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Microwave-assisted rapid synthesis of NiCo2S4 nanotube arrays on Ni foam for high-cycling-stability supercapacitors
Journal of Alloys and Compounds 780 (2019) 164e169
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microwave-assisted rapid synthesis of NiCo2S4 nanotube arrays on Ni foam for high-cycling-stability supercapacitors Mingliang Guo, Haixing Gao, Wei Huang, Jieqiong Wang, Zheng Liu, Changhong Zhan, Lei Ding*, Jinchun Tu State Key Laboratory of Marine Resource Utilization in South China Sea, College of Materials and Chemical Engineering, Hainan University, Haikou 570228, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2018 Received in revised form 14 November 2018 Accepted 25 November 2018 Available online 28 November 2018
In this work, NiCo2S4 nanotube arrays are grown in suit on a Ni foam through a rapid microwave-assisted sulfidation process from analogous structure of Ni-Co precursors. This uniform nanotube array structure yields a high specific capacitance of 8.6 F cm2 at 10 mA cm2, and superb cycle property without loss of function after 48000 cycles at 50 mA cm2. Benefitted by the nanotube structure and the unique triangular supporting system from the electrochemical activation, the cycling stability of ternary nickel cobalt sulfide is maintained at a high level with superior specific capacitance. Therefore, this novel structure has some reference significance for further research of supercapacitors. © 2018 Elsevier B.V. All rights reserved.
Keywords: Supercapacitors NiCo2S4 Nanotube arrays Cycling stability
1. Introduction In the past several years, there is a soaring number of researches in new-style developing and efficient energy-storage devices for energy crisis [1e5]. Supercapacitors, as an energy storage device with splendid power density and moderate energy density, shorter charging time, and wider working temperature range, have been extensively studied recently [6e8]. Based on the energy storage mechanism, supercapacitors can be divided into electrical doublelayer capacitors (EDLCs) [9e11] and pseudocapacitors [12e15]. Compared with EDLCs, pseudocapacitors have a greater potential in specific capacitance due to its higher reversible redox reactions on the surface of electrode. Therefore, it is crucial in the selection of electrode active materials for the study of pseudocapacitors. As we known, the previous studies regarding pseudocapacitors have demonstrated that transition-metal oxides [16,17], hydroxides [18,19], sulfides [20e22], and conducting polymers [23,24], frequently encountered problems such as low conductivity and poor stability, which largely hindered their commercial applications and promotions. Fortunately, among these promising candidates, NiCo2S4 [25e30] has attracted more attention for two
* Corresponding author. E-mail address: [email protected] (L. Ding). https://doi.org/10.1016/j.jallcom.2018.11.340 0925-8388/© 2018 Elsevier B.V. All rights reserved.
reasons: (1) They can provide multiple redox reactions during the electrochemical processes, and (2) they possess a higher electronic conductivity than the homologous metal oxides [31]. The NiCo2S4 porous spheres were successfully synthesized by Cheng et al. [32], who produced a nice capacitive property of 1870.2 F g1 at 2 A g1 (about 9% loss after 3000 cycles at 10 A g1). Zhang et al. [33] prepared the composite electrode of NiCo2S4 with 3D graphene foam which exhibited an excellent electrochemical performance of 1454.6 F g1 at 1.3 A g1 (about 4% loss after 3000 cycles at 13 A g1). Cockscomb flower-like NiCo2S4 nanostructures grown on Ni foam disclosed a unit mass capacitance of 1707 F g1 at 1 A g1 (about 11% loss after 4000 cycles at 5 A g1) [34]. However, NiCo2S4 electrodes are often constrained by poor rate capacity and cycling property due to the inefficient ion transmission from electrolytes to the active material and the structural degradation during electrochemical reaction. Hereby, we are excited to proclaim that these limitations are tackled in the present work via microwave-assisted rapid synthesis method due to its considerable boost in electrochemical performance, especially in terms of cycle stability. In this work, NiCo2S4 nanotube arrays are fabricated on a Ni foam (NF) via the rapid microwave-assisted sulfidation process to serve as a freestanding electrode. Then, the unique nanotube structure can supply abundant redox reaction sites, afford plenty of passages for ion transmission, and form a triangular structure to consolidate the cycle-ability. Based on the above results, it corroborates that
M. Guo et al. / Journal of Alloys and Compounds 780 (2019) 164e169
NiCo2S4 nanotube arrays on Ni foam are a type of prospective electrode with fortified cycling stability for high-performance supercapacitor applications.
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determined by using FESEM (Hitachi S4800) and TEM (JEM 2100). In addition, their phases and chemical states were observed through XRD (D8 Advanced XRD, Cu-Ka radiation, 40 kV, 40 mA), and XPS (AXIS SUPRA, C1s 284.8 eV).
2. Experimental section 2.3. Electrochemical measurements 2.1. Synthesis of NiCo2S4 composite electrode The chemicals for the experiment were used without processed and the experiment was operated in two steps. Step 1, the precursor was prepared by typical hydrothermal synthesis [35]. First, Ni foam (2.5 cm 3 cm 1.5 mm) substrates were pre-cleaned with dilute hydrochloric acid (HCl), deionized (DI) water, and ethanol to eliminate impurities and nickel oxides. Then, 2 mmol CoCl2$6H2O, 1 mmol NiCl2$6H2O, and 6 mmol urea were added into 18 ml DI water. After stirring for a few minutes, the prepared solution and the treated NF were transferred into a 30 ml Teflon-lined autoclave, heated at 120 C for 6 h for a hydrothermal reaction. Finally, the synthesized precursors were cleaned ultrasonically, and dried at 55 C overnight. Step 2, the precursors were converted into NiCo2S4 by the microwave-assisted method. First, a total of 5 mmol Na2S$9H2O was added into 50 ml DI water to form a mixed solution. Afterwards, the mixture was poured into a 100 ml microwave reaction vessel with the as-obtained precursors and treated with 800 W microwave irradiation in a dual-control microwave digestion apparatus at 110 C for 0.5 h. After restored to normal temperature, the black product loaded on the Ni foam was obtained through washing with DI water and ethanol for several times and dried at 60 C overnight. The final mass loading of NiCo2S4 on the Ni foam was approximately 6 mg cm2. 2.2. Materials characterization The microstructure of the as-prepared NiCo2S4 electrodes was
A series of electrochemical performance of the NiCo2S4 electrodes (1 cm 1 cm) were characterized by the Cyclic voltammetry (CV), Galvanostatic chargeedischarge (GCD), and electrochemical impedance spectra (EIS) using a traditional three-electrode system in 1 M KOH solution. The synthesized composites, platinum plate (2 cm 2 cm), and standard calomel electrode were used as the working, counter, and reference electrodes, respectively. The CV tests were carried out at various scan rates ranging from 1 mV s1 to 25 mV s1 at a potential window of 0 Ve0.6 V (vs. SCE). The GCD curves were conducted with the potential window of 0e0.4 V at different current densities varying from 5 mA cm2 to 50 mA cm2. The EIS measurements were tested in a frequency range from 100 kHz to 0.01 Hz at the open circuit potential with an AC potential amplitude of 10 mV. 3. Results and discussion As illustrated in Fig. 1, a facile microwave-assisted route is developed to synthesize NiCo2S4 nanotube-assembled arrays via an anion-exchange reaction of Ni-Co precursors [36e38]. The Ni foam is selected as the supporter of active material because of its excellent electrical conductivity and large specific surface area provided by the 3D porous structure (Fig. 1a). Initially, uniform Ni-Co precursor nanowires are grown on 3D conductive substrate via a simple hydrothermal reaction (Fig. 1b). Subsequently, the asfabricated precursors are completely converted into spinel NiCo2S4 though a microwave-assisted sulfidation process (Fig. 1c). Evidently, the NiCo2S4 nanotube arrays can entirely preserve the
Fig. 1. Schematic of the synthesized process of NiCo2S4 composite electrode. (a) Three-dimensional Ni foam; (b) Ni-Co precursor (inset, high resolution image); (c) NiCo2S4 nanotube arrays, (inset, high resolution image).
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morphology of the Ni-Co precursor. A close observation (inset in Fig. 1c) unfolds that the hybrid products possess an average diameter of 150 nm and are rougher than the precursors (inset in Fig. 1b). During the anion exchange reaction, microwave heating is employed for its faster and more uniform heating effect than that of traditional hydrothermal reactions. Through that way, there is a great acceleration in the exchange rate of sulfur anions with CO2 3 and OH anions in the precursors, and in better acquisition of fully
converted homogeneous NiCo2S4 nanotube arrays on the Ni foam. Nuanced morphologies of NiCo2S4 nanotube was investigated by TEM, as shown in Fig. 2. Fig. 2a manifests that NiCo2S4 nanotube has a diameter of approximately 150 nm and which is in accordance with the FESEM results. An amplified view of an individual nanotube in Fig. 2b evinces that the micro structure was comprised of random oriented nanoparticles. The SAED pattern in the inset of Fig. 2d displays the polycrystalline nature of the NiCo2S4 nanotube,
Fig. 2. (aec) Low-resolution TEM images, (d) HR-TEM image, (inset, SAED pattern), and (eeh) A local element mapping of composite electrode of S, Ni, and Co.
Fig. 3. (a) XRD of the NiCo2S4 composite electrode and XPS analysis of (b) Co 2p, (c) Ni 2p, and (d) S 2p.
M. Guo et al. / Journal of Alloys and Compounds 780 (2019) 164e169
which is composed of many nanocrystallites (Fig. 2c). The elemental maps (Fig. 2eeh) substantiate the existence of S, Ni, and Co elementals and implicit a uniform spatial elemental distribution on the composite electrode. Further examinations of the crystal structure and elements state of the electrode materials are implemented by XRD and XPS analyses. As shown in Fig. 3a, the diffraction peaks at around 31.5 , 38.1, 47.7, 50.6 , and 54.9 are consistent with (311), (400), (422), (511), and (440) planes of the cubic NiCo2S4 phase, respectively (JCPDS no. 43-1477) [39]. Moreover, three peaks at 44.4 , 51.8 , and 76.4 are concerted with the metallic Ni (JCPDS no. 03-1043). Furthermore, XPS is also conducted to dissect the chemical composition and surface state of NiCo2S4 sample. The survey spectrum (Fig. S1) represents the presence of Co, Ni and S elements, which is reconciled with the obtained typical ternary metal sulfide
167
product of NiCo2S4. The Co 2p XPS spectrum in Fig. 3b manifests the peaks at 782.6 eV in Co 2p3/2, and 797.4 eV in Co 2p1/2 are consistent with Co2þ. In addition, 781.1 eV in Co 2p3/2 and 795.3 eV in Co 2p1/2 can be ascribed to Co3þ. Similarly, as illustrated in Fig. 3c, the peaks of the spectrum expound the presence of both Ni2þ and Ni3þ. With regard to the S 2p XPS pattern (Fig. 3d), the binding energy at 163.5 eV (S 2p1/2) and 161.8 eV (S 2p3/2) are connected with the typical metal-sulfur bonding [40], and the S2 in low-coordination [41], respectively. The XPS results are relatively concordant with the preceding XRD analysis. In the light of the analysis of TEM, XRD and XPS, it can be concluded that the array structure is composed of NiCo2S4 nanotubes. The electrochemical performance was characterized by CV, GCD, EIS, and cycling stability tests. Fig. 4a elucidates the CV curves of NiCo2S4 at various scan rates from 1 to 25 mV s1. A couple of redox
Fig. 4. Electrochemical measurements of the NiCo2S4 composite electrode. (a) CV curves; (b) Charge-discharge curves; (c) The specific capacitance of the NiCo2S4 electrode at different current densities; (d) Nyquist plots before and after charge-discharge test (inset is the illustration of the NiCo2S4 triangular system on Ni foam); (e) Cycling performance of NiCo2S4 electrode at 50 mA cm2, (inset is SEM images of NiCo2S4 sample after 48000 cycles in the triangular structure).
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M. Guo et al. / Journal of Alloys and Compounds 780 (2019) 164e169
peaks are apparent in Fig. 4a, which can be ascribed to the redox reactions of NiCo2S4 on the basis of the following reactions [42]:
structure is of referential significance for the commercial application of supercapacitors.
NiCo2S4 þ OH 4 NiSOH þ CoSOH þ e
(1)
Acknowledgements
CoSOH þ OH 4 CoSO þ H2O þ e
(2)
This work was supported by the National Natural Science Foundation of China (No.51461014 and 51762012), Key Research and Development Project of Hainan Province (No. ZDYF2018106).
GCD tests are also carried out to analyze the capacitive performance of the composite electrode. The GCD curve in Fig. 4b explicates that the potential-time curves at various current density from 5 to 50 mA cm2 are nearly symmetric, which affirms the brilliant Coulomb efficiency in the electrode. Moreover, the appearance of two visible voltage platforms corroborates the pseudocapacitive characteristic of NiCo2S4 [43,44], which conforms to the results acquired from the CV curve. The capacitive performance of the NiCo2S4 electrode was calculated from the following equation [45]: Ca¼ IDt/(SDV). In this equation, Ca (F cm2), I (mA), Dt (s), DV (V) and S (cm2) were areal capacitance, constant discharge current, discharge time, window voltage upon discharging, and area of the samples, respectively. It is noticeable that the NiCo2S4 electrode exhibits a remarkable areal capacitance of 5.7 F cm2 at 50 mA cm2, insinuating its extraordinary rate capacity (Fig. 4c). The cycling performance, a crucial requirement for supercapacitors in commercial application, is also investigated by repeated charge and discharge experiments at a rated current of 50 mA cm2, as shown in Fig. 4e. Interestingly, there is approximately no loss after 48000 successive cycles. These results verify the superb cycle stability of the NiCo2S4 electrode. Furthermore, SEM images of the NiCo2S4 nanotubes after the cycling tests demonstrate that the microstructure of NiCo2S4 became slightly thicker and rougher even after 48000 cycles (Fig. S2). The EIS measurements (Fig. 4d) are performed to further explain the excellent electrochemical performance of the NiCo2S4 electrode. The charge-transfer resistance (Rct) of the NiCo2S4 sample was slightly decreased after 48000 cycles, further indicated extraordinary cycling stability. Surprisingly, there was a smaller ion diffusion resistance in the electrode after the test, which proved that the triangulation system was more conducive to ion diffusion than other electrodes. The superior capacitive property and cycling stability of the NiCo2S4 electrode can be ascribed to the following reasons: (1) The as-prepared uniform and separated NiCo2S4 nanotube structure provides enough connections between the electrolyte and the active materials, which considerably stimulates the overall utilization of the electrode. (2) The nanotubes are closer enough to form a triangular structure (inset in Fig. 4e) during the electrochemical reaction process, which not only alleviates the stress on the internal nanotubes induced by abrupt volume changes but also supplies a great number of channels for ion and electron diffusion (inset in Fig. 4d). Hence, the triangular system assembled by NiCo2S4 nanotubes is a jarless structure for reinforcing electrochemical performance and cycling stability. 4. Conclusions A simple and fast microwave-assisted sulfidation strategy of synthesizing uniform NiCo2S4 nanotube arrays on a threedimensional Ni conductive substrate is affirmed in this work. The composite electrodes composed of NiCo2S4 nanotubes disclose a marvelous electrochemical performance with a high specific capacitance of 8.6 F cm2 at 10 mA cm2, and the special triangular structure demonstrates an eminent cycling stability of approximately 100% retention after 48000 cycles. The low-cost and rapid synthetic process facilitates the promotion of commercial capacitors, and the prominent cycling stability generated by this unique
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microwave-assisted rapid synthesis of NiCo2S4 nanotube arrays on Ni foam for high-cycling-stability supercapacitors Mingliang Guo, Haixing Gao, Wei Huang, Jieqiong Wang, Zheng Liu, Changhong Zhan, Lei Ding*, Jinchun Tu State Key Laboratory of Marine Resource Utilization in South China Sea, College of Materials and Chemical Engineering, Hainan University, Haikou 570228, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2018 Received in revised form 14 November 2018 Accepted 25 November 2018 Available online 28 November 2018
In this work, NiCo2S4 nanotube arrays are grown in suit on a Ni foam through a rapid microwave-assisted sulfidation process from analogous structure of Ni-Co precursors. This uniform nanotube array structure yields a high specific capacitance of 8.6 F cm2 at 10 mA cm2, and superb cycle property without loss of function after 48000 cycles at 50 mA cm2. Benefitted by the nanotube structure and the unique triangular supporting system from the electrochemical activation, the cycling stability of ternary nickel cobalt sulfide is maintained at a high level with superior specific capacitance. Therefore, this novel structure has some reference significance for further research of supercapacitors. © 2018 Elsevier B.V. All rights reserved.
Keywords: Supercapacitors NiCo2S4 Nanotube arrays Cycling stability
1. Introduction In the past several years, there is a soaring number of researches in new-style developing and efficient energy-storage devices for energy crisis [1e5]. Supercapacitors, as an energy storage device with splendid power density and moderate energy density, shorter charging time, and wider working temperature range, have been extensively studied recently [6e8]. Based on the energy storage mechanism, supercapacitors can be divided into electrical doublelayer capacitors (EDLCs) [9e11] and pseudocapacitors [12e15]. Compared with EDLCs, pseudocapacitors have a greater potential in specific capacitance due to its higher reversible redox reactions on the surface of electrode. Therefore, it is crucial in the selection of electrode active materials for the study of pseudocapacitors. As we known, the previous studies regarding pseudocapacitors have demonstrated that transition-metal oxides [16,17], hydroxides [18,19], sulfides [20e22], and conducting polymers [23,24], frequently encountered problems such as low conductivity and poor stability, which largely hindered their commercial applications and promotions. Fortunately, among these promising candidates, NiCo2S4 [25e30] has attracted more attention for two
* Corresponding author. E-mail address: [email protected] (L. Ding). https://doi.org/10.1016/j.jallcom.2018.11.340 0925-8388/© 2018 Elsevier B.V. All rights reserved.
reasons: (1) They can provide multiple redox reactions during the electrochemical processes, and (2) they possess a higher electronic conductivity than the homologous metal oxides [31]. The NiCo2S4 porous spheres were successfully synthesized by Cheng et al. [32], who produced a nice capacitive property of 1870.2 F g1 at 2 A g1 (about 9% loss after 3000 cycles at 10 A g1). Zhang et al. [33] prepared the composite electrode of NiCo2S4 with 3D graphene foam which exhibited an excellent electrochemical performance of 1454.6 F g1 at 1.3 A g1 (about 4% loss after 3000 cycles at 13 A g1). Cockscomb flower-like NiCo2S4 nanostructures grown on Ni foam disclosed a unit mass capacitance of 1707 F g1 at 1 A g1 (about 11% loss after 4000 cycles at 5 A g1) [34]. However, NiCo2S4 electrodes are often constrained by poor rate capacity and cycling property due to the inefficient ion transmission from electrolytes to the active material and the structural degradation during electrochemical reaction. Hereby, we are excited to proclaim that these limitations are tackled in the present work via microwave-assisted rapid synthesis method due to its considerable boost in electrochemical performance, especially in terms of cycle stability. In this work, NiCo2S4 nanotube arrays are fabricated on a Ni foam (NF) via the rapid microwave-assisted sulfidation process to serve as a freestanding electrode. Then, the unique nanotube structure can supply abundant redox reaction sites, afford plenty of passages for ion transmission, and form a triangular structure to consolidate the cycle-ability. Based on the above results, it corroborates that
M. Guo et al. / Journal of Alloys and Compounds 780 (2019) 164e169
NiCo2S4 nanotube arrays on Ni foam are a type of prospective electrode with fortified cycling stability for high-performance supercapacitor applications.
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determined by using FESEM (Hitachi S4800) and TEM (JEM 2100). In addition, their phases and chemical states were observed through XRD (D8 Advanced XRD, Cu-Ka radiation, 40 kV, 40 mA), and XPS (AXIS SUPRA, C1s 284.8 eV).
2. Experimental section 2.3. Electrochemical measurements 2.1. Synthesis of NiCo2S4 composite electrode The chemicals for the experiment were used without processed and the experiment was operated in two steps. Step 1, the precursor was prepared by typical hydrothermal synthesis [35]. First, Ni foam (2.5 cm 3 cm 1.5 mm) substrates were pre-cleaned with dilute hydrochloric acid (HCl), deionized (DI) water, and ethanol to eliminate impurities and nickel oxides. Then, 2 mmol CoCl2$6H2O, 1 mmol NiCl2$6H2O, and 6 mmol urea were added into 18 ml DI water. After stirring for a few minutes, the prepared solution and the treated NF were transferred into a 30 ml Teflon-lined autoclave, heated at 120 C for 6 h for a hydrothermal reaction. Finally, the synthesized precursors were cleaned ultrasonically, and dried at 55 C overnight. Step 2, the precursors were converted into NiCo2S4 by the microwave-assisted method. First, a total of 5 mmol Na2S$9H2O was added into 50 ml DI water to form a mixed solution. Afterwards, the mixture was poured into a 100 ml microwave reaction vessel with the as-obtained precursors and treated with 800 W microwave irradiation in a dual-control microwave digestion apparatus at 110 C for 0.5 h. After restored to normal temperature, the black product loaded on the Ni foam was obtained through washing with DI water and ethanol for several times and dried at 60 C overnight. The final mass loading of NiCo2S4 on the Ni foam was approximately 6 mg cm2. 2.2. Materials characterization The microstructure of the as-prepared NiCo2S4 electrodes was
A series of electrochemical performance of the NiCo2S4 electrodes (1 cm 1 cm) were characterized by the Cyclic voltammetry (CV), Galvanostatic chargeedischarge (GCD), and electrochemical impedance spectra (EIS) using a traditional three-electrode system in 1 M KOH solution. The synthesized composites, platinum plate (2 cm 2 cm), and standard calomel electrode were used as the working, counter, and reference electrodes, respectively. The CV tests were carried out at various scan rates ranging from 1 mV s1 to 25 mV s1 at a potential window of 0 Ve0.6 V (vs. SCE). The GCD curves were conducted with the potential window of 0e0.4 V at different current densities varying from 5 mA cm2 to 50 mA cm2. The EIS measurements were tested in a frequency range from 100 kHz to 0.01 Hz at the open circuit potential with an AC potential amplitude of 10 mV. 3. Results and discussion As illustrated in Fig. 1, a facile microwave-assisted route is developed to synthesize NiCo2S4 nanotube-assembled arrays via an anion-exchange reaction of Ni-Co precursors [36e38]. The Ni foam is selected as the supporter of active material because of its excellent electrical conductivity and large specific surface area provided by the 3D porous structure (Fig. 1a). Initially, uniform Ni-Co precursor nanowires are grown on 3D conductive substrate via a simple hydrothermal reaction (Fig. 1b). Subsequently, the asfabricated precursors are completely converted into spinel NiCo2S4 though a microwave-assisted sulfidation process (Fig. 1c). Evidently, the NiCo2S4 nanotube arrays can entirely preserve the
Fig. 1. Schematic of the synthesized process of NiCo2S4 composite electrode. (a) Three-dimensional Ni foam; (b) Ni-Co precursor (inset, high resolution image); (c) NiCo2S4 nanotube arrays, (inset, high resolution image).
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morphology of the Ni-Co precursor. A close observation (inset in Fig. 1c) unfolds that the hybrid products possess an average diameter of 150 nm and are rougher than the precursors (inset in Fig. 1b). During the anion exchange reaction, microwave heating is employed for its faster and more uniform heating effect than that of traditional hydrothermal reactions. Through that way, there is a great acceleration in the exchange rate of sulfur anions with CO2 3 and OH anions in the precursors, and in better acquisition of fully
converted homogeneous NiCo2S4 nanotube arrays on the Ni foam. Nuanced morphologies of NiCo2S4 nanotube was investigated by TEM, as shown in Fig. 2. Fig. 2a manifests that NiCo2S4 nanotube has a diameter of approximately 150 nm and which is in accordance with the FESEM results. An amplified view of an individual nanotube in Fig. 2b evinces that the micro structure was comprised of random oriented nanoparticles. The SAED pattern in the inset of Fig. 2d displays the polycrystalline nature of the NiCo2S4 nanotube,
Fig. 2. (aec) Low-resolution TEM images, (d) HR-TEM image, (inset, SAED pattern), and (eeh) A local element mapping of composite electrode of S, Ni, and Co.
Fig. 3. (a) XRD of the NiCo2S4 composite electrode and XPS analysis of (b) Co 2p, (c) Ni 2p, and (d) S 2p.
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which is composed of many nanocrystallites (Fig. 2c). The elemental maps (Fig. 2eeh) substantiate the existence of S, Ni, and Co elementals and implicit a uniform spatial elemental distribution on the composite electrode. Further examinations of the crystal structure and elements state of the electrode materials are implemented by XRD and XPS analyses. As shown in Fig. 3a, the diffraction peaks at around 31.5 , 38.1, 47.7, 50.6 , and 54.9 are consistent with (311), (400), (422), (511), and (440) planes of the cubic NiCo2S4 phase, respectively (JCPDS no. 43-1477) [39]. Moreover, three peaks at 44.4 , 51.8 , and 76.4 are concerted with the metallic Ni (JCPDS no. 03-1043). Furthermore, XPS is also conducted to dissect the chemical composition and surface state of NiCo2S4 sample. The survey spectrum (Fig. S1) represents the presence of Co, Ni and S elements, which is reconciled with the obtained typical ternary metal sulfide
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product of NiCo2S4. The Co 2p XPS spectrum in Fig. 3b manifests the peaks at 782.6 eV in Co 2p3/2, and 797.4 eV in Co 2p1/2 are consistent with Co2þ. In addition, 781.1 eV in Co 2p3/2 and 795.3 eV in Co 2p1/2 can be ascribed to Co3þ. Similarly, as illustrated in Fig. 3c, the peaks of the spectrum expound the presence of both Ni2þ and Ni3þ. With regard to the S 2p XPS pattern (Fig. 3d), the binding energy at 163.5 eV (S 2p1/2) and 161.8 eV (S 2p3/2) are connected with the typical metal-sulfur bonding [40], and the S2 in low-coordination [41], respectively. The XPS results are relatively concordant with the preceding XRD analysis. In the light of the analysis of TEM, XRD and XPS, it can be concluded that the array structure is composed of NiCo2S4 nanotubes. The electrochemical performance was characterized by CV, GCD, EIS, and cycling stability tests. Fig. 4a elucidates the CV curves of NiCo2S4 at various scan rates from 1 to 25 mV s1. A couple of redox
Fig. 4. Electrochemical measurements of the NiCo2S4 composite electrode. (a) CV curves; (b) Charge-discharge curves; (c) The specific capacitance of the NiCo2S4 electrode at different current densities; (d) Nyquist plots before and after charge-discharge test (inset is the illustration of the NiCo2S4 triangular system on Ni foam); (e) Cycling performance of NiCo2S4 electrode at 50 mA cm2, (inset is SEM images of NiCo2S4 sample after 48000 cycles in the triangular structure).
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peaks are apparent in Fig. 4a, which can be ascribed to the redox reactions of NiCo2S4 on the basis of the following reactions [42]:
structure is of referential significance for the commercial application of supercapacitors.
NiCo2S4 þ OH 4 NiSOH þ CoSOH þ e
(1)
Acknowledgements
CoSOH þ OH 4 CoSO þ H2O þ e
(2)
This work was supported by the National Natural Science Foundation of China (No.51461014 and 51762012), Key Research and Development Project of Hainan Province (No. ZDYF2018106).
GCD tests are also carried out to analyze the capacitive performance of the composite electrode. The GCD curve in Fig. 4b explicates that the potential-time curves at various current density from 5 to 50 mA cm2 are nearly symmetric, which affirms the brilliant Coulomb efficiency in the electrode. Moreover, the appearance of two visible voltage platforms corroborates the pseudocapacitive characteristic of NiCo2S4 [43,44], which conforms to the results acquired from the CV curve. The capacitive performance of the NiCo2S4 electrode was calculated from the following equation [45]: Ca¼ IDt/(SDV). In this equation, Ca (F cm2), I (mA), Dt (s), DV (V) and S (cm2) were areal capacitance, constant discharge current, discharge time, window voltage upon discharging, and area of the samples, respectively. It is noticeable that the NiCo2S4 electrode exhibits a remarkable areal capacitance of 5.7 F cm2 at 50 mA cm2, insinuating its extraordinary rate capacity (Fig. 4c). The cycling performance, a crucial requirement for supercapacitors in commercial application, is also investigated by repeated charge and discharge experiments at a rated current of 50 mA cm2, as shown in Fig. 4e. Interestingly, there is approximately no loss after 48000 successive cycles. These results verify the superb cycle stability of the NiCo2S4 electrode. Furthermore, SEM images of the NiCo2S4 nanotubes after the cycling tests demonstrate that the microstructure of NiCo2S4 became slightly thicker and rougher even after 48000 cycles (Fig. S2). The EIS measurements (Fig. 4d) are performed to further explain the excellent electrochemical performance of the NiCo2S4 electrode. The charge-transfer resistance (Rct) of the NiCo2S4 sample was slightly decreased after 48000 cycles, further indicated extraordinary cycling stability. Surprisingly, there was a smaller ion diffusion resistance in the electrode after the test, which proved that the triangulation system was more conducive to ion diffusion than other electrodes. The superior capacitive property and cycling stability of the NiCo2S4 electrode can be ascribed to the following reasons: (1) The as-prepared uniform and separated NiCo2S4 nanotube structure provides enough connections between the electrolyte and the active materials, which considerably stimulates the overall utilization of the electrode. (2) The nanotubes are closer enough to form a triangular structure (inset in Fig. 4e) during the electrochemical reaction process, which not only alleviates the stress on the internal nanotubes induced by abrupt volume changes but also supplies a great number of channels for ion and electron diffusion (inset in Fig. 4d). Hence, the triangular system assembled by NiCo2S4 nanotubes is a jarless structure for reinforcing electrochemical performance and cycling stability. 4. Conclusions A simple and fast microwave-assisted sulfidation strategy of synthesizing uniform NiCo2S4 nanotube arrays on a threedimensional Ni conductive substrate is affirmed in this work. The composite electrodes composed of NiCo2S4 nanotubes disclose a marvelous electrochemical performance with a high specific capacitance of 8.6 F cm2 at 10 mA cm2, and the special triangular structure demonstrates an eminent cycling stability of approximately 100% retention after 48000 cycles. The low-cost and rapid synthetic process facilitates the promotion of commercial capacitors, and the prominent cycling stability generated by this unique
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