precursors.27 Research has proved that the anisotropic low dimensional growth may be preferable at low supersaturation. While using solvents with high polarity and solubility, a higher degree of supersaturation is achieved, and the supersaturation will favor multi-dimensional growth. Thus, both the concentration and supersaturation of the precursor are responsible for the formation of different morphologies.28 Therefore, the shape of the active materials' precursors can be precisely controlled by adjusting the ratio of different solvents. In summary, the morphologies of the nal products were determined by the structures of precursors. With the S2 substitution reaction, the precursors changed to specically structured sulde materials. Transmission electron microscope (TEM) characterization was further carried out to provide more insight into the detailed microstructures of the NiCo2S4 samples. It can be seen from Fig. 4a that for the urchin-like sample, the diameter of the nanotube is about 150 nm with a relatively smooth surface. However, the scattered tube-like NiCo2S4 has a diameter of 70– 100 nm, as shown in Fig. 4b. The difference of diameter between the urchin- and tube-like NiCo2S4 further indicates the scattered tubes are not the same kind as those in the assembled urchin structure. Fig. 4c shows that the ower-like NiCo2S4 particles are made up of very thin NiCo2S4 nanosheets. The crystal phase of NiCo2S4 was examined by XRD with the results shown in Fig. 5. It can be clearly seen that all the welldened diffraction peaks of ower-, cubic-, urchin- and tubelike samples can be perfectly indexed to the cubic phase of NiCo2S4 (JCPDS card no. 20-0782). The peaks at 26.7 , 31.6 , 38.4 , 47.4 , 50.5 , and 55.3 correspond to the respective (220), (311), (400), (511) and (440) planes of NiCo2S4, respectively. The absence of other noticeable diffraction peaks further conrms the phase purity of NiCo2S4. In order to systematically explore the properties of NiCo2S4, the nitrogen adsorption–desorption method was used for the characterization of the inner structure of NiCo2S4. Among the four samples, the tube-like NiCo2S4 achieved the largest BET surface area (around 34.7 m2 g1) (Fig. 5b), which is larger than that previously reported for NiCo2S4.20,21
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Fig. 4 TEM images of (a) urchin-, (b) tube-, and (c) flower-NiCo2S4.
Fig. 5 (a) X-ray diffraction (XRD) patterns of NiCo2S4 with different morphologies. (b) Nitrogen adsorption–desorption isotherms of tube-like NiCo2S4.
Fig. 6 (a) CV curves of different morphologies of NiCo2S4 samples at the scan rate of 3.0 mV s1. (b) Discharge curves of different morphologies of NiCo2S4 samples at the current density of 6.0 A g1.
The electrochemical performances of all the samples were systematically investigated by cyclic voltammograms (CV) and galvanostatic charge–discharge measurement with 3 M KOH as electrolyte. Fig. 6a shows the CV curves of the NiCo2S4 samples recorded at a scan rate of 30 mV s1. It can be clearly observed that there is one distinct pair of redox peaks during the anodic and cathodic sweeps, which may mainly result from the reversible Faradaic reactions within the electrode materials. It is known that the redox reactions in NiCo2O4 can be described in the following equations:29,30 NiCo2O4 + OH + H2O 4 NiOOH + 2CoOOH + 2e
(1)
CoOOH + OH 4 CoO2 + H2O + e
(2)
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As known, sulfur is in the same family as oxygen. As reported in literature, the OH ions also participate in the electrochemical oxidation and reduction of cobalt suldes and nickel suldes.31 Therefore, the redox reaction mechanism of NiCo2S4 in alkaline electrolyte is similar to the redox reactions in NiCo2O4, which form NiSOH, CoSOH, and CoSO during the charge–discharge process.32,33 It is also evident that the NiCo2S4 samples present obvious pseudocapacitive properties. The CV curves in Fig. 6a also indicate that the tube-like NiCo2S4 structure presents the highest electrochemical performance. This phenomenon may be attributed to the scattered tube structure, facilitating the effective reaction of the electrolyte and the active materials. Fig. 6b shows the galvanostatic charge–discharge curves of NiCo2S4 structures at the current density of 6.0 A g1. It should
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also be noted that the discharge curves of the NiCo2S4 electrodes exhibit a typical pseudocapacitive behavior, which is different from that of the linear characteristics of electric double layer capacitors.34 Moreover, the specic capacitance can be calculated by the following formula:35 C ¼ IDt/mDV, where I (A) is the discharge current, Dt (s) is the discharge time, m (g) is the mass of the active material and DV (V) is the voltage
Fig. 7 Nyquist plots of different morphologies of NiCo2S4 samples.
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change for the discharge–charge process. According to the equation, the specic capacitance value of the tube-, urchin-, ower- and cubic-like NiCo2S4 can be calculated from the discharge curves to be 1050 F g1, 795 F g1, 450 F g1, and 195 F g1, respectively, over the voltage of 0–0.4 V, at the current density of 6.0 A g1. It also indicates that the scattered tube-like NiCo2S4 structure possesses better electrochemical performance than other kinds of NiCo2S4 structures, which coincides with the results of CV measurements. Because it is well known that the morphology of the material can greatly affect the performance of the material, the above results can be easily explained. Since the supersaturation for nucleation and crystal growth is determined by the solvent polarity and solubility, employing different components of the mixed solvents can control the morphologies of the nal products. Every morphology can afford different specic areas for active redox reactions, so the differences lead to variation in the electrochemical performance. In addition, the exact explanation for the different electrochemical behavior of the four electrodes may be elaborated as differences in the individual electronic transition of Co and Ni ions or the combined effect of transition states of Co2+/Co3+ and Ni2+/Ni3+, which can be affected by diverse morphologies and crystallinity.36 Fig. 7 shows the Nyquist plots of tube-, urchin-, and cubiclike NiCo2S4 electrodes obtained from an equivalent circuit from 1–100 000 Hz. The electrochemical impedance
Fig. 8 Electrochemical performance of the tube-like NiCo2S4 electrode. (a) CV curves at scan rates of 10, 20, 30, 40, and 50 mV s1. (b) Discharge curves at various current densities. (c) Effects of current density on specific capacitance. (d) Cycling performance of 5000 cycles at the current density of 10.0 A g1. The inset shows the charge–discharge curves at a current density of 10.0 A g1.
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spectroscopy (EIS) data were tted from an equivalent circuit containing a solution resistance Rs, a charge-transfer Rct and a pseudocapacitive component Cp as shown in the inset of Fig. 8. The internal resistance can be valued from the intercepts of the high frequency semicircle with the x axis. The Rct corresponds to Faradaic reactions, and its value can be estimated from the diameter of the semicircle.37 From Fig. 7, it can be observed that the axis intercepts in the high frequency range of the three electrodes are almost the same, indicating the three electrodes present nearly equal solution resistance. However, the semicircles in the high frequency range are signicantly different. The tube-like sample possesses the smallest diameter, indicating that the tube-like NiCo2S4 has the lowest interfacial charge-transfer impedance Rct.38 It is favorable for electrolyte penetration and fast ion/electron transfer.39 Besides, the tubelike NiCo2S4 has the most vertical line in the low frequency range, suggesting the best capacitive behavior and the lowest electrolyte diffusion impedance.40 These results also coincide with CV and discharge results, which conrms that the scattered tube-like NiCo2S4 has the best electrochemical performance among the four NiCo2S4 samples. Fig. 8a shows the CV curves of the scattered tube-like NiCo2S4 at the scan rates from 10 to 50 mV s1. A pair of redox peaks is clearly observed in the CV curves, indicating the capacitance properties are mainly governed by Faradaic redox reactions. Meanwhile, the peak current increases with the scan rate, suggesting the rates of electronic and ionic transport are rapid enough in the applied scan rates.41 Fig. 8b presents the discharge curves of the tube-like NiCo2S4 electrode at different current densities. Impressively, the tube-like NiCo2S4 electrode exhibits excellent capacitance values of 1048, 868, 750, 620, and 525 F g1 at the current density of 3.0, 4.0, 6.0, 8.0, and 10.0 A g1, respectively. The calculated specic capacitance values as a function of the discharge current densities from discharge curves are collected and plotted in Fig. 8c. It can be seen that the specic capacitance decreases as the current density increases. This phenomenon may come from the fact that during the charge–discharge process, the diffusion of OH anions is not fast enough to immediately reach the electrode–electrolyte interface when a higher current density is applied.42 In addition, the cycling stability of the tube-like NiCo2S4 electrode was also investigated by a repeated charge–discharge measurement at a constant current density of 10.0 A g1 (Fig. 8d). It indicates that the specic capacitance decreases at the rst 200 cycles, which may relate to the degradation of the active material in the process of OH insertion (extraction) during oxidation (reduction).43 Then, the NiCo2S4 electrode remains at 75.9% of its initial capacitance even aer 5000 cycles. It can be concluded that the unique hollow structure of the tube-like NiCo2S4 electrode can provide massive electroactive sites and facilitate the transport of the electrolyte. The inset of Fig. 8d is the discharge– charge curves of the tube-like NiCo2S4. It can be observed that the charge–discharge curves remain symmetrical even aer long cycles. Therefore, the tube-like NiCo2S4 electrode not only exhibits large specic capacitance but also excellent cycling stability at high current density.
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4. Conclusions In summary, urchin-, ower-, tube-, and cubic-like NiCo2S4 architectures have been successfully synthesized by a facile shape-controlled hydrothermal route by employing different solvents. The electrochemical measurements indicate that the scattered tube-like NiCo2S4 sample exhibits the best electrochemical performance with large specic capacitance and excellent cycling stability. It suggests that the morphologies of the active material, such as NiCo2S4, can be easily controlled by adjusting the ratio of different solvents during the hydrothermal process. It also demonstrates that shaped-controlled synthesis could be the most promising route for a low-cost production of oxide and sulde materials for high-performance supercapacitors.
Acknowledgements The project was supported by Jiangsu Provincial Founds for Distinguished Young Scholars (BK20130046), Qing Lan Project, the NNSF of China (21275076, 61328401), the Key Project of Chinese Ministry of Education (2012058), Program for New Century Excellent Talents in University (NCET-13-0853), Research Fund for the Doctoral Program of Higher Education of China (20123223110008), Synergetic Innovation Center for Organic Electronics and Information Displays, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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