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Ni–Co layered double hydroxide nanocomposite prepared by a vapor-phase hydrothermal method for electrochemical capacitor application
Journal of Alloys and Compounds 705 (2017) 349e355
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
NiCo2S4/NieCo layered double hydroxide nanocomposite prepared by a vapor-phase hydrothermal method for electrochemical capacitor application Le Luo a, *, Bowen He b, Wei Kong b, Zhenghua Wang b a
Department of Chemical Engineering, Wanjiang College of Anhui Normal University, Wuhu 241008, PR China Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2016 Received in revised form 30 January 2017 Accepted 18 February 2017 Available online 20 February 2017
In this work, NiCo2S4/NieCo layered double hydroxide (LDH) nanocomposites were prepared by using Ni eCo LDH nanosheets as precursor through a vapor-phase hydrothermal method. The NiCo2S4/NieCo LDH nanocomposites have a hierarchical structure in which interconnected NiCo2S4 nanosheets with smaller sizes are loaded on NieCo LDH nanosheets. Electrochemical performances of the NiCo2S4/NieCo LDH nanocomposites were measured by cyclic voltammetry and galvanostatic chargeedischarge techniques. The NiCo2S4/NieCo LDH electrode achieved a capacity of 1765 F g1 at 1 A g1 and a capacity of 940 F g1 at 10 A g1. Control experiments showed that the capacities of the NiCo2S4/NieCo LDH are higher than NieCo LDH and NiCo2S4 nanosheets. The better electrochemical performances of the NiCo2S4/NieCo LDH electrode can be attributed to the unique hierarchical structure which was formed during the vapor-phase hydrothermal process. © 2017 Elsevier B.V. All rights reserved.
Keywords: NiCo2S4 Hierarchical structure Vapor-phase hydrothermal synthesis Supercapacitor
1. Introduction The development of clean and renewable energies is regarded as an important way to solve the problems of energy scarcity and environmental pollution. However, some of the clean and renewable energies such as solar, wind and tide energies are unstable and intermittent, and superior energy storage devices are usually needed for efficient utilization of these energies. Supercapacitors as a new emerging energy storage device have aroused intensive attentions due to their merits such as high power density, ultrafast chargingedischarging rate, wide operation temperature range and long cycle life [1e6]. Taking the chargeestorage mechanism into account, supercapacitors are commonly classified into two kinds, the one is electrical double layer capacitors (EDLCs) and the other is pseudocapacitors. For the EDLCs, charges are stored electrostatically at the interface of the electrode and electrolyte and limited specific capacitances can be realized [7,8]. On the other hand, pseudocapacitors store charges through fast and reversible Faradaic redox reactions [9,10]. In recent years, battery-type Faradaic
* Corresponding author. E-mail address: [email protected] (L. Luo). http://dx.doi.org/10.1016/j.jallcom.2017.02.188 0925-8388/© 2017 Elsevier B.V. All rights reserved.
electrode materials such as the transition metal oxides, hydroxides and sulfides have been extensively studied for electrochemical energy storage [11e18]. The battery-type materials are very similar to pseudocapacitive materials in terms of the chargeestorage mechanism, and are considered as promising electrode materials for high-performance hybrid supercapacitors [19e21]. Recently, spinel NiCo2S4 has attracted much interest as an active electrode material for electrochemical energy storage due to its rich redox chemistry and excellent conductivity. Many NiCo2S4 nanostructures with high capability have been reported, such as the 3D porous nanonetworks [22], hollow hexagonal nanoplates [23], nanotube arrays [24], nanosheets [25], flaky arrays [26], mesoporous nanoparticles [27] and hollow nanoboxes [28]. Besides the above mentioned single-component nanostructures, some NiCo2S4 nanocomposites such as the 3D hierarchical NiCo2S4@MnO2 coreshell nanosheet arrays [29], hierarchical NiCo2S4@NiMoO4 core/ shell nanospheres [30], NiCo2S4@Co(OH)2 core-shell nanotube arrays [31], 3D hierarchical mesoporous NiCo2S4@Ni(OH)2 core-shell nanosheet arrays [32] and hierarchically constructed NiCo2S4@Ni1exCox(OH)2 core/shell nanoarrays [33] have also been reported for electrochemical energy storage applications. In this work, we report the fabrication of NiCo2S4/NieCo layered double hydroxide (LDH) nanocomposite and its application for
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2. Experimental section 2.1. Chemicals Cobalt chloride hexahydrate (Co(NO3)2$6H2O), nickel chloride hexahydrate (Ni(NO3)2$6H2O), hexamethylenetetramine (HMT, C6H12N4), thioacetamide (TAA, CH3CSNH2) and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co. with analytical grade and used without further purification. 2.2. Synthesis of NieCo LDH on nickel foam
Fig. 1. (a) XRD patterns of the NieCo LDH sample, (b) XRD pattern of the NiCo2S4/ NieCo LDH sample.
electrochemical energy storage. The NiCo2S4/NieCo LDH nanocomposite was synthesized by using NieCo LDH as precursor through a vapor-phase hydrothermal method. SEM observations show that ultrathin NiCo2S4 nanosheets with smaller sizes are grown on NieCo LDH nanosheets, such a hierarchical structure can avoid the aggregation of the nanosheets and therefore improve the electrochemical activities of the materials. The NiCo2S4/NieCo LDH was directly grown on nickel foam, which can be applied as a binder-free electrode without further treatment. The assynthesized NiCo2S4/NieCo LDH electrode shows higher capability than the NieCo LDH nanosheets and NiCo2S4 nanosheets.
In a typical procedure, Co(NO3)2$6H2O (1.1645 g, 4 mmol), Ni(NO3)2$6H2O (0.581 g, 2 mmol) and HMT (1.12 g, 8 mmol) were added to 50 mL deionized water under stirring. After the chemicals were completely dissolved, the solution was transferred into a 60 mL Teflon-lined stainless-steel autoclave. A piece of nickel foam (2 cm 2 cm) which has been washed with acetone, ethanol and deionized water was immersed into the above solution. The autoclave was sealed and heated at 100 C for 10 h. After that, the NieCo LDH loaded nickel foam was repeatedly washed with deionized water for further use. 2.3. Synthesis of NiCo2S4/NieCo LDH on nickel foam Firstly, thioacetamide (0.038 g, 5 mmol) was dissolved in 30 mL distilled water. Then the solution was transferred into a 60 mL Teflon-lined stainless-steel autoclave. The NieCo LDH loaded nickel foam was placed on a Teflon support above the thioacetamide solution. The autoclave was sealed and heated at 107 C for 10 h. After that, the nickel foam was repeatedly washed with deionized water, and dried under vacuum at 50 C for 2 h. The mass loading of NiCo2S4/NieCo LDH on nickel foam is about 3.8 mg cm2.
Fig. 2. XPS spectra of (a) Ni 2p, (b) Co 2p, (c) S 2p and (d) O 1s.
L. Luo et al. / Journal of Alloys and Compounds 705 (2017) 349e355
2.4. Material characterizations The crystallographic information of the samples was measured by X-ray powder diffraction (XRD, Bruker D8 Advance). Elemental composition was analyzed by X-ray photoelectron spectroscopies (XPS, ESCALab MKII). The morphologies of the samples were observed by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, FEI Tecnai G2 20). 2.5. Electrochemical measurements Electrochemical properties of the electrodes were measured with a three-electrode cell. The active materials loaded nickel foam (1.0 cm 1.0 cm) acted as the working electrode, a platinum plate served as the counter electrode and an Hg/HgO electrode as the reference electrodes. KOH aqueous solution with a concentration of 3 mol L1 served as the electrolyte. 3. Results and discussion The NiCo2S4/NieCo LDH nanocomposite was prepared in twosteps. In the first step, the hydrolysis of HMT released many OH
351
which act as precipitant for the production of NieCo LDH. In the second step, the hydrolysis of TAA released H2S, and the reaction between NieCo LDH and H2S in the vapor phase result in the formation of NiCo2S4 on the surface of NieCo LDH. XRD technique was applied to analyze the crystallographic information of the asobtained samples. In order to avoid the affection of nickel foam, samples were separated from nickel foam by ultrasonication. Fig. 1a shows a typical XRD pattern of the NieCo LDH, the diffraction peaks can be indexed as a mixture of rhombohedral phase a-Ni(OH)2 (JCPDS No. 38-715) and a-Co(OH)2 [34]. XRD pattern of the NiCo2S4/ NieCo LDH sample is shown in Fig. 1b, in addition to the diffraction peaks from NieCo LDH, three weak peaks at 31.1, 38.0 and 54.9 can be indexed to cubic phase NiCo2S4 (JCPDS no. 43-1477). The weak diffraction peaks of NiCo2S4 indicate the low content of NiCo2S4 in NiCo2S4/NieCo LDH composite. The surface elements of NiCo2S4/NieCo LDH sample are analyzed by XPS. Fig. S1 shows a survey XPS spectrum of the NiCo2S4/NieCo LDH sample. The peaks at 163, 531, 781 and 855 eV correspond to S 2p, O 1s, Co 2p and Ni 2p, respectively, which indicates the presence of S, O, Co and Ni elements in the NiCo2S4/ NieCo LDH sample. The high-resolution XPS spectra of each element are shown in Fig. 2. The XPS peaks in these spectra were computer fitted. Fig. 2a shows a XPS spectrum of Co 2p region, the
Fig. 3. (a,b) SEM and (c) TEM images of NieCo LDH nanosheets; (d) SEM, (e) TEM and (f) HRTEM images of NiCo2S4/NieCo LDH nanocomposites.
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strong peaks at 781.2 and 796.4 eV are attributed to Co 2p3/2 and Co 2p1/2, while the weak and broad peaks at 784.3 and 803.6 eV are attributed to the satellite peaks [35]. Both the Co 2p3/2 and Co 2p1/2 peaks can be fitted into two peaks corresponding to Co3þ and Co2þ, respectively, indicating the coexistence of Co3þ and Co2þ in the sample. Fig. 2b shows the XPS spectrum of Ni 2p, the Ni 2p3/2 at 855.6 eV, Ni 2p1/2 at 873.1 eV and satellite peaks at 861.1 and 879.2 eV can be clearly seen. Similar to Co element, the Ni 2p3/2 and Ni 2p1/2 peaks can also be fitted into peaks corresponding to Ni3þ and Ni2þ, suggesting the coexistence of both Ni2þ and Ni3þ [35]. The atomic ratio of Ni and Co is 1:1.72, which is close to the ratio in NiCo2S4. Fig. 2c shows the O 1s XPS spectrum at 531.2 eV which can be fitted into two peaks. The fitted peak at 531.1 eV can be attributed to hydroxyl ions and the peak at 532.0 eV can be attributed to metal oxide [36]. XPS spectrum of S 2p is shown in Fig. 2d, the peaks at 162.0, 163.5 and 168.0 eV can be assigned to S 2p3/2, S 2p1/2 and the satellite peaks, respectively, consistent with the previous report [35]. The morphology of the samples is observed by SEM and TEM. Fig. 3a displays a low-magnification SEM image of the NieCo LDH loaded nickel foam, suggesting that the surface of nickel foam is uniformly covered with the as-synthesized sample. The corresponding high-resolution SEM image shown in Fig. 3b indicates that the NieCo LDH sample is composed of many thin nanosheets and these nanosheets are interconnected to each other. Fig. 3c shows a TEM image of the NieCo LDH nanosheets in which the nanosheets seem transparent under the illumination of electron beams, indicating their ultrathin nature. SEM image of NiCo2S4/ NieCo LDH sample is shown in Fig. 3d. It is obvious that many smaller nanosheets are present. Through the comparison with the NieCo LDH nanosheets shown in Fig. 3b, it can be concluded that these nanosheets are grown on the basis of the NieCo LDH nanosheets. TEM image of the NiCo2S4/NieCo LDH sample (Fig. 3e) shows some small nanosheets on larger nanosheets. HRTEM image
(Fig. 3f) taken from the smaller nanosheets shows lattice fringes of 0.54 nm which matches with the (111) lattice planes of NiCo2S4. The formation of NiCo2S4/NieCo LDH nanocomposite is according to the following reactions: C6H12N4 þ H2O / HCHO þ NH3
(1)
NH3 þ H2O 4 NHþ 4 þ OH
(2)
Co2þ þ Ni2þ þ OH / Ni(OH)2$Co(OH)2
(3)
CH3CSNH2 þ H2O / CH3CONH2 þ H2S
(4)
H2S þ Ni(OH)2$Co(OH)2 / NiCo2S4 þ H2O
(5)
The reaction between NieCo LDH and H2S occurred in vapor atmosphere. The vapor atmosphere is crucial for the formation of the hierarchical NiCo2S4/NieCo LDH nanocomposite. Control experiment shows that when NieCo LDH was immersed in thioacetamide solution at the same reacting temperature, porous NiCo2S4 nanosheets instead of NiCo2S4/NieCo LDH nanocomposite can be obtained within 2 h, as shown in Fig. S2. This result indicates that the reaction in vapor phase is much slower than in solution. The reason can be attributed to the slower feeding of reacting materials and the slower diffusion speed in vapor phase. The unique reacting condition leads to the formation of the hierarchical structure. The electrochemical performances of the NiCo2S4/NieCo LDH as a battery-type electrode were measured in a three-electrode cell by using a Pt plate counter electrode and a Hg/HgO reference electrode in 3 mol L1 KOH electrolyte. Fig. 4a shows the representative cyclic voltammogram (CV) curves of the NiCo2S4/NieCo LDH electrode in a potential window of 0e0.5 V at scan rates ranging from 10 to 100 mV s1. At low scan rates, a pair of redox peaks can be seen in
Fig. 4. (a) CV curves of the NiCo2S4/NieCo LDH at different scan rates; (b) galvanostatic chargeedischarge curves of the NiCo2S4/NieCo LDH at different current densities; (c) specific capacitance of the NiCo2S4/NieCo LDH electrode vs. current densities; (d) comparative galvanostatic chargeedischarge curves at a current density of 2 A g1.
L. Luo et al. / Journal of Alloys and Compounds 705 (2017) 349e355
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potential window of 0e0.5 V at current densities of 1e10 A g1. Fig. 4b shows a series of chargeedischarge curves at various current densities. Different from the triangular curves of EDLCs, here the curves show obvious platforms during the charging and discharging process, which reveals the occurring of Faradaic redox reactions. The capabilities of the electrodes are calculated according to the following equations [39]:
Cs ¼
I Dt m DV
(6)
Ca ¼
I Dt S DV
(7)
In the equation the Cs (F g1) is the specific capacitance, Ca (mF cm ) is the areal capacitance, I (A) is the discharge current, Dt (s) is the discharge time, m (g) is the active mass, S (cm2) is the electrode geometrical area and DV (V) is the potential window. The specific capacitance of the NiCo2S4/NieCo LDH electrode vs. current densities is shown in Fig. 4c. The NiCo2S4/NieCo LDH electrode exhibits a high specific capacitance of 1765 F g1 (6707 mF cm2) at a current density of 1 A g1 (3.8 mA cm2). With the increase of current densities, the specific capacitances decrease. This is a common phenomenon observed in supercapacitors. At lower current densities, the electrolyte ions can fully diffused into the active materials, which lead to higher specific capacitances; whereas at higher current densities, the electrolyte ions can only absorbed on the outer layer or insufficient portion of the active materials, which result in lower specific capacitances. At a current density of 10 A g1 (38 mA cm2), the specific capacitance of the NiCo2S4/NieCo LDH electrode is 940 F g1 (3572 mF cm2), which is 53.3% of that at 1 A g1. The specific capacitances of the NiCo2S4/NieCo LDH, the precursor NieCo LDH and the porous NiCo2S4 nanosheets which were obtained by treating the precursor in thioacetamide solution were compared. Fig. 5d displays galvanostatic chargeedischarge curves of the three samples tested at a current density of 2 A g1, it is clear that the discharge time of NiCo2S4/NieCo LDH is the longest. Specific capacitances calculated from these discharge curves are 1546, 1239 and 1275 F g1 for the NiCo2S4/NieCo LDH, NieCo LDH and NiCo2S4 electrodes. The NiCo2S4/NieCo LDH shows the best capacitive performances. Furthermore, the specific capacitances of the NiCo2S4/NieCo LDH electrode are also comparable or better than those of previously reported NiCo2S4 electrodes, as shown in Table 1 [22e26]. Cycling stability of the NiCo2S4/NieCo LDH electrode was tested at a current density of 5 A g1 over 2000 successive cycles, and the capacitance retention vs. cycles are shown in Fig. 5a. At the initial 500 cycles, there is an obvious deterioration of the capacitance. After that, the decay tendency of capacitance turns weak. The capacitance retention is 56% after 2000 cycles. In previous reports, the cycling stability of transition metal hydroxides is not so good [40], and the hierarchical structure does not helpful for improve the cycling stability. Electrochemical impedance spectra (EIS) were measured in a frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV. The EIS of the NiCo2S4/NieCo LDH, NieCo LDH and NiCo2S4 electrodes are shown in Fig. 5b. In the high frequency region there exists a semicircle which reflects charge-transfer resistance in the electrodeeelectrolyte interface. The little radius of the semicircle reflects the lower charge-transfer resistance [41]. As learn from the spectra, the charge-transfer resistance of the NiCo2S4 electrode is negligible, and the charge-transfer resistance of the NiCo2S4/NieCo LDH electrode is a little lower than that of the NieCo LDH electrode. The sloping straight lines in the low frequency region reflect the Warburg impedance, and the slop of the lines reveals the diffusion 2
Fig. 5. (a) Cycling performance of the NiCo2S4/NieCo LDH electrode; (b) EIS of the NiCo2S4/NieCo LDH, NieCo LDH and NiCo2S4 electrodes.
the CV curves, which reflects the Faradaic behavior of battery-type electrodes. The separation of the oxidation and reduction peaks increases with the increase of scan rates, which may ascribed to the polarization effect [37,38]. Fig. S3 shows CV curves of the NiCo2S4/ NieCo LDH electrode and pure Ni foam at a scan rate of 100 mV s1. The CV curve of the Ni foam is a straight line, revealing that the capacitive performance contributed from Ni foam is negligible. Galvanostatic chargeedischarge tests were carried out in a
Table 1 Comparison of the NiCo2S4/NieCo LDH electrode with references. Electrode material
Mass loading (mg cm2)
Current density (A g1)
Specific capacitance (F g1)
Ref.
3D porous NiCo2S4 nanonetworks Hollow hexagonal NiCo2S4 nanoplates NiCo2S4 nanotube arrays NiCo2S4 nanosheets Mesoporous NiCo2S4 nanoparticles NiCo2S4/NieCo LDH
2.0
1
1501.2
[22]
2.4
1
437
[23]
4.2
4
738
[24]
4.76 4.28
2 2
1231 972
[25] [27]
3.8
1
1765
This work
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rates of electrolyte ions in the active materials [42]. The slope of the three lines is similar, demonstrating similar electrolyte ion diffusion rates in the three electrode materials. The higher specific capacitances of the NiCo2S4/NieCo LDH electrode than the NieCo LDH and NiCo2S4 electrodes can be ascribed to the following aspects. First, the hierarchical structure avoids the possible aggregation of the thin nanosheets, therefore more active sites for ion adsorption and Faradaic redox reactions are provided. Second, the NiCo2S4 nanosheets possess good conductivity and rich redox chemistry. All in all, the vapor-phase hydrothermal approach provide a different way for fabricate nanocomposite for supercapacitor applications.
[12]
[13]
[14]
[15]
[16]
4. Conclusions In summary, hierarchical NiCo2S4/NieCo LDH nanocomposites were synthesized by a vapor-phase hydrothermal method. The unique reacting condition leads to the formation of the hierarchical structure. The NiCo2S4/NieCo LDH nanocomposites were directly loaded on nickel foam, and can be applied for supercapacitor application without further treatment. Electrochemical measurements show that the hierarchical NiCo2S4/NieCo LDH has a high specific capacitance, and the capacitive performances of the hierarchical NiCo2S4/NieCo LDH are better than the NieCo LDH nanosheets and the porous NiCo2S4 nanosheets. The vapor-phase hydrothermal method may provide a feasible way for construct nanocomposites for supercapacitor applications. Acknowledgment Financial support from the National Natural Science Foundation of China (NSFC no. 21671007) and the foundation from Wanjiang college, Anhui Normal University (WJKY-201517) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.02.188.
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[21] [22]
[23]
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[28]
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He, NiCo2S4@Co(OH)2 core-shell nanotube arrays in situ grown on Ni foam for high performances asymmetric supercapacitors, J. Power Sources 312 (2016) 156e164. V.H. Nguyen, C. Lamiel, J.J. Shim, 3D hierarchical mesoporous NiCo2S4@ Ni(OH)2 core-shell nanosheet arrays for high performance supercapacitors, New J. Chem. 40 (2016) 4810e4817. W.W. Zhou, K. Yu, D. Wang, J. Chu, J.Y. Li, L.M. Zhao, C.Y. Ding, Y. Du, X.T. Jia, H.T. Wang, G.W. Wen, Hierarchically constructed NiCo2S4@Ni(1-x)Cox(OH)2 core/shell nanoarrays and their application in energy storage, Nanotechnology 27 (2016) 235402. Z.A. Hu, Y.L. Xie, Y.X. Wang, H.Y. Wu, Y.Y. Yang, Z.Y. Zhang, Synthesis and electrochemical characterization of mesoporous CoxNi1x layered double hydroxides as electrode materials for supercapacitors, Electrochim. Acta 54 (2009) 2737e2741. W. Kong, C.C. Lu, W. Zhang, J. Pub, Z.H. Wang, Homogeneous core-shell NiCo2S4 nanostructures supported on nickel foam for supercapacitors, J. Mater. Chem. A 3 (2015) 12452e12460. J.C. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Systematic XPS studies of metal oxides, hydroxides and peroxides, Phys. Chem. Chem. Phys. 2 (2000) 1319e1324. X. Cong, C. Cheng, Y.M. Liao, Y.F. Ye, C. Dong, H. Sun, X. Ji, W.Q. Zhang, P.L. Fang, L. Miao, J.J. Jiang, Intrinsic charge storage capability of transition metal dichalcogenides as pseudocapacitor electrodes, J. Phys. Chem. C 119 (2015) 20864e20870. C. Wu, J.J. Cai, Q.B. Zhang, X. Zhou, Y. Zhu, P.K. Shen, K.L. Zhang, Hierarchical
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
NiCo2S4/NieCo layered double hydroxide nanocomposite prepared by a vapor-phase hydrothermal method for electrochemical capacitor application Le Luo a, *, Bowen He b, Wei Kong b, Zhenghua Wang b a
Department of Chemical Engineering, Wanjiang College of Anhui Normal University, Wuhu 241008, PR China Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2016 Received in revised form 30 January 2017 Accepted 18 February 2017 Available online 20 February 2017
In this work, NiCo2S4/NieCo layered double hydroxide (LDH) nanocomposites were prepared by using Ni eCo LDH nanosheets as precursor through a vapor-phase hydrothermal method. The NiCo2S4/NieCo LDH nanocomposites have a hierarchical structure in which interconnected NiCo2S4 nanosheets with smaller sizes are loaded on NieCo LDH nanosheets. Electrochemical performances of the NiCo2S4/NieCo LDH nanocomposites were measured by cyclic voltammetry and galvanostatic chargeedischarge techniques. The NiCo2S4/NieCo LDH electrode achieved a capacity of 1765 F g1 at 1 A g1 and a capacity of 940 F g1 at 10 A g1. Control experiments showed that the capacities of the NiCo2S4/NieCo LDH are higher than NieCo LDH and NiCo2S4 nanosheets. The better electrochemical performances of the NiCo2S4/NieCo LDH electrode can be attributed to the unique hierarchical structure which was formed during the vapor-phase hydrothermal process. © 2017 Elsevier B.V. All rights reserved.
Keywords: NiCo2S4 Hierarchical structure Vapor-phase hydrothermal synthesis Supercapacitor
1. Introduction The development of clean and renewable energies is regarded as an important way to solve the problems of energy scarcity and environmental pollution. However, some of the clean and renewable energies such as solar, wind and tide energies are unstable and intermittent, and superior energy storage devices are usually needed for efficient utilization of these energies. Supercapacitors as a new emerging energy storage device have aroused intensive attentions due to their merits such as high power density, ultrafast chargingedischarging rate, wide operation temperature range and long cycle life [1e6]. Taking the chargeestorage mechanism into account, supercapacitors are commonly classified into two kinds, the one is electrical double layer capacitors (EDLCs) and the other is pseudocapacitors. For the EDLCs, charges are stored electrostatically at the interface of the electrode and electrolyte and limited specific capacitances can be realized [7,8]. On the other hand, pseudocapacitors store charges through fast and reversible Faradaic redox reactions [9,10]. In recent years, battery-type Faradaic
* Corresponding author. E-mail address: [email protected] (L. Luo). http://dx.doi.org/10.1016/j.jallcom.2017.02.188 0925-8388/© 2017 Elsevier B.V. All rights reserved.
electrode materials such as the transition metal oxides, hydroxides and sulfides have been extensively studied for electrochemical energy storage [11e18]. The battery-type materials are very similar to pseudocapacitive materials in terms of the chargeestorage mechanism, and are considered as promising electrode materials for high-performance hybrid supercapacitors [19e21]. Recently, spinel NiCo2S4 has attracted much interest as an active electrode material for electrochemical energy storage due to its rich redox chemistry and excellent conductivity. Many NiCo2S4 nanostructures with high capability have been reported, such as the 3D porous nanonetworks [22], hollow hexagonal nanoplates [23], nanotube arrays [24], nanosheets [25], flaky arrays [26], mesoporous nanoparticles [27] and hollow nanoboxes [28]. Besides the above mentioned single-component nanostructures, some NiCo2S4 nanocomposites such as the 3D hierarchical NiCo2S4@MnO2 coreshell nanosheet arrays [29], hierarchical NiCo2S4@NiMoO4 core/ shell nanospheres [30], NiCo2S4@Co(OH)2 core-shell nanotube arrays [31], 3D hierarchical mesoporous NiCo2S4@Ni(OH)2 core-shell nanosheet arrays [32] and hierarchically constructed NiCo2S4@Ni1exCox(OH)2 core/shell nanoarrays [33] have also been reported for electrochemical energy storage applications. In this work, we report the fabrication of NiCo2S4/NieCo layered double hydroxide (LDH) nanocomposite and its application for
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2. Experimental section 2.1. Chemicals Cobalt chloride hexahydrate (Co(NO3)2$6H2O), nickel chloride hexahydrate (Ni(NO3)2$6H2O), hexamethylenetetramine (HMT, C6H12N4), thioacetamide (TAA, CH3CSNH2) and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co. with analytical grade and used without further purification. 2.2. Synthesis of NieCo LDH on nickel foam
Fig. 1. (a) XRD patterns of the NieCo LDH sample, (b) XRD pattern of the NiCo2S4/ NieCo LDH sample.
electrochemical energy storage. The NiCo2S4/NieCo LDH nanocomposite was synthesized by using NieCo LDH as precursor through a vapor-phase hydrothermal method. SEM observations show that ultrathin NiCo2S4 nanosheets with smaller sizes are grown on NieCo LDH nanosheets, such a hierarchical structure can avoid the aggregation of the nanosheets and therefore improve the electrochemical activities of the materials. The NiCo2S4/NieCo LDH was directly grown on nickel foam, which can be applied as a binder-free electrode without further treatment. The assynthesized NiCo2S4/NieCo LDH electrode shows higher capability than the NieCo LDH nanosheets and NiCo2S4 nanosheets.
In a typical procedure, Co(NO3)2$6H2O (1.1645 g, 4 mmol), Ni(NO3)2$6H2O (0.581 g, 2 mmol) and HMT (1.12 g, 8 mmol) were added to 50 mL deionized water under stirring. After the chemicals were completely dissolved, the solution was transferred into a 60 mL Teflon-lined stainless-steel autoclave. A piece of nickel foam (2 cm 2 cm) which has been washed with acetone, ethanol and deionized water was immersed into the above solution. The autoclave was sealed and heated at 100 C for 10 h. After that, the NieCo LDH loaded nickel foam was repeatedly washed with deionized water for further use. 2.3. Synthesis of NiCo2S4/NieCo LDH on nickel foam Firstly, thioacetamide (0.038 g, 5 mmol) was dissolved in 30 mL distilled water. Then the solution was transferred into a 60 mL Teflon-lined stainless-steel autoclave. The NieCo LDH loaded nickel foam was placed on a Teflon support above the thioacetamide solution. The autoclave was sealed and heated at 107 C for 10 h. After that, the nickel foam was repeatedly washed with deionized water, and dried under vacuum at 50 C for 2 h. The mass loading of NiCo2S4/NieCo LDH on nickel foam is about 3.8 mg cm2.
Fig. 2. XPS spectra of (a) Ni 2p, (b) Co 2p, (c) S 2p and (d) O 1s.
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2.4. Material characterizations The crystallographic information of the samples was measured by X-ray powder diffraction (XRD, Bruker D8 Advance). Elemental composition was analyzed by X-ray photoelectron spectroscopies (XPS, ESCALab MKII). The morphologies of the samples were observed by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, FEI Tecnai G2 20). 2.5. Electrochemical measurements Electrochemical properties of the electrodes were measured with a three-electrode cell. The active materials loaded nickel foam (1.0 cm 1.0 cm) acted as the working electrode, a platinum plate served as the counter electrode and an Hg/HgO electrode as the reference electrodes. KOH aqueous solution with a concentration of 3 mol L1 served as the electrolyte. 3. Results and discussion The NiCo2S4/NieCo LDH nanocomposite was prepared in twosteps. In the first step, the hydrolysis of HMT released many OH
351
which act as precipitant for the production of NieCo LDH. In the second step, the hydrolysis of TAA released H2S, and the reaction between NieCo LDH and H2S in the vapor phase result in the formation of NiCo2S4 on the surface of NieCo LDH. XRD technique was applied to analyze the crystallographic information of the asobtained samples. In order to avoid the affection of nickel foam, samples were separated from nickel foam by ultrasonication. Fig. 1a shows a typical XRD pattern of the NieCo LDH, the diffraction peaks can be indexed as a mixture of rhombohedral phase a-Ni(OH)2 (JCPDS No. 38-715) and a-Co(OH)2 [34]. XRD pattern of the NiCo2S4/ NieCo LDH sample is shown in Fig. 1b, in addition to the diffraction peaks from NieCo LDH, three weak peaks at 31.1, 38.0 and 54.9 can be indexed to cubic phase NiCo2S4 (JCPDS no. 43-1477). The weak diffraction peaks of NiCo2S4 indicate the low content of NiCo2S4 in NiCo2S4/NieCo LDH composite. The surface elements of NiCo2S4/NieCo LDH sample are analyzed by XPS. Fig. S1 shows a survey XPS spectrum of the NiCo2S4/NieCo LDH sample. The peaks at 163, 531, 781 and 855 eV correspond to S 2p, O 1s, Co 2p and Ni 2p, respectively, which indicates the presence of S, O, Co and Ni elements in the NiCo2S4/ NieCo LDH sample. The high-resolution XPS spectra of each element are shown in Fig. 2. The XPS peaks in these spectra were computer fitted. Fig. 2a shows a XPS spectrum of Co 2p region, the
Fig. 3. (a,b) SEM and (c) TEM images of NieCo LDH nanosheets; (d) SEM, (e) TEM and (f) HRTEM images of NiCo2S4/NieCo LDH nanocomposites.
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strong peaks at 781.2 and 796.4 eV are attributed to Co 2p3/2 and Co 2p1/2, while the weak and broad peaks at 784.3 and 803.6 eV are attributed to the satellite peaks [35]. Both the Co 2p3/2 and Co 2p1/2 peaks can be fitted into two peaks corresponding to Co3þ and Co2þ, respectively, indicating the coexistence of Co3þ and Co2þ in the sample. Fig. 2b shows the XPS spectrum of Ni 2p, the Ni 2p3/2 at 855.6 eV, Ni 2p1/2 at 873.1 eV and satellite peaks at 861.1 and 879.2 eV can be clearly seen. Similar to Co element, the Ni 2p3/2 and Ni 2p1/2 peaks can also be fitted into peaks corresponding to Ni3þ and Ni2þ, suggesting the coexistence of both Ni2þ and Ni3þ [35]. The atomic ratio of Ni and Co is 1:1.72, which is close to the ratio in NiCo2S4. Fig. 2c shows the O 1s XPS spectrum at 531.2 eV which can be fitted into two peaks. The fitted peak at 531.1 eV can be attributed to hydroxyl ions and the peak at 532.0 eV can be attributed to metal oxide [36]. XPS spectrum of S 2p is shown in Fig. 2d, the peaks at 162.0, 163.5 and 168.0 eV can be assigned to S 2p3/2, S 2p1/2 and the satellite peaks, respectively, consistent with the previous report [35]. The morphology of the samples is observed by SEM and TEM. Fig. 3a displays a low-magnification SEM image of the NieCo LDH loaded nickel foam, suggesting that the surface of nickel foam is uniformly covered with the as-synthesized sample. The corresponding high-resolution SEM image shown in Fig. 3b indicates that the NieCo LDH sample is composed of many thin nanosheets and these nanosheets are interconnected to each other. Fig. 3c shows a TEM image of the NieCo LDH nanosheets in which the nanosheets seem transparent under the illumination of electron beams, indicating their ultrathin nature. SEM image of NiCo2S4/ NieCo LDH sample is shown in Fig. 3d. It is obvious that many smaller nanosheets are present. Through the comparison with the NieCo LDH nanosheets shown in Fig. 3b, it can be concluded that these nanosheets are grown on the basis of the NieCo LDH nanosheets. TEM image of the NiCo2S4/NieCo LDH sample (Fig. 3e) shows some small nanosheets on larger nanosheets. HRTEM image
(Fig. 3f) taken from the smaller nanosheets shows lattice fringes of 0.54 nm which matches with the (111) lattice planes of NiCo2S4. The formation of NiCo2S4/NieCo LDH nanocomposite is according to the following reactions: C6H12N4 þ H2O / HCHO þ NH3
(1)
NH3 þ H2O 4 NHþ 4 þ OH
(2)
Co2þ þ Ni2þ þ OH / Ni(OH)2$Co(OH)2
(3)
CH3CSNH2 þ H2O / CH3CONH2 þ H2S
(4)
H2S þ Ni(OH)2$Co(OH)2 / NiCo2S4 þ H2O
(5)
The reaction between NieCo LDH and H2S occurred in vapor atmosphere. The vapor atmosphere is crucial for the formation of the hierarchical NiCo2S4/NieCo LDH nanocomposite. Control experiment shows that when NieCo LDH was immersed in thioacetamide solution at the same reacting temperature, porous NiCo2S4 nanosheets instead of NiCo2S4/NieCo LDH nanocomposite can be obtained within 2 h, as shown in Fig. S2. This result indicates that the reaction in vapor phase is much slower than in solution. The reason can be attributed to the slower feeding of reacting materials and the slower diffusion speed in vapor phase. The unique reacting condition leads to the formation of the hierarchical structure. The electrochemical performances of the NiCo2S4/NieCo LDH as a battery-type electrode were measured in a three-electrode cell by using a Pt plate counter electrode and a Hg/HgO reference electrode in 3 mol L1 KOH electrolyte. Fig. 4a shows the representative cyclic voltammogram (CV) curves of the NiCo2S4/NieCo LDH electrode in a potential window of 0e0.5 V at scan rates ranging from 10 to 100 mV s1. At low scan rates, a pair of redox peaks can be seen in
Fig. 4. (a) CV curves of the NiCo2S4/NieCo LDH at different scan rates; (b) galvanostatic chargeedischarge curves of the NiCo2S4/NieCo LDH at different current densities; (c) specific capacitance of the NiCo2S4/NieCo LDH electrode vs. current densities; (d) comparative galvanostatic chargeedischarge curves at a current density of 2 A g1.
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potential window of 0e0.5 V at current densities of 1e10 A g1. Fig. 4b shows a series of chargeedischarge curves at various current densities. Different from the triangular curves of EDLCs, here the curves show obvious platforms during the charging and discharging process, which reveals the occurring of Faradaic redox reactions. The capabilities of the electrodes are calculated according to the following equations [39]:
Cs ¼
I Dt m DV
(6)
Ca ¼
I Dt S DV
(7)
In the equation the Cs (F g1) is the specific capacitance, Ca (mF cm ) is the areal capacitance, I (A) is the discharge current, Dt (s) is the discharge time, m (g) is the active mass, S (cm2) is the electrode geometrical area and DV (V) is the potential window. The specific capacitance of the NiCo2S4/NieCo LDH electrode vs. current densities is shown in Fig. 4c. The NiCo2S4/NieCo LDH electrode exhibits a high specific capacitance of 1765 F g1 (6707 mF cm2) at a current density of 1 A g1 (3.8 mA cm2). With the increase of current densities, the specific capacitances decrease. This is a common phenomenon observed in supercapacitors. At lower current densities, the electrolyte ions can fully diffused into the active materials, which lead to higher specific capacitances; whereas at higher current densities, the electrolyte ions can only absorbed on the outer layer or insufficient portion of the active materials, which result in lower specific capacitances. At a current density of 10 A g1 (38 mA cm2), the specific capacitance of the NiCo2S4/NieCo LDH electrode is 940 F g1 (3572 mF cm2), which is 53.3% of that at 1 A g1. The specific capacitances of the NiCo2S4/NieCo LDH, the precursor NieCo LDH and the porous NiCo2S4 nanosheets which were obtained by treating the precursor in thioacetamide solution were compared. Fig. 5d displays galvanostatic chargeedischarge curves of the three samples tested at a current density of 2 A g1, it is clear that the discharge time of NiCo2S4/NieCo LDH is the longest. Specific capacitances calculated from these discharge curves are 1546, 1239 and 1275 F g1 for the NiCo2S4/NieCo LDH, NieCo LDH and NiCo2S4 electrodes. The NiCo2S4/NieCo LDH shows the best capacitive performances. Furthermore, the specific capacitances of the NiCo2S4/NieCo LDH electrode are also comparable or better than those of previously reported NiCo2S4 electrodes, as shown in Table 1 [22e26]. Cycling stability of the NiCo2S4/NieCo LDH electrode was tested at a current density of 5 A g1 over 2000 successive cycles, and the capacitance retention vs. cycles are shown in Fig. 5a. At the initial 500 cycles, there is an obvious deterioration of the capacitance. After that, the decay tendency of capacitance turns weak. The capacitance retention is 56% after 2000 cycles. In previous reports, the cycling stability of transition metal hydroxides is not so good [40], and the hierarchical structure does not helpful for improve the cycling stability. Electrochemical impedance spectra (EIS) were measured in a frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV. The EIS of the NiCo2S4/NieCo LDH, NieCo LDH and NiCo2S4 electrodes are shown in Fig. 5b. In the high frequency region there exists a semicircle which reflects charge-transfer resistance in the electrodeeelectrolyte interface. The little radius of the semicircle reflects the lower charge-transfer resistance [41]. As learn from the spectra, the charge-transfer resistance of the NiCo2S4 electrode is negligible, and the charge-transfer resistance of the NiCo2S4/NieCo LDH electrode is a little lower than that of the NieCo LDH electrode. The sloping straight lines in the low frequency region reflect the Warburg impedance, and the slop of the lines reveals the diffusion 2
Fig. 5. (a) Cycling performance of the NiCo2S4/NieCo LDH electrode; (b) EIS of the NiCo2S4/NieCo LDH, NieCo LDH and NiCo2S4 electrodes.
the CV curves, which reflects the Faradaic behavior of battery-type electrodes. The separation of the oxidation and reduction peaks increases with the increase of scan rates, which may ascribed to the polarization effect [37,38]. Fig. S3 shows CV curves of the NiCo2S4/ NieCo LDH electrode and pure Ni foam at a scan rate of 100 mV s1. The CV curve of the Ni foam is a straight line, revealing that the capacitive performance contributed from Ni foam is negligible. Galvanostatic chargeedischarge tests were carried out in a
Table 1 Comparison of the NiCo2S4/NieCo LDH electrode with references. Electrode material
Mass loading (mg cm2)
Current density (A g1)
Specific capacitance (F g1)
Ref.
3D porous NiCo2S4 nanonetworks Hollow hexagonal NiCo2S4 nanoplates NiCo2S4 nanotube arrays NiCo2S4 nanosheets Mesoporous NiCo2S4 nanoparticles NiCo2S4/NieCo LDH
2.0
1
1501.2
[22]
2.4
1
437
[23]
4.2
4
738
[24]
4.76 4.28
2 2
1231 972
[25] [27]
3.8
1
1765
This work
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rates of electrolyte ions in the active materials [42]. The slope of the three lines is similar, demonstrating similar electrolyte ion diffusion rates in the three electrode materials. The higher specific capacitances of the NiCo2S4/NieCo LDH electrode than the NieCo LDH and NiCo2S4 electrodes can be ascribed to the following aspects. First, the hierarchical structure avoids the possible aggregation of the thin nanosheets, therefore more active sites for ion adsorption and Faradaic redox reactions are provided. Second, the NiCo2S4 nanosheets possess good conductivity and rich redox chemistry. All in all, the vapor-phase hydrothermal approach provide a different way for fabricate nanocomposite for supercapacitor applications.
[12]
[13]
[14]
[15]
[16]
4. Conclusions In summary, hierarchical NiCo2S4/NieCo LDH nanocomposites were synthesized by a vapor-phase hydrothermal method. The unique reacting condition leads to the formation of the hierarchical structure. The NiCo2S4/NieCo LDH nanocomposites were directly loaded on nickel foam, and can be applied for supercapacitor application without further treatment. Electrochemical measurements show that the hierarchical NiCo2S4/NieCo LDH has a high specific capacitance, and the capacitive performances of the hierarchical NiCo2S4/NieCo LDH are better than the NieCo LDH nanosheets and the porous NiCo2S4 nanosheets. The vapor-phase hydrothermal method may provide a feasible way for construct nanocomposites for supercapacitor applications. Acknowledgment Financial support from the National Natural Science Foundation of China (NSFC no. 21671007) and the foundation from Wanjiang college, Anhui Normal University (WJKY-201517) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.02.188.
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