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NiS composite as an advanced electrode material for high-performance supercapacitors
Journal of Energy Storage 28 (2020) 101199
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Synthesis of CNTs on ZnO/NiS composite as an advanced electrode material for high-performance supercapacitors
T
S. Srinivasa Rao School of Mechanical and Mechatronics Engineering, KyungSung University, 309 Suyeong-ro Nam-gu Busan, 48434, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Supercapacitors ZnO/NiS/CNT composite Galvanostatic charge discharge Cyclic voltammetry Electrochemical impedance spectroscopy
The design and preparation of an efficient electrode material with a multifunctional surface structure is a key challenge for high-performance supercapacitors. In this study, ZnO/NiS and ZnO/NiS/CNT nanocomposites were synthesized on a nickel foam substrate and used as an electrode material. The surface morphology and structure of the electrodes were analyzed by field emission–scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The prepared nanocomposite of ZnO/NiS/CNT exhibited a high specific capacitance of 879.0 F g−1 at a current density of 1.5 A g−1, along with remarkable cycling stability of 89.7% capacitance retention after 5000 cycles, which was superior to that of ZnO/NiS (805.75 F g−1 and 74.59% retention). In addition, the ZnO/NiS/CNT electrode also exhibited good rate capability, faster ion and electron transfer ability, and higher surface area, highlighting its potential for electrochemical energy storage.
1. Introduction Supercapacitors, also known as electrochemical capacitors, are attracting increasing attention in energy storage owing to their rapid charge/discharge rate, high power density, and excellent cycling performance [1–3]. On the other hand, their energy density needs to be improved to meet the ever-increasing requirements for electronic devices, such as portable electronics and electric vehicles [4,5]. Transition metal sulfides, such as NiS, CoS, CuS, and MnS, are new types of energy storage materials that have been studied extensively owing to their excellent electrochemical performance. Metal sulfide-based materials have attracted enormous interest because of their better conductivity. The electronegativity of sulfur is lower than that of oxygen, which allows easier electron transport in the structure. The oxygen can be replaced with sulfur, which provides flexibility in the fabrication of nanomaterials [6,7]. Transition metal sulfides have promising applications in the areas of solar cells, lithium ion batteries, and SCs because of their unique optical and electrical properties. Metal oxide/metal sulfide-based electrode materials are more interesting for SC applications owing to their better capacitance and potential as electroactive materials [8]. A range of sulfide-based electrode materials, such as CoS, NiS, Ni3S2, SnS2, CuS, MnS, NiCo2S4, and Co3S4, have been used as emerging electrode materials. Different types of nanostructures will be responsible for the most effective energy storage, which improves the electrochemical
performance of electrode materials. Nickel-cobalt sulfides have attracted considerable attention because of their richer redox reactions [9,10]. On the other hand, their capacitance at a high rate is inadequate because of the low intrinsic electronic conductivity and unfavorable ion transport length. NiS is a semiconducting material with a variety of compositions that can be utilized in many applications, e.g., SCs, dye and quantum-dot sensitized solar cells. Zhang et al. reported the one pot synthesis of Ni3S4@MoS2 [11]. Zhu et al. synthesized Ni3S2@ multi-walled carbon nanotubes (CNTs) and Krishnamoorthy et al. used a solvent thermal synthesis to synthesize Ni3S2/Ni foam [12,13]. Previous reports explained that NiS is suitable as an electrode material for SCs owing to its excellent electrochemical performance. On the other hand, it still requires a base carrier, such as CNTs and Ni foam, to fully maximize the advantages of electrode materials. NiS have attracted considerable research interest and exhibit good electrochemical performance for applications in pseudo-capacitors, dye-sensitized solar cells, and catalysts [14]. Thus far, great success has been achieved in the synthesis of nanostructured NiS, but their electrochemical performance, particularly cycling stability, is still unsatisfactory [15]. Therefore, improving the electrochemical performance of nickel sulfide is a major research focus. To overcome this, reducing the device size to the nanoscale to increase its active area and mechanical stability is a key concept, and hybridizing the nanostructured pseudo-capacitive materials as a shell with highly conductive
E-mail addresses: [email protected], [email protected]. https://doi.org/10.1016/j.est.2020.101199 Received 13 November 2019; Received in revised form 27 December 2019; Accepted 4 January 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
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vacuum oven at 55 °C for 4 h. The CNT slurry was prepared by mixing CNT powder with the appropriate amount of ethanol and the resulting slurry was loaded on the ZnO/NiS films and dried overnight at 60 °C.
materials as a core can improve the performance of energy storage devices [16,17]. Considerable effort has made to synthesize core/shell nanostructures for high performance SCs. The excellent conductivity and regular tubular framework for anchoring pseudo-capacitive electrode materials makes CNTs an ideal material for core/shell structural design. On the other hand, double-layer capacitance produced by the bare CNT-based materials is very low due to the serous aggregation caused by strong π- π stacking effect. The double-layer capacitance can be enhanced by introducing metal sulfide and metal oxides [18–20]. With this base, the authors developed highly efficient thin-film electrodes by forming an aligned compact of ZnO/NiS/CNT composite on a Ni foam substrate, in which the NiS nanoparticles were bundled tightly by chains of ZnO and CNTs and could maintain good surface contact with each other. The ZnO/NiS/CNT composite, as the active thin film electrode, exhibited a high specific capacitance of 879.0 F g−1 at a current density of 1.5 A g−1, good rate performance (38.11% capacitance retention from 1.5 to 8 A g−1), and cycling stability (89.7% capacitance retention after 5000 cycles). To the best of the authors’ knowledge, the composite exhibited much better electrochemical performance than that of ZnO/NiS as well as other similar materials reported in the literature.
2.4. Electrochemical measurement Electrochemical measurements were performed using a Bio-LogicSP150 workstation interfaced to a computer system with electrochemical software. A 10-mg sample of the as-prepared electrodes was incorporated into nickel foam (1.0 cm × 1.0 cm). A three-electrode glass cell setup was used in the experiment consisting of a working electrode, platinum wire as the counter electrode, and saturated Ag/ AgCl as the reference electrode at 25 °C in 3 M KOH. The electrodes were soaked in a KOH solution and degassed in a vacuum for 5 h before the electrochemical test. All electrodes were tested in a 3 M KOH aqueous solution with a potential range between −0.5 to 0.7 V (vs. Ag/ AgCl). The galvanostatic charge/discharge curves were obtained at current densities of 1.5, 2.0, 2.5, 3.0, 3.5, 4, 5, 6, 7 and 8 A g−1. The specific capacitance of the electrode was calculated from the galvanostatic charge/discharge curves using Eq. (1):
C= 2. Materials and methods
I × Δt m ×V
(1) −1
Where C = specific capacitance (F g ), m = mass of the electrode (g), V = voltage window during the discharge process (V), i = discharge current, Δt = discharge time difference.
2.1. Preparation of Nickel foam substrate Zinc acetate dehydrate (Zn (CH3COOH)2.2H2O), zinc nitrate hexahydrate (Zn (NO3)2.6H2O), Nickel (II) chloride hexahydrate (NiCl2.6H2O), hexamethylenetetramine (HMT), Nickel(II) chloride hexahydrate (NiCl2.6H2O), thiourea (CH4N2S), 3-Mercaptopropionic acid (3-MPA), and ethylene glycol were purchased from Aldrich and used as received. Before the deposition of ZnO/NiS and ZnO/NiS/CNT, the nickel foam (approximately 1 cm × 1 cm) was cleaned carefully in a concentrated HCl solution (37 wt. %) by sonication for 10 min to remove the surface oxide layer. The foam was then washed sequentially with acetone, ethanol, and deionized water for 10 min each to ensure that the surface of the Ni foam was well cleaned and dried with a hair dryer.
2.5. Characterization The microstructure of the synthesized samples was measured by Schottky emission scanning electron microscopy (SEM; SU-70, Hitachi) at the Busan KBSI. The elemental compositions of the electrodes were investigated by field emission scanning electron microscopy (FE-SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX) operated at 15 kV. The crystalline structure and morphology of the prepared electrodes were investigated by high-resolution transmission electron microscopy (HRTEM; Jem 2011, Jeol cop.). High-resolution X-ray diffraction (XRD) was performed on a D8 ADVANCE with a DAVINCI (Bruker AXS) diffractometer using Cu Kα radiation and operated at 40 kV and 40 mA. X-ray photoelectron spectroscopy was also conducted (XPS, PHI 5000C ESCA System). Prior to the measurement, the samples were dehydrated at 90 °C for 5 h. Transmission electron microscopy (TEM, Jeol, JEM 2011) was performed at the Busan KBSI center and operated at 200 kV using a CCD camera 4k × 4k.
2.2. Synthesis of ZnO nanorods ZnO NRs were prepared using a two-step facile chemical bath deposition method. The first step was to coat a ZnO seed layer on the Ni foam. The ZnO seed layer was prepared using zinc acetate dihydrate 10 mM [Zn (CH3COOH)2. 2H2O] as a precursor, dissolved in ethanol and stirred for 30 min. After growth of the seed layer, the samples were annealed at 65 °C for 5 h in ambient air. The second step was to grow the ZnO NRs on the seeded Ni foam using a chemical bath deposition method. The seeded substrate was suspended upside down in an aqueous solution containing 0.015 M zinc nitrate hexahydrate and HMT at 95 °C for 15 h to grow the ZnO NRs. After cooling naturally to room temperature, the prepared ZnO NRs on Ni foam substrate was washed with deionized water and dried in an oven at 65 °C for 5 h.
3. Results and discussion HR-SEM (Fig. 1a–d) revealed the formation of ZnO/NiS and (Fig. 1e and f) ZnO/NiS/CNT on the nickel foam at high and low magnifications. CNTs were introduced to suppress the agglomeration of the ZnO/ NiS nanocomposite. The CNTs act as nucleation sites and might be responsible for the inhibition in their ZnO/NiS/CNT [21]. The previous studies suggested that a small quantity of individually dispersed carbon nanotubes can affect the active material orientation and crystallization [22]. Moreover, this might have a synergic effect and improve the electrical connection between ZnO, NiS, and the conducting substrate. The growth of such uniformly distributed and porous nanostructures provides a large surface area that enhances the electrochemical performance [23,24]. The agglomeration problem was resolved by adding CNTs to the ZnO/NiS nanocomposite. The free space between the interconnected CNTs could facilitate the transport of electrolyte and ions. In addition, the CNT network framework with a NiS rough surface endows the electrode with a large specific surface area that favors charge carrier transport. The uniform distribution of NiS nanoparticles arose from the growth
2.3. Synthesis of NiS and CNTs on the ZnO NRs The host precursor materials, such as 0.1 M NiCl2.6H2O and 0.4 M thiourea, were dissolved in 40 mL of ethylene glycol. Subsequently, 0.2 M 3-MPA was added to the above mixture with strirring. The mixture was kept stirring for 60 min until complete dissolution. The mixture was then transferred to a Teflon-lined autoclave and ZnO-deposited Ni foams were placed into the reaction mixture. The hydrothermal reaction system was conducted at 150 °C for 10 h. The autoclave was cooled naturally to room temperature and the as-obtained ZnO/NiS material was filtered and washed sequentially with DI water and ethanol two to three times each. The ZnO/NiS were dried in a 2
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Fig. 1. High-resolution HR-SEM images of (a), (b), (c), (d) ZnO/NiS and (e), (f) of ZnO/NiS/CNT and their corresponding high magnification images.
Fig. 3. XRD patterns of ZnO/NiS and ZnO/NiS/CNT electrodes on nickel foam substrates.
was analyzed further by XRD. As shown in Fig. 3, the peaks at (100), (002), (101), and (103) were assigned to the ZnO phase (JCPDS no: 361451). The peaks at 30.2°, 34.8°, 37.9°, 53.5°, and 73.3° 2θ were attributed to the NiS phase (JCPDS no: 02-1280) and some other diffraction peaks located at 44.5°, 51.8°, and 76.4° were obtained from the nickel foam substrate. The XRD peaks for NiS and ZnO either disappeared or the intensity of the peaks decreased dramatically after deposition with CNTs. XPS is an analytical technique that can be used not only for elemental identification within a sample but also to identify the oxidation state of the element. This technique can be used to investigate areas, such as corrosion, oxidation, surface contamination and modification, absorption and deposition of chemical species, catalysis, and many other nanoscale surface processes. The survey spectrum of ZnO/NiS/ CNT (Fig. 4b) revealed S2p, C1s, O1s, Ni2p3, Zn2p3, and N1s at 162.88, 284.6, 531.14, 855.5, 1045.5 and 399.72 eV, respectively. The O1s, C1s, and N1s peaks were attributed to exposure of the fabricated electrode to oxygen molecules or with moisture. The high resolution XPS spectra of Zn2p, S2p, Ni2p, and C1s elements in ZnO/NiS/CNT composite are shown in Fig. 5. Fig. 5a shows the two main peaks of Zn2p located at 1022.24 eV and 1045.55 eV, which were assigned to Zn2p3/2 and Zn2p1/2, respectively. The strong resolution Ni2p spectrum (Fig. 5b) at 855.5 eV could be deconvoluted
Fig. 2. TEM images of (a), (b) ZnO/NiS and (e), (f) of ZnO/NiS/CNT on a nickel foam substrate.
mechanism and NiS stems formed on the ZnO, which increases the number of reactive sites for the reaction. Consequently, the void spaces between the ZnO/NiS stems were filled with CNTs, which formed a network wrapping. This increases the electrical conductivity between the material, yielding more defects and an enhanced charge transport process. HR-SEM suggested that during the CNT coating on the networks on the ZnO/NiS stem-like nanostructure, the particle sizes were reduced significantly and coated uniformly on entire Ni foam substrate which may increases the surface area. The nanoparticles on the porous structures can enable the transport of electrolyte and ensure efficient contact between the active material and the KOH electrolyte for the electrodes. TEM was used to reveal the detailed structural and morphological properties of the metal sulfides with CNTs, as shown in Fig. 2. The crystal structure of the ZnO/NiS and ZnO/NiS/CNT electrodes 3
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Fig. 4. XPS survey spectrum of ZnO/NiS and ZnO/NiS/CNT electrodes on a nickel foam substrate.
which was higher than that of the ZnO/NiS (9.38 m2/g). These results indicate that the mesoporous structure combined with large surface area of ZnO/NiS/CNT composite is quite advantageous to endow ample electroactive sites and shorten the charge/ion transport path which is tremendously helpful for Faradaic redox reaction [28]. The electrochemical properties of the as-prepared ZnO/NiS and ZnO/NiS/CNT electrodes were analyzed by cyclic-voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) using a three-electrode system. Both electrodes were evaluated as binder-free electrodes for supercapacitors in a 3 M KOH electrolyte. Fig. 7a shows the CV curves of the ZnO/NiS and ZnO/NiS/ CNT composites at 20 mV s−1. The CV curves comprise two pairs of well-defined redox couples in ZnO/NiS and ZnO/NiS/CNT composites, which could be ascribed to the Faradaic redox reaction in KOH solution.
to S2p3/2 (855.45 eV) and S2p1/2 (873.42 eV) and the two main peaks separated by 17.97 eV. As shown in Fig. 5c, the two main peaks separated by 4.8 eV at 162.88 and 167.68 eV were indexed to sulfide [25,26]. Fig 5d shows the XPS spectra of C 1s for all the CNTs in the ZnO/NiS/CNT samples. The C 1 s spectra were composed of several characteristic peaks: two peaks due to the carbon-carbon interactions, including CeC s2p bonds at binding energies of 284.4–284.7 eV and CeC sp3 bonds at 285.1–285.5 eV; and two relatively weak peaks due to the carbon-oxygen interactions, including C-O bonds at 286.4–286.7 eV and C]O bonds at 287.8–288.1 eV [27]. To investigate the specific surface are of ZnO/NiS and ZnO/NiS/CNT composite were investigated by nitrogen adsorption/desorption isotherms (BET) and the results are shown in Fig. 6. The BET specific surface area of the prepared ZnO/NiS/CNT composite is calculated to be 10.77 m2/g,
Fig. 5. High-resolution scan of the (a) Zn2p, (b) Ni2p, (c) S2p and (d) C1s peaks in ZnO/NiS/CNT composite. 4
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area were observed, which illustrates the higher energy storage performance of the ZnO/NiS/CNT composite compared to ZnO/NiS composite. In addition, the oxidation peaks were shifted to more positive side and reduction peaks were shifted to more negative potential with increment of scan rates, demonstrating the good ion diffusion rate and lower resistance of the ZnO/NiS/CNT composite during the electrochemical redox reaction [29,30]. To check the electrochemical effect for the Ni foam substrate, we also performed CV test for bare Ni foam (Fig. S1 and Fig. S2) at 50 mV s−1 and GCD test in KOH solution. Compared to the ZnO/NiS and ZnO/NiS/CNT composite, the oxidation and reduction current of Ni foam is quite small and almost negligible, indicating that the Ni foam contributes very little to the total specific capacitance of the electrode. Fig. 8 presents typical GCD curves of the ZnO/NiS and ZnO/NiS/ CNT electrodes at various current densities. The GCD curve measured at 1.5 A g−1 showed a larger enclosed area for the ZnO/NiS/CNT (Fig. 8b), which is higher than that of the ZnO/NiS-based electrode (Fig. 8a). The specific capacitance of the ZnO/NiS and ZnO/NiS/CNT electrodes were 805.75, 744.56, 702.75, 669.8, and 622.3 F g−1 and 879, 812.25, 766.63, 730.69 and 678.87 F g−1 at a current density of 1.5, 2, 2.5, 3, and 3.5 A g−1, respectively. The specific capacitance of the ZnO/NiS/CNT electrodes was much higher than that of the ZnO/ NiS. Moreover, when the current density was increased from 1.5 to 8 A g−1, the ZnO/NiS/CNT electrode exhibited good rate capability and capacitance retention of 38.11%, which was higher than that of the ZnO/NiS (28.24%). These observations have many explanations. First, there is abundant space between the materials and highly uniform nanowire structure on the nickel foam substrate, which can act as “ion-
Fig. 6. The Brunauer–Emmett–Teller (BET) surface area obtained from nitrogen adsorption-desorption isotherms.
The CV integrated area, oxidation and reduction currents of the ZnO/ NiS/CNT electrode were much higher than those of the ZnO/NiS electrode, highlighting the significant energy storage performance of the ZnO/NiS/CNT. Moreover, as the scan rate was increased from 10 to 60 mV s−1 for ZnO/NiS/CNT composite (Fig. 7c), similar CV shapes with a pair of redox peaks, higher peak current values with enlarged CV
Fig. 7. CV curves of (a) ZnO/NiS and ZnO/NiS/CNT electrodes at 20 mV s−1 in 3 M KOH solution, CV curves of (b) ZnO/NiS and (c) ZnO/NiS/CNT at various current rates (10–60 mV s−1). 5
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Fig. 8. GCD curves of (a) ZnO/NiS and (b) ZnO/NiS/CNT electrodes at a current density of 1.5, 2.0, 2.5, 3.0, and 3.5 A g−1 in a 3 M KOH solution. (c) Specific capacitance at different current densities. (d) Long-term cycling stability of the ZnO/NiS and ZnO/NiS/CNT electrodes for 5000 GCD cycles in 3 M KOH solution.
performance of ZnO/NiS and ZnO/NiS/CNT electrodes at a current density of 1.5 A g−1 for 5000 cycles in a 3 M KOH solution. The ZnO/ NiS electrode showed a specific capacitance of 601.0 F g−1 after 5000 GCD cycles and 74.59% capacitance retention. In contrast, the ZnO/ NiS/CNT electrode showed higher capacitance retention of 89.9% after 5000 cycles. Moreover, the slight increment of specific capacitance was observed at the beginning of few cycles is probably due to the activation process of electroactive material. The higher capacitance retention of ZnO/NiS/CNT could be attributed mainly to the stable electroactive sites and strong surface morphology on a nickel foam substrate even when the ions were intercalated/extracted into/out of the structure. EIS was conducted to evaluate the kinetic properties of the electrodes. The ZnO/NiS and ZnO/NiS/CNT electrodes are comprised of two parts (Fig. 9), i.e., semicircle part in the high frequency region and vertical line in the low frequency region. The spectra of the ZnO/NiS/ CNT electrode showed a more vertical line in the low-frequency region and was almost perpendicular to the real axis, revealing a more efficient
buffering reservoirs” for ions and may reduce the diffusion distance from the KOH electrolyte to the interior surface [31]. Second, the higher electrical conductivity of the CNTs can reduce the charge transfer resistance of the entire electrode effectively. Third, the ZnO/ NiS/CNT electrode avoids the use of conducting additives and polymer binders, enhancing the consumption of the active material. Fourth, the CNT nanowires afford an enormous number of active sites for the ZnO/ NiS nanostructure, and great mechanical adhesion on the nickel foam substrate and electrical connection of the active material to the current collector, which can lead to a larger surface area [32]. The electrochemical properties of the ZnO/NiS/CNT electrode in this study were also similar or superior to than those in other studies (Table 1), such as pristine Co3O4 (590 F g−1 at 10 A g−1), NiCoXOY (507. F g−1 at 1 A g−1), Zn-WO3 (35.70 F g−1), Ag-doped PEDOT:PSS/ CNT (85.3 F g−1), and NiCoO2/rGO composite (742 F g−1 at 10 A g−1) [33–37]. The long-term stability is an important factor to identify the stability of active material in supercapacitor. Fig. 8d shows the cycling
Table 1 Comparison of specific capacitance performance of ZnO/NiS/CNT composite with various literature. Material
Electrolyte
Specific capacitance
Current density/scan rates
Ref
TiO2-MWCNT Ag-doped PEDOT:PSS/CNT Fe-TiO2/C nanofibers WS2/CFC WO3-X@MnO2 NWs Fe2O3@LiCoO2 NiCoO2/rGO composite ZnMn2O4@Mn3O4 CC@NiCo2O4@ZnWO4 ZnO/NiS/CNT
1 – 1 1 – 3 – 1 – 3
160 F g−1 85.3 F g−1 137 F g−1 399 F g−1 341 F g−1 489 F g−1 742 F g−1 321.34 F g−1 872.0 F g−1 879.0 F g−1
1 A g−1 – 5 mV s−1 1 A g−1 10 mV s−1 5 mA cm−2 10 A g−1 1 mV s−1 1 A g−1 1.5 A g−1
[38] [36] [39] [40] [41] [42] [37] [43] [44] This study
M H2SO4 M KOH M KCL M KOH M Na2SO4 M KOH
6
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Acknowledgment This research was supported by Kyungsung University Research Grants in 2019. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2020.101199. References [1] C. Meng, C. Liu, L. Chen, C. Hu, S. Fan, Highly flexible and all-solid-state paperlike polymer supercapacitors, Nano Lett. 10 (2010) 4025–4031. [2] B. Liu, X.F. Wang, H.T. Chen, Z.R. Wang, D. Chen, Y.B. Cheng, C.W. Zhou, G.Z. Shen, Hierarchical silicon nanowires-carbon textiles matrix as a binder-free anode for high-performance advanced lithium-ion batteries, Sci. Rep. 3 (2013) 1622. [3] I.K. Durga, S.S. Rao, A.E. Reddy, C.V.V.M. Gopi, H.J. Kim, Achieving copper sulfide leaf like nanostructure electrode for high performance supercapacitor and quantum-dot sensitized solar cells, Appl. Surface Sci. 435 (2018) 666–675. [4] P. Simon, Y. Gogotsi, Capacitive energy storage in nanostructured carbon–electrolyte systems, Acc. Chem. 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Fig. 9. EIS of ZnO/NiS and ZnO/NiS/CNT electrodes over the frequency range of 100 kHz to 0.1 Hz.
electrolyte, proton diffusion, and lower ion transfer resistance in the electrode. These properties can be attributed to the uniform nanowires of ZnO/NiS/CNT with a higher surface area and abundant open spaces, which increased the use of the active electrode and the supply of OH− ions [45]. In the high-frequency region, ZnO/NiS/CNT showed a negligible series resistance and lower charge transfer resistance than that of the ZnO/NiS electrode, demonstrating the high electron transfer rate at the electrode/electrolyte interface and the extremely high electrochemical activity for energy storage. The significant enhancement of specific capacitance may be attributed to the following points: (a) the Ni foam substrate with great surface area can increases the rapid ion/ electron transfer rate due to its high conductivity and also acts as a scaffold for fabrication of high-performance electroactive materials; (b) synergetic effect of the unique structure; (c) ZnO and NiS nanoparticles are well decorated on Ni foam substrate with CNT that provide more active sides for redox reactions; (d) direct growth of ZnO/NiS/CNT composite on Ni foam without any polymer binders and conductive additives, which can control/reduce the dead mass and hence improves the electron transportation. Accordingly, the ZnO/NiS/CNT composite contains higher electron and ionic conductivity, which leads to improved energy storage performance.
4. Conclusions ZnO/NiS and ZnO/NiS/CNT nanostructures were prepared on a nickel foam substrate and used as an electrode material for supercapacitors. The ZnO/NiS/CNT nanocomposite exhibited greater electrochemical performance with a higher specific capacitance of 879.0 F g−1 at a current density of 1.5 A g−1 and excellent cycling stability (89.7% after 5000 cycles at 1.5 A g−1), and 678.87 F g−1 was maintained at 3.5 A g−1. The ZnO/NiS/CNT electrode exhibited higher electrochemical performance than the ZnO/NiS, which can be explained by the following. First, there was abundant space between the materials and highly uniform nanowire structure on the nickel foam substrate, which can act as “ion-buffering reservoirs” for ions and may reduce the diffusion distance from the KOH electrolyte to the interior surface. Second, the higher electrical conductivity of the CNTs can reduce the charge transfer resistance of the entire electrode effectively. These features make the ZnO/NiS/CNT composite an appropriate and auspicious electrode material for efficient supercapacitor applications.
Declaration of Competing Interest The authors declare no conflict of interest. 7
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Compd. 755 (2018) 15. [35] R.D. Kumar, Y. Andou, S. Karuppuchamy, Microwave-assisted synthesis of Zn–WO3 and ZnWO4 for pseudocapacitor applications, J. Phys. Chem. Solids 92 (2016) 94–99. [36] Y. Zhu, L. Ning, L. Tian, Y. Yao, H. Peng, J. Shi, S. Cao, T. Chen, Ag-Doped PEDOT:PSS/CNT composites for thin-film all-solid-state supercapacitors with a stretchability of 480%, J. Mater. Chem. A 6 (2018) 941–947. [37] X.H. Guan, M. Li, H.Z. Zhang, L. Yang, G.S. Wang, Template-assisted synthesis of NiCoO2 nanocages/reduced graphene oxide composites as high-performance electrodes for supercapacitors, RSC Adv. 8 (2018) 16902–16909. [38] A.L.M. Reddy, S. Ramaprabhu, Nanocrystalline metal oxides dispersed multiwalled carbon nanotubes as supercapacitor electrodes, J. Phys. Chem. C 111 (2007) 7727–7734. [39] G. Tolba, M. Motlak, A.M. bastaweesy, E.A. Ashour, W. Abdelmoez, M. El-Newehy, N. barakat, Synthesis of novel Fe-doped amorphous TiO2/C nanofibers for supercapacitors applications, Int. J. Electrochem. Sci. 10 (2015) 3117–3123. [40] X. Shang, J.Q. Chi, S.S. Lu, J.X. Gou, B. Dong, X. Li, Y.R. Liu, K.L. Yan, Y.M. Chai, C.G. Liu, Carbon fiber cloth supported interwoven WS2 nanosplates with highly enhanced performances for supercapacitors, Appl. Surf. Sci. 392 (2017) 708–714. [41] X. Lu, T. Zhai, X. Zhang, Y. Shen, L. Yuan, B. Hu, L. Gong, J. Chen, Y. Gao, J. Zhou, Y. Tong, Z.L. Wang, WO3–x@Au@MnO2 core–shell nanowires on carbon fabric for high-performance flexible supercapacitors, Adv. Mater. 24 (2012) 938–944. [42] C.V.V.M. Gopi, A. Somasekha, A.E. Reddy, S.K. Kim, H.J. Kim, One-step facile hydrothermal synthesis of Fe2O3@LiCoO2 composite as excellent supercapacitor electrode materials, Appl. Surf. Sci. 435 (2018) 462–467. [43] B. Ameri, S.S.H. Davarani, H.R. Moazami, H. Darjazi, Cathodic electrosynthesis of ZnMn2O4/Mn3O4 composite nanostructures for high performance supercapacitor applications, J. Alloys Compd. 720 (2017) 408–416. [44] K. Zhang, L. Lin, S. Hussain, S. Han, Core–shell NiCo2O4@ZnWO4 nanosheets arrays electrode material deposited at carbon-cloth for flexible electrochemical supercapacitors, J. Mater. Sci. 29 (2018) 12871–12877. [45] C. Guan, J. Liu, C. Cheng, H. Li, X. Li, W. Zhou, H. Zhang, H.J. Fan, Hybrid structure of cobalt monoxide nanowire @ nickel hydroxidenitratenanoflake aligned on nickel foam for high-rate supercapacitor, Energy Environ. Sci. 4 (2011) 4496–4499.
nickel sulfide (NiS) nanostructures, New J. Chem. 42 (2018) 2733–2742. [24] I.K. Durga, S.S. Rao, J.W. Ahn, T.Y. Park, J.S. Bak, C.I. Ho, P. Kandasamy, H.J. Kim, Dice-like nanostructure of a CuS@PbS composite for high-performance supercapacitor electrode applications, Energies 11 (7) (2018) 1624. [25] C. Lamiel, V.H. Nguyen, D.R. Kumar, J.J. Shim, Microwave-assisted binder-free synthesis of 3D Ni-Co-Mn oxide nanoflakes@Ni foam electrode for supercapacitor applications, Chem. Eng. J. 316 (2017) 1091–1102. [26] Z. Zhang, Q. Wang, C. Zhao, S. Min, X. Qian, One-step hydrothermal synthesis of 3D petal-like Co9S8/RGO/Ni3S2 composite on nickel foam for high-performance supercapacitors, ACS Appl. Mater. Interf. 7 (2015) 4861–4868. [27] C. Min, Z. He, H. Song, D. Liu, W. Jia, J. Qian, Y. Jin, L. Guo, Fabrication of novel CeO2/GO/CNTs ternary nanocomposites with enhanced tribological performance, Appl. Sci. 9 (2019) 170. [28] F. Zhao, W. Huang, H. Zhang, D. Zhou, Facile synthesis of CoNi2S4/Co9S8 composites as advanced electrode materials for supercapacitors, Applied Surface Science 426 (2017) 1206–1212. [29] A.E. Reddy, T. Anitha, C.V.V.M. Gopi, I.K. Durga, H.J. Kim, Facile synthesis of hierarchical ZnMn2O4@ZnFe2O4 microspheres on nickel foam for high-performance supercapacitor applications, New J. Chem. 42 (2018) 2964–2969. [30] S.S. Rao, I.K. Durga, N. Bandari, J.S. Bak, T.N.V. Krishna, C.I. Ho, J.W. Ahn, H.J. Kim, One-pot hydrothermal synthesis of novel Cu-MnS with PVP cabbage-like nanostructures for high-performance supercapacitors, Energies 11 (6) (2018) 1590. [31] D. Punnoose, S.S. Rao, H.J. Kim, Solution processed metal-doped NiS/PEDOT:PSS composite thin films as an efficient electrode for quantum-dot sensitized solar cells, Mater, Res. Bull 102 (2018) 369–378. [32] W. Zhou, X. Cao, Z. Zeng, W. Shi, Y. Zhu, Q. Yan, H. Liu, J. Wang, H. Zhang, Onestep synthesis of Ni3S2 nanorod@Ni(OH)2nanosheet core–shell nanostructures on a three-dimensional graphene network for high-performance supercapacitors, Energy Environ. Sci. 6 (2013) 2216–2221. [33] D. Cai, D. Wang, B. Liu, L. Wang, Y. Liu, L. Han, Y. Wang, L. Qiuhong, T. Wang, Three-dimensional Co3O4@NiMoO4 Core/shell nanowire arrays on Ni foam for electrochemical energy storage, ACS Appl. Mater. Interf. 6 (2014) 5050–5055. [34] F. Zhao, W. Huang, D. Zhou, Chemical bath deposition synthesis of nickel cobalt oxides/sulfides for high-performance supercapacitors electrode materials, J. Alloys
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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Synthesis of CNTs on ZnO/NiS composite as an advanced electrode material for high-performance supercapacitors
T
S. Srinivasa Rao School of Mechanical and Mechatronics Engineering, KyungSung University, 309 Suyeong-ro Nam-gu Busan, 48434, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Supercapacitors ZnO/NiS/CNT composite Galvanostatic charge discharge Cyclic voltammetry Electrochemical impedance spectroscopy
The design and preparation of an efficient electrode material with a multifunctional surface structure is a key challenge for high-performance supercapacitors. In this study, ZnO/NiS and ZnO/NiS/CNT nanocomposites were synthesized on a nickel foam substrate and used as an electrode material. The surface morphology and structure of the electrodes were analyzed by field emission–scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The prepared nanocomposite of ZnO/NiS/CNT exhibited a high specific capacitance of 879.0 F g−1 at a current density of 1.5 A g−1, along with remarkable cycling stability of 89.7% capacitance retention after 5000 cycles, which was superior to that of ZnO/NiS (805.75 F g−1 and 74.59% retention). In addition, the ZnO/NiS/CNT electrode also exhibited good rate capability, faster ion and electron transfer ability, and higher surface area, highlighting its potential for electrochemical energy storage.
1. Introduction Supercapacitors, also known as electrochemical capacitors, are attracting increasing attention in energy storage owing to their rapid charge/discharge rate, high power density, and excellent cycling performance [1–3]. On the other hand, their energy density needs to be improved to meet the ever-increasing requirements for electronic devices, such as portable electronics and electric vehicles [4,5]. Transition metal sulfides, such as NiS, CoS, CuS, and MnS, are new types of energy storage materials that have been studied extensively owing to their excellent electrochemical performance. Metal sulfide-based materials have attracted enormous interest because of their better conductivity. The electronegativity of sulfur is lower than that of oxygen, which allows easier electron transport in the structure. The oxygen can be replaced with sulfur, which provides flexibility in the fabrication of nanomaterials [6,7]. Transition metal sulfides have promising applications in the areas of solar cells, lithium ion batteries, and SCs because of their unique optical and electrical properties. Metal oxide/metal sulfide-based electrode materials are more interesting for SC applications owing to their better capacitance and potential as electroactive materials [8]. A range of sulfide-based electrode materials, such as CoS, NiS, Ni3S2, SnS2, CuS, MnS, NiCo2S4, and Co3S4, have been used as emerging electrode materials. Different types of nanostructures will be responsible for the most effective energy storage, which improves the electrochemical
performance of electrode materials. Nickel-cobalt sulfides have attracted considerable attention because of their richer redox reactions [9,10]. On the other hand, their capacitance at a high rate is inadequate because of the low intrinsic electronic conductivity and unfavorable ion transport length. NiS is a semiconducting material with a variety of compositions that can be utilized in many applications, e.g., SCs, dye and quantum-dot sensitized solar cells. Zhang et al. reported the one pot synthesis of Ni3S4@MoS2 [11]. Zhu et al. synthesized Ni3S2@ multi-walled carbon nanotubes (CNTs) and Krishnamoorthy et al. used a solvent thermal synthesis to synthesize Ni3S2/Ni foam [12,13]. Previous reports explained that NiS is suitable as an electrode material for SCs owing to its excellent electrochemical performance. On the other hand, it still requires a base carrier, such as CNTs and Ni foam, to fully maximize the advantages of electrode materials. NiS have attracted considerable research interest and exhibit good electrochemical performance for applications in pseudo-capacitors, dye-sensitized solar cells, and catalysts [14]. Thus far, great success has been achieved in the synthesis of nanostructured NiS, but their electrochemical performance, particularly cycling stability, is still unsatisfactory [15]. Therefore, improving the electrochemical performance of nickel sulfide is a major research focus. To overcome this, reducing the device size to the nanoscale to increase its active area and mechanical stability is a key concept, and hybridizing the nanostructured pseudo-capacitive materials as a shell with highly conductive
E-mail addresses: [email protected], [email protected]. https://doi.org/10.1016/j.est.2020.101199 Received 13 November 2019; Received in revised form 27 December 2019; Accepted 4 January 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
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vacuum oven at 55 °C for 4 h. The CNT slurry was prepared by mixing CNT powder with the appropriate amount of ethanol and the resulting slurry was loaded on the ZnO/NiS films and dried overnight at 60 °C.
materials as a core can improve the performance of energy storage devices [16,17]. Considerable effort has made to synthesize core/shell nanostructures for high performance SCs. The excellent conductivity and regular tubular framework for anchoring pseudo-capacitive electrode materials makes CNTs an ideal material for core/shell structural design. On the other hand, double-layer capacitance produced by the bare CNT-based materials is very low due to the serous aggregation caused by strong π- π stacking effect. The double-layer capacitance can be enhanced by introducing metal sulfide and metal oxides [18–20]. With this base, the authors developed highly efficient thin-film electrodes by forming an aligned compact of ZnO/NiS/CNT composite on a Ni foam substrate, in which the NiS nanoparticles were bundled tightly by chains of ZnO and CNTs and could maintain good surface contact with each other. The ZnO/NiS/CNT composite, as the active thin film electrode, exhibited a high specific capacitance of 879.0 F g−1 at a current density of 1.5 A g−1, good rate performance (38.11% capacitance retention from 1.5 to 8 A g−1), and cycling stability (89.7% capacitance retention after 5000 cycles). To the best of the authors’ knowledge, the composite exhibited much better electrochemical performance than that of ZnO/NiS as well as other similar materials reported in the literature.
2.4. Electrochemical measurement Electrochemical measurements were performed using a Bio-LogicSP150 workstation interfaced to a computer system with electrochemical software. A 10-mg sample of the as-prepared electrodes was incorporated into nickel foam (1.0 cm × 1.0 cm). A three-electrode glass cell setup was used in the experiment consisting of a working electrode, platinum wire as the counter electrode, and saturated Ag/ AgCl as the reference electrode at 25 °C in 3 M KOH. The electrodes were soaked in a KOH solution and degassed in a vacuum for 5 h before the electrochemical test. All electrodes were tested in a 3 M KOH aqueous solution with a potential range between −0.5 to 0.7 V (vs. Ag/ AgCl). The galvanostatic charge/discharge curves were obtained at current densities of 1.5, 2.0, 2.5, 3.0, 3.5, 4, 5, 6, 7 and 8 A g−1. The specific capacitance of the electrode was calculated from the galvanostatic charge/discharge curves using Eq. (1):
C= 2. Materials and methods
I × Δt m ×V
(1) −1
Where C = specific capacitance (F g ), m = mass of the electrode (g), V = voltage window during the discharge process (V), i = discharge current, Δt = discharge time difference.
2.1. Preparation of Nickel foam substrate Zinc acetate dehydrate (Zn (CH3COOH)2.2H2O), zinc nitrate hexahydrate (Zn (NO3)2.6H2O), Nickel (II) chloride hexahydrate (NiCl2.6H2O), hexamethylenetetramine (HMT), Nickel(II) chloride hexahydrate (NiCl2.6H2O), thiourea (CH4N2S), 3-Mercaptopropionic acid (3-MPA), and ethylene glycol were purchased from Aldrich and used as received. Before the deposition of ZnO/NiS and ZnO/NiS/CNT, the nickel foam (approximately 1 cm × 1 cm) was cleaned carefully in a concentrated HCl solution (37 wt. %) by sonication for 10 min to remove the surface oxide layer. The foam was then washed sequentially with acetone, ethanol, and deionized water for 10 min each to ensure that the surface of the Ni foam was well cleaned and dried with a hair dryer.
2.5. Characterization The microstructure of the synthesized samples was measured by Schottky emission scanning electron microscopy (SEM; SU-70, Hitachi) at the Busan KBSI. The elemental compositions of the electrodes were investigated by field emission scanning electron microscopy (FE-SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX) operated at 15 kV. The crystalline structure and morphology of the prepared electrodes were investigated by high-resolution transmission electron microscopy (HRTEM; Jem 2011, Jeol cop.). High-resolution X-ray diffraction (XRD) was performed on a D8 ADVANCE with a DAVINCI (Bruker AXS) diffractometer using Cu Kα radiation and operated at 40 kV and 40 mA. X-ray photoelectron spectroscopy was also conducted (XPS, PHI 5000C ESCA System). Prior to the measurement, the samples were dehydrated at 90 °C for 5 h. Transmission electron microscopy (TEM, Jeol, JEM 2011) was performed at the Busan KBSI center and operated at 200 kV using a CCD camera 4k × 4k.
2.2. Synthesis of ZnO nanorods ZnO NRs were prepared using a two-step facile chemical bath deposition method. The first step was to coat a ZnO seed layer on the Ni foam. The ZnO seed layer was prepared using zinc acetate dihydrate 10 mM [Zn (CH3COOH)2. 2H2O] as a precursor, dissolved in ethanol and stirred for 30 min. After growth of the seed layer, the samples were annealed at 65 °C for 5 h in ambient air. The second step was to grow the ZnO NRs on the seeded Ni foam using a chemical bath deposition method. The seeded substrate was suspended upside down in an aqueous solution containing 0.015 M zinc nitrate hexahydrate and HMT at 95 °C for 15 h to grow the ZnO NRs. After cooling naturally to room temperature, the prepared ZnO NRs on Ni foam substrate was washed with deionized water and dried in an oven at 65 °C for 5 h.
3. Results and discussion HR-SEM (Fig. 1a–d) revealed the formation of ZnO/NiS and (Fig. 1e and f) ZnO/NiS/CNT on the nickel foam at high and low magnifications. CNTs were introduced to suppress the agglomeration of the ZnO/ NiS nanocomposite. The CNTs act as nucleation sites and might be responsible for the inhibition in their ZnO/NiS/CNT [21]. The previous studies suggested that a small quantity of individually dispersed carbon nanotubes can affect the active material orientation and crystallization [22]. Moreover, this might have a synergic effect and improve the electrical connection between ZnO, NiS, and the conducting substrate. The growth of such uniformly distributed and porous nanostructures provides a large surface area that enhances the electrochemical performance [23,24]. The agglomeration problem was resolved by adding CNTs to the ZnO/NiS nanocomposite. The free space between the interconnected CNTs could facilitate the transport of electrolyte and ions. In addition, the CNT network framework with a NiS rough surface endows the electrode with a large specific surface area that favors charge carrier transport. The uniform distribution of NiS nanoparticles arose from the growth
2.3. Synthesis of NiS and CNTs on the ZnO NRs The host precursor materials, such as 0.1 M NiCl2.6H2O and 0.4 M thiourea, were dissolved in 40 mL of ethylene glycol. Subsequently, 0.2 M 3-MPA was added to the above mixture with strirring. The mixture was kept stirring for 60 min until complete dissolution. The mixture was then transferred to a Teflon-lined autoclave and ZnO-deposited Ni foams were placed into the reaction mixture. The hydrothermal reaction system was conducted at 150 °C for 10 h. The autoclave was cooled naturally to room temperature and the as-obtained ZnO/NiS material was filtered and washed sequentially with DI water and ethanol two to three times each. The ZnO/NiS were dried in a 2
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Fig. 1. High-resolution HR-SEM images of (a), (b), (c), (d) ZnO/NiS and (e), (f) of ZnO/NiS/CNT and their corresponding high magnification images.
Fig. 3. XRD patterns of ZnO/NiS and ZnO/NiS/CNT electrodes on nickel foam substrates.
was analyzed further by XRD. As shown in Fig. 3, the peaks at (100), (002), (101), and (103) were assigned to the ZnO phase (JCPDS no: 361451). The peaks at 30.2°, 34.8°, 37.9°, 53.5°, and 73.3° 2θ were attributed to the NiS phase (JCPDS no: 02-1280) and some other diffraction peaks located at 44.5°, 51.8°, and 76.4° were obtained from the nickel foam substrate. The XRD peaks for NiS and ZnO either disappeared or the intensity of the peaks decreased dramatically after deposition with CNTs. XPS is an analytical technique that can be used not only for elemental identification within a sample but also to identify the oxidation state of the element. This technique can be used to investigate areas, such as corrosion, oxidation, surface contamination and modification, absorption and deposition of chemical species, catalysis, and many other nanoscale surface processes. The survey spectrum of ZnO/NiS/ CNT (Fig. 4b) revealed S2p, C1s, O1s, Ni2p3, Zn2p3, and N1s at 162.88, 284.6, 531.14, 855.5, 1045.5 and 399.72 eV, respectively. The O1s, C1s, and N1s peaks were attributed to exposure of the fabricated electrode to oxygen molecules or with moisture. The high resolution XPS spectra of Zn2p, S2p, Ni2p, and C1s elements in ZnO/NiS/CNT composite are shown in Fig. 5. Fig. 5a shows the two main peaks of Zn2p located at 1022.24 eV and 1045.55 eV, which were assigned to Zn2p3/2 and Zn2p1/2, respectively. The strong resolution Ni2p spectrum (Fig. 5b) at 855.5 eV could be deconvoluted
Fig. 2. TEM images of (a), (b) ZnO/NiS and (e), (f) of ZnO/NiS/CNT on a nickel foam substrate.
mechanism and NiS stems formed on the ZnO, which increases the number of reactive sites for the reaction. Consequently, the void spaces between the ZnO/NiS stems were filled with CNTs, which formed a network wrapping. This increases the electrical conductivity between the material, yielding more defects and an enhanced charge transport process. HR-SEM suggested that during the CNT coating on the networks on the ZnO/NiS stem-like nanostructure, the particle sizes were reduced significantly and coated uniformly on entire Ni foam substrate which may increases the surface area. The nanoparticles on the porous structures can enable the transport of electrolyte and ensure efficient contact between the active material and the KOH electrolyte for the electrodes. TEM was used to reveal the detailed structural and morphological properties of the metal sulfides with CNTs, as shown in Fig. 2. The crystal structure of the ZnO/NiS and ZnO/NiS/CNT electrodes 3
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Fig. 4. XPS survey spectrum of ZnO/NiS and ZnO/NiS/CNT electrodes on a nickel foam substrate.
which was higher than that of the ZnO/NiS (9.38 m2/g). These results indicate that the mesoporous structure combined with large surface area of ZnO/NiS/CNT composite is quite advantageous to endow ample electroactive sites and shorten the charge/ion transport path which is tremendously helpful for Faradaic redox reaction [28]. The electrochemical properties of the as-prepared ZnO/NiS and ZnO/NiS/CNT electrodes were analyzed by cyclic-voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) using a three-electrode system. Both electrodes were evaluated as binder-free electrodes for supercapacitors in a 3 M KOH electrolyte. Fig. 7a shows the CV curves of the ZnO/NiS and ZnO/NiS/ CNT composites at 20 mV s−1. The CV curves comprise two pairs of well-defined redox couples in ZnO/NiS and ZnO/NiS/CNT composites, which could be ascribed to the Faradaic redox reaction in KOH solution.
to S2p3/2 (855.45 eV) and S2p1/2 (873.42 eV) and the two main peaks separated by 17.97 eV. As shown in Fig. 5c, the two main peaks separated by 4.8 eV at 162.88 and 167.68 eV were indexed to sulfide [25,26]. Fig 5d shows the XPS spectra of C 1s for all the CNTs in the ZnO/NiS/CNT samples. The C 1 s spectra were composed of several characteristic peaks: two peaks due to the carbon-carbon interactions, including CeC s2p bonds at binding energies of 284.4–284.7 eV and CeC sp3 bonds at 285.1–285.5 eV; and two relatively weak peaks due to the carbon-oxygen interactions, including C-O bonds at 286.4–286.7 eV and C]O bonds at 287.8–288.1 eV [27]. To investigate the specific surface are of ZnO/NiS and ZnO/NiS/CNT composite were investigated by nitrogen adsorption/desorption isotherms (BET) and the results are shown in Fig. 6. The BET specific surface area of the prepared ZnO/NiS/CNT composite is calculated to be 10.77 m2/g,
Fig. 5. High-resolution scan of the (a) Zn2p, (b) Ni2p, (c) S2p and (d) C1s peaks in ZnO/NiS/CNT composite. 4
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area were observed, which illustrates the higher energy storage performance of the ZnO/NiS/CNT composite compared to ZnO/NiS composite. In addition, the oxidation peaks were shifted to more positive side and reduction peaks were shifted to more negative potential with increment of scan rates, demonstrating the good ion diffusion rate and lower resistance of the ZnO/NiS/CNT composite during the electrochemical redox reaction [29,30]. To check the electrochemical effect for the Ni foam substrate, we also performed CV test for bare Ni foam (Fig. S1 and Fig. S2) at 50 mV s−1 and GCD test in KOH solution. Compared to the ZnO/NiS and ZnO/NiS/CNT composite, the oxidation and reduction current of Ni foam is quite small and almost negligible, indicating that the Ni foam contributes very little to the total specific capacitance of the electrode. Fig. 8 presents typical GCD curves of the ZnO/NiS and ZnO/NiS/ CNT electrodes at various current densities. The GCD curve measured at 1.5 A g−1 showed a larger enclosed area for the ZnO/NiS/CNT (Fig. 8b), which is higher than that of the ZnO/NiS-based electrode (Fig. 8a). The specific capacitance of the ZnO/NiS and ZnO/NiS/CNT electrodes were 805.75, 744.56, 702.75, 669.8, and 622.3 F g−1 and 879, 812.25, 766.63, 730.69 and 678.87 F g−1 at a current density of 1.5, 2, 2.5, 3, and 3.5 A g−1, respectively. The specific capacitance of the ZnO/NiS/CNT electrodes was much higher than that of the ZnO/ NiS. Moreover, when the current density was increased from 1.5 to 8 A g−1, the ZnO/NiS/CNT electrode exhibited good rate capability and capacitance retention of 38.11%, which was higher than that of the ZnO/NiS (28.24%). These observations have many explanations. First, there is abundant space between the materials and highly uniform nanowire structure on the nickel foam substrate, which can act as “ion-
Fig. 6. The Brunauer–Emmett–Teller (BET) surface area obtained from nitrogen adsorption-desorption isotherms.
The CV integrated area, oxidation and reduction currents of the ZnO/ NiS/CNT electrode were much higher than those of the ZnO/NiS electrode, highlighting the significant energy storage performance of the ZnO/NiS/CNT. Moreover, as the scan rate was increased from 10 to 60 mV s−1 for ZnO/NiS/CNT composite (Fig. 7c), similar CV shapes with a pair of redox peaks, higher peak current values with enlarged CV
Fig. 7. CV curves of (a) ZnO/NiS and ZnO/NiS/CNT electrodes at 20 mV s−1 in 3 M KOH solution, CV curves of (b) ZnO/NiS and (c) ZnO/NiS/CNT at various current rates (10–60 mV s−1). 5
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Fig. 8. GCD curves of (a) ZnO/NiS and (b) ZnO/NiS/CNT electrodes at a current density of 1.5, 2.0, 2.5, 3.0, and 3.5 A g−1 in a 3 M KOH solution. (c) Specific capacitance at different current densities. (d) Long-term cycling stability of the ZnO/NiS and ZnO/NiS/CNT electrodes for 5000 GCD cycles in 3 M KOH solution.
performance of ZnO/NiS and ZnO/NiS/CNT electrodes at a current density of 1.5 A g−1 for 5000 cycles in a 3 M KOH solution. The ZnO/ NiS electrode showed a specific capacitance of 601.0 F g−1 after 5000 GCD cycles and 74.59% capacitance retention. In contrast, the ZnO/ NiS/CNT electrode showed higher capacitance retention of 89.9% after 5000 cycles. Moreover, the slight increment of specific capacitance was observed at the beginning of few cycles is probably due to the activation process of electroactive material. The higher capacitance retention of ZnO/NiS/CNT could be attributed mainly to the stable electroactive sites and strong surface morphology on a nickel foam substrate even when the ions were intercalated/extracted into/out of the structure. EIS was conducted to evaluate the kinetic properties of the electrodes. The ZnO/NiS and ZnO/NiS/CNT electrodes are comprised of two parts (Fig. 9), i.e., semicircle part in the high frequency region and vertical line in the low frequency region. The spectra of the ZnO/NiS/ CNT electrode showed a more vertical line in the low-frequency region and was almost perpendicular to the real axis, revealing a more efficient
buffering reservoirs” for ions and may reduce the diffusion distance from the KOH electrolyte to the interior surface [31]. Second, the higher electrical conductivity of the CNTs can reduce the charge transfer resistance of the entire electrode effectively. Third, the ZnO/ NiS/CNT electrode avoids the use of conducting additives and polymer binders, enhancing the consumption of the active material. Fourth, the CNT nanowires afford an enormous number of active sites for the ZnO/ NiS nanostructure, and great mechanical adhesion on the nickel foam substrate and electrical connection of the active material to the current collector, which can lead to a larger surface area [32]. The electrochemical properties of the ZnO/NiS/CNT electrode in this study were also similar or superior to than those in other studies (Table 1), such as pristine Co3O4 (590 F g−1 at 10 A g−1), NiCoXOY (507. F g−1 at 1 A g−1), Zn-WO3 (35.70 F g−1), Ag-doped PEDOT:PSS/ CNT (85.3 F g−1), and NiCoO2/rGO composite (742 F g−1 at 10 A g−1) [33–37]. The long-term stability is an important factor to identify the stability of active material in supercapacitor. Fig. 8d shows the cycling
Table 1 Comparison of specific capacitance performance of ZnO/NiS/CNT composite with various literature. Material
Electrolyte
Specific capacitance
Current density/scan rates
Ref
TiO2-MWCNT Ag-doped PEDOT:PSS/CNT Fe-TiO2/C nanofibers WS2/CFC WO3-X@MnO2 NWs Fe2O3@LiCoO2 NiCoO2/rGO composite ZnMn2O4@Mn3O4 CC@NiCo2O4@ZnWO4 ZnO/NiS/CNT
1 – 1 1 – 3 – 1 – 3
160 F g−1 85.3 F g−1 137 F g−1 399 F g−1 341 F g−1 489 F g−1 742 F g−1 321.34 F g−1 872.0 F g−1 879.0 F g−1
1 A g−1 – 5 mV s−1 1 A g−1 10 mV s−1 5 mA cm−2 10 A g−1 1 mV s−1 1 A g−1 1.5 A g−1
[38] [36] [39] [40] [41] [42] [37] [43] [44] This study
M H2SO4 M KOH M KCL M KOH M Na2SO4 M KOH
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Acknowledgment This research was supported by Kyungsung University Research Grants in 2019. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2020.101199. References [1] C. Meng, C. Liu, L. Chen, C. Hu, S. Fan, Highly flexible and all-solid-state paperlike polymer supercapacitors, Nano Lett. 10 (2010) 4025–4031. [2] B. Liu, X.F. Wang, H.T. Chen, Z.R. Wang, D. Chen, Y.B. Cheng, C.W. Zhou, G.Z. Shen, Hierarchical silicon nanowires-carbon textiles matrix as a binder-free anode for high-performance advanced lithium-ion batteries, Sci. Rep. 3 (2013) 1622. [3] I.K. Durga, S.S. Rao, A.E. Reddy, C.V.V.M. Gopi, H.J. Kim, Achieving copper sulfide leaf like nanostructure electrode for high performance supercapacitor and quantum-dot sensitized solar cells, Appl. Surface Sci. 435 (2018) 666–675. [4] P. Simon, Y. Gogotsi, Capacitive energy storage in nanostructured carbon–electrolyte systems, Acc. Chem. 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Fig. 9. EIS of ZnO/NiS and ZnO/NiS/CNT electrodes over the frequency range of 100 kHz to 0.1 Hz.
electrolyte, proton diffusion, and lower ion transfer resistance in the electrode. These properties can be attributed to the uniform nanowires of ZnO/NiS/CNT with a higher surface area and abundant open spaces, which increased the use of the active electrode and the supply of OH− ions [45]. In the high-frequency region, ZnO/NiS/CNT showed a negligible series resistance and lower charge transfer resistance than that of the ZnO/NiS electrode, demonstrating the high electron transfer rate at the electrode/electrolyte interface and the extremely high electrochemical activity for energy storage. The significant enhancement of specific capacitance may be attributed to the following points: (a) the Ni foam substrate with great surface area can increases the rapid ion/ electron transfer rate due to its high conductivity and also acts as a scaffold for fabrication of high-performance electroactive materials; (b) synergetic effect of the unique structure; (c) ZnO and NiS nanoparticles are well decorated on Ni foam substrate with CNT that provide more active sides for redox reactions; (d) direct growth of ZnO/NiS/CNT composite on Ni foam without any polymer binders and conductive additives, which can control/reduce the dead mass and hence improves the electron transportation. Accordingly, the ZnO/NiS/CNT composite contains higher electron and ionic conductivity, which leads to improved energy storage performance.
4. Conclusions ZnO/NiS and ZnO/NiS/CNT nanostructures were prepared on a nickel foam substrate and used as an electrode material for supercapacitors. The ZnO/NiS/CNT nanocomposite exhibited greater electrochemical performance with a higher specific capacitance of 879.0 F g−1 at a current density of 1.5 A g−1 and excellent cycling stability (89.7% after 5000 cycles at 1.5 A g−1), and 678.87 F g−1 was maintained at 3.5 A g−1. The ZnO/NiS/CNT electrode exhibited higher electrochemical performance than the ZnO/NiS, which can be explained by the following. First, there was abundant space between the materials and highly uniform nanowire structure on the nickel foam substrate, which can act as “ion-buffering reservoirs” for ions and may reduce the diffusion distance from the KOH electrolyte to the interior surface. Second, the higher electrical conductivity of the CNTs can reduce the charge transfer resistance of the entire electrode effectively. These features make the ZnO/NiS/CNT composite an appropriate and auspicious electrode material for efficient supercapacitor applications.
Declaration of Competing Interest The authors declare no conflict of interest. 7
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