English

• 0.   Introduction

  With the development of science and technology, the increasing demands of high efficiency energy storage units in modern electronics are becoming more salient^[1-4]. In the last few years, the supercapacitor has become one of the promising effective and practical energy storage devices for its high power density, good cycle stability and fast charging rate^[5-7]. The performance and application of supercapacitors are mainly determined by the electrode materials. Therefore, improving the performance of the electrode materials has become a hotspot in the field of energy storage^[8-9].

  Transition metal sulfide, a new type of electrode material, has been extensively researched due to its superior electrochemical performance^[10-12]. Among all sufides, CoNi₂S₄ has received increasing attention because of the synergistic effect of nickel sulfide and cobaltous sulfide^[13-16]. Compared with the oxidation products (CoNi₂O₄), the extension of chemical bonds in CoNi₂S₄ is beneficial to form a more flexible structure and makes it easier for ion transport^[17-20]. However, the rapid decay of specific capacitance during the chargedischarge cycles restricts the further application in energy storage^[21-24]. To overcome this shortcoming, developing composite materials has been proven to be the most efficient way and has been widely used in the preparation of the electrode materials^[25-26]. It has been reported that the hierarchical CoNi₂S₄@CC nanowire is successfully designed and synthesized by the hydrothermal process, which shows excellent specific capacitance (1 872 F·g^-1 at 1 A·g^-1), fantastic rate capability and superior cycling stability when utilized as the elec- trode material of supercapacitors^[27]. Furthermore, Co_0.85Se@CoNi₂S₄/GF (graphene foam) nanotubes, applied to the electrode material of supercapacitors, are successfully prepared by a concise one-step electrochemical method, which have excellent interface effect and hollow structure and show outstanding specific capacitance of 5.25 F·cm^-2 at 1 mA·cm^-2, remarkable charge storage capacity and superior rate perfor- mance^[28].

  In this work, CoNi₂S₄@NiS nanocomposites were successfully synthesized by combining the hydrothermal and electrodeposition methods. The synthesized CoNi₂S₄@NiS electrode showed an excellent performance of 1 433 F·g^-1 at 1 A·g^-1, which is superior to the CoNi₂S₄ and NiS electrodes. Moreover, a flexible solid-state asymmetric supercapacitor CoNi₂S₄@NiS// rGO was assembled by CoNi₂S₄@NiS and reduced graphene oxide (rGO), which exhibits an outstanding electrochemical performance and has promising potential for application in supercapacitors.

  1.   Experimental

  1.1   Synthesis of CoNi₂S₄ on carbon fiber cloth(CoNi₂S₄@CC)

  The carbon fiber cloth (CC, 1.0 cm×2.0 cm) was ultrasonically cleaned with 0.5 mol·L^-1 KMnO₄ for 30 min individually and then was washed with ethanol and deionized water for several times and desiccated in a vacuum oven at 70 ℃ for 12 h. The CoNi₂S₄ was prepared by a hydrothermal reaction. 0.291 g Co(NO₃)₂· 6H₂O, 0.237 g NiCl₂·6H₂O, 0.060 g CO(NH₂)₂ and 0.300 g thioacetamide (TAA) were used as sources, respectively. Under the continuous magnetic stirring for 30 min, the above reagents were immersed in 30 mL deionized water to get a uniform solution. Subsequently, the uniform solution was transferred into 50 mL Teflon - lined autoclave and the treated CC was immersed into the solution, then the autoclave was heated at 180 ℃ for 24 h. The final product was ultrasonically rinsed with deionized water and ethanol, respectively. After dried at 70 ℃ for 12 h, the product was denoted as CoNi₂S₄@CC.

  1.2   Synthesis of CoNi₂S₄@NiS

  The NiS was synthesized by facile and effective three-electrode system electrodeposition. 2.376 g NiCl₂·6H₂O and 7.612 g CH₄N₂S were mixed in 100 mL deionized water and stirred for 30 min to obtain a homogenous solution. Then, the electrodeposition process was conducted for 5 min at an invariable voltage of 0.9 V, while the CoNi₂S₄@CC was served as the work electrode. After that, the samples were washed with ethanol and deionized water separately, and the products were dried in a vacuum environment at 70 ℃. For comparison, the pure NiS without CoNi₂S₄ was also synthesized on the CC under the same procedure.

  1.3   Characterization

  The crystalline and structural of the synthesized samples were examined by X - ray diffraction (XRD) using Bruker D8 Advance diffractometer with Cu Kα radiation (0.154 nm) at 40 kV and 30 mA, and at a scan rate of 6 (°)·min^-1 in the 2θ range from 10° to 80°. The microstructure of the samples was investigated using scanning microscopy (SEM) at 5 kV, transmission electron microscopy (TEM) with an accelerating voltage of 200 kV, high - resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). X-ray photoelectron spectroscopy (XPS, 1 486.7 eV) was used to observe the elemental analysis and chemical valence state of the lased irradiated samples.

  1.4   Electrochemical measurements

  The electrochemical performance of the sample was measured on an electrochemical workstation (CHI660E). Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were conducted as the main paths to exhibit the electrochemical behaviors. The electrochemical test was proceeded in a three-electro configuration in 2 mol·L^-1 KOH electrolyte and the positive and the negative electrode were the as-sample and the Pt, the Hg/HgO serve as the reference electrode, respectively. The specific capacitance can be calculated from the GCD curves by the following equation (1):

  $ {C_{\rm{A}}} = \frac{{I\Delta t}}{{m\Delta V}} $

  (1)

  Where I (A) represents discharge current, m (g) repre- sents the accurate weight of the active material, Δt (s) represents the discharge time, and ΔV represents the potential window, respectively.

  1.5   Fabrication and electrochemical measure-ments of asymmetric supercapacitor

  The all-solid-state asymmetric hybrid supercapac- itor (ASC) device was assembled by using the CoNi₂S₄ @CC as the positive electrode and rGO as the negative electrode, while the PVA-KOH gel (PVA=polyvinyl alcohol) performed as the electrolyte. The positive and negative electrode were dissolved in the PVA-KOH gel solution, then two electrodes were combined at room temperature and dry until the electrolyte is completely cured, and the solid-state supercapacitor was prepared. So as to obtain an ASC with excellent electrochemical properties, it is required to balance the relationship (q₊=q_-) of the two electrodes charge. As the stored charge of the electrode, the q can be calculated by the equation (2):

  $ q = Cm\Delta V $

  (2)

  where C (F·g^-1) represents the specific capacitance, m (g) is the mass of the active material and ΔV (V) is the potential window. Meanwhile, the ideal mass ratio can be calculated by the equation (3):

  $ \frac{{{m_ + }}}{{{m_ - }}} = \frac{{{C_ - }\Delta {V_ - }}}{{{C_ + }\Delta {V_ + }}} $

  (3)

  Where, C₊ (F·g^-1) and C_- (F·g^-1) represent the specific capacitance of CoNi₂S₄@NiS and rGO electrode. ΔV+ (V) and ΔV- (V) represent the voltage range of CoNi₂S₄ @NiS and rGO electrode, respectively. The power density (P, W·kg^-1) and the energy density (E, Wh·kg^-1) of CoNi₂S₄@NiS//rGO ASC device can be calculated by the equations (4, 5):

  $ E = \frac{1}{2}C{\left( {\Delta V} \right)^2} $

  (4)

  $ P = \frac{E}{{\Delta t}} $

  (5)

  Where Δt (s) is the discharge time, ΔV (V) is the voltage range and C (F·g^-1) is the specific capacitance of CoNi₂S₄@NiS//rGO ASC device.

  2.   Result and discussions

  2.1   Structural and morphological characteriza-tion

  The XRD patterns of NiS, CoNi₂S₄@CC and CoNi₂S₄@NiS composite are illustrated in Fig. 1. The XRD pattern of the NiS had the same diffraction peaks with the CoNi₂S₄@NiS at 2θ =30.31°, 34.77°, 46.08° and 53.58°, which can be attributed to the (100), (101), (102) and (110) planes of the NiS (PDF No.75-0613). The patterns of the CoNi₂S₄@CC and CoNi₂S₄@NiS show the same peaks at 2θ =16.28°, 26.82°, 31.52°, 38.30°, 47.33°, 50.29° and 55.22°, which are indexed to the (111), (220), (311), (400), (422), (511) and (440) planes of the CoNi₂S₄ (PDF No.24-0334), respectively. In addition, the XRD patterns of the three samples exhibited the extra diffraction peak at 2θ =26°, which can be contributed to the carbon fiber cloth substrate (PDF No.26-1080). Moreover, there were no other impurity peaks on the patterns, indicating that the successful synthesis of CoNi₂S₄@NiS on the carbon fiber cloth.

  图 1

  

  Figure 1.  XRD patterns of the NiS, CoNi₂S₄@CC and CoNi₂S₄@NiS

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  The surface element analysis and chemical valence state of the CoNi₂S₄@NiS sample were further confirmed by XPS as plotted in Fig. 2. Fig. 2a exhibited the survey spectrum and revealed the presence of Ni, Co, S and C elements in the multiple materials. The Co2p XPS spectrum of the CoNi₂S₄@NiS is shown in Fig. 2b. The peaks situated at 779.78 and 795.15 eV are attributed to the Co2p_3/2 and Co2p_1/2 levels of Co²⁺. The peaks situated at 778.55 and 793.33 eV reveal the Co2p_3/2 and Co2p_1/2 levels of Co³⁺. It proves that the coexistence of Co²⁺ and Co³⁺ in the CoNi₂S₄@NiS composite^[29]. The Ni2p spectrum is shown in Fig. 2c, the diffraction peaks situated at 853.45 and 872.38 eV are attributed to Ni²⁺ and the peak at 856.16 and 876.21 eV are related to Ni^{3+ [30]}. The S2p spectrum is displayed in Fig. 2d, the diffraction peaks located at 162.98 and 161.68 eV can be assigned to S2p_1/2 and S2p_3/2^[18]. Moreover, the peak at 169.13 eV indicates that the existence of S-O^[31].

  图 2

  

  Figure 2.  (a) XPS survey spectrum of CoNi₂S₄@NiS; (b~d) XPS spectra of Co2p, Ni2p and S2p

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  The morphology of NiS, CoNi₂S₄/CC and CoNi₂S₄ @NiS electrode materials can be observed in SEM images (Fig. 3). As exhibited in Fig. 3a and 3b, the Co- Ni₂S₄@CC presented a hexagonal flaky cubic structure and were tightly attached to the CC. Fig. 3c and 3d exhibit the morphology of the NiS, which presented a granular structure with a size of about 50~200 nm. These cross-linked nanoparticles would provide a higher electrode/electrolyte active sites for reaction and a shorter ion diffusion way^[32-33]. The microstructure of the CoNi₂S₄@NiS is shown in Fig. 3e and 3f, the NiS nanoparticles were anchored onto the surface of Co- Ni₂S₄@NiS and form a dense film. The unique structure provides a large specific surface area, which enhance the active sites and would effectively enhance the specific capacitance of composite materials.

  图 3

  

  Figure 3.  SEM images of (a, b) CoNi₂S₄@CC, (c, d) NiS and (e, f) CoNi₂S₄@NiS

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  To better understand the chemical composite and detailed structures of the synthesized CoNi₂S₄@NiS, HRTEM and element mapping analyses were conducted. The HRTEM images of the CoNi₂S₄@NiS are shown in Fig. 4a and 4b. The interplanar spacing can be measured to be 0.20 and 0.28 nm, which can be ascribe to the (102) lattice plane of NiS and (311) lattice plane of CoNi₂S₄, respectively. The consequences are match with the XRD and XPS tests. Fig. 4c and 4f displays the elemental mappings of the Co, Ni, Co/Ni and S in the CoNi₂S₄@NiS samples. The distribution area of the Ni element was slightly larger than the Co element. The Ni and Co element coexisted in the central region of the sample, while in the outside of the sample there is only Ni element left. In consideration of that CoNi₂S₄ contained Ni and Co element while NiS had no Co element, it can be deduced that the outer layer of the composite is NiS which wraps the inner CoNi₂S₄.

  图 4

  

  Figure 4.  (a, b) TEM images of CoNi₂S₄@NiS; (c~f) Element mappings of the Co, Ni, Co/Ni and S

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  2.2   Electrochemical performance

  The electrochemical performance of CoNi₂S₄@CC, NiS, and CoNi₂S₄@NiS electrodes were tested on a three - electrode configuration with 2 mol·L^-1 KOH electrolyte. Fig. 5a shows the CV curves for CoNi₂S₄@CC, NiS, and CoNi₂S₄@NiS electrodes measured at a scan rate of 10 mV·s^-1. The CoNi₂S₄@NiS exhibited superior specific capacitance and the redox peaks can be regard as the symbol of Faradaic feature. The improvement of the specific capacitance of the CoNi₂S₄@NiS is mainly contributed to the fact that the elements in the two substances have multiple valence states, which can carry out the redox reaction more effectively^[34]. The CV curves of CoNi₂S₄@NiS electrode at different scan rates from 10 to 50 mV·s^-1 are shown in Fig. 5b. The trend of the CV curves was basically maintained with the scan rate increasing, indicating the CoNi₂S₄@NiS electrode possess ideal pseudocapacitance characteristic and superior rate performance. The large deviation of the shape in large scan rate can be explained by the mismatch between charge transfer and diffusion. It can be observed that the cathode peak moved to a lower potential, and meanwhile, the anode peak moved to a higher potential when the scan rate continued to increase, which can be explained by the polarization in different scan rates^[35]. As displayed in Fig. 5c, the GCD curves of CoNi₂S₄@CC, NiS, and CoNi₂S₄@NiS electrode were measured at a current density of 1 A·g^-1 to confirm the advantage of the CoNi₂S₄@NiS. The discharge time of CoNi₂S₄@NiS was larger than NiS and CoNi₂S₄@CC, suggesting the composite structure is conducive to enhance the specific capacitance. Comparing to the NiS (1 245 F·g^-1 at 1 A·g^-1) and CoNi₂S₄/CC (1 165 F·g^-1 at 1 A·g^-1), CoNi₂S₄@NiS (1 433 F·g^-1 at 1 A·g^-1) exhibited higher specific capacitances. Fig. 5d illustrates the GCD curves of CoNi₂S₄@NiS at different current densities to further investigate charge and discharge mechanism. It can be found that the curves show an apparent voltage platform, which is characteristic of typical pseudocapacitor behavior. The result can supplement the above conclusion. Moreover, the nonlinear curves of the GCD maintained the similarity and symmetry indicating the good stability. The specific capacitances of NiS, CoNi₂S₄@CC and CoNi₂S₄@NiS calculat- ed are illustrated in Fig. 5e. The specific capacitances of CoNi₂S₄@NiS were 1 433, 1 284, 1 248, 1 170, 1 073 and 998 F·g^-1 at 1, 2, 3, 5, 8 and 10 A·g^-1, which possess better rate stability compared with the NiS and CoNi₂S₄@CC. The electrode cannot fully participate in the reaction when the current density increases, and the utilization rate of the electrochemically active material is insufficient, so the specific capacitance will decrease at a higher current density. As is shown in Fig. 5f, the EIS curve of CoNi₂S₄@CC, NiS, and CoNi₂S₄ @NiS were fitted using the equivalent circuit model, where CPE is the constant phase angle original and ZW is the Warburg resistance. The equivalent series resistance (R_s) value of NiS, CoNi₂S₄@CC and CoNi₂S₄@NiS were 1.12, 1.56 and 1.01 Ω, indicating that the CoNi₂S₄ @CC electrode had the lowest internal impedance. Moreover, the value of charge transfer resistance (R_ct) can be fitted to be 0.25, 1.62 and 0.56 Ω for the NiS, CoNi₂S₄@CC and CoNi₂S₄@NiS, suggesting that the CoNi₂S₄@CC electrode had much large R_ct than that of the NiS and CoNi₂S₄@NiS electrode. Besides, the slope of the samples was greater than 45° in the low frequency region, indicating the ions and electrolyte are effectively diffused in the entire system, resulting in a re- duction in the diffusion resistance of the NiS, Co- Ni₂S₄@CC and CoNi₂S₄@NiS electrodes^[36].

  图 5

  

  Figure 5.  Electrochemical performance of CoNi₂S₄, NiS and CoNi₂S₄@NiS: (a) CV curves of the CoNi₂S₄, NiS and CoNi₂S₄@NiS samples at a scan rate of 10 mV·s^-1; (b) CV curves of the CoNi₂S₄@NiS sample at various scan rates; (c) GCD curves of the CoNi ₂S₄CC, NiS and CoNi₂S₄@NiS samples at a current density of 1 A·g^-1; (d) GCD curves of the CoNi₂S₄@NiS at various current densities; (e) Comparison of specific capacitance; (f) EIS Nyquist plots of the CoNi₂S₄, NiS and CoNi₂S₄@NiS samples

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  A flexible solid - state asymmetric supercapacitor (ASC) device (CoNi₂S₄@NiS//rGO) was assembled to confirm the energy storage properties for practical application. Fig. 6a is the CV curves of the CoNi₂S₄@NiS and rGO electrode under the three-electrode configuration at the scan rate of 10 mV·s^-1. Obviously, the potential windows of the positive and negative electrode were connected, indicating that the loss of potential is nonexistent. The CV curves of CoNi₂S₄@NiS//rGO at different scan rates (10~100 mV·s^-1) are displayed in Fig. 6b. Significantly, the curves maintained the similar trend with the scan rate increase, and the polarization phenomenon was minimal even at the scan rate of 100 mV·s^-1, suggesting the device has excellent electrochemical reversibility. Fig. 6c exhibits the GCD curves of CoNi₂S₄@NiS//rGO at different current densities, which possessed good symmetry and had no obvious electrochemical reaction platform. Fig. 6d exhibits an excellent specific capacitance of the CoNi₂S₄@NiS// rGO ASC (103.43 F·g^-1 at 1 A·g^-1 and maintained 61.25 F·g^-1 at 10 A·g^-1), revealing excellent rate capa- bility. The EIS of the CoNi₂S₄@NiS//rGO ASC device is shown in Fig. 6e. In the high-frequency region, the R_s and R_ct can be calculated to be 1.019 and 4.89 Ω. Furthermore, cycling performance is also a significant indicator to evaluate the practical application of supercapacitor electrode materials. Fig. 6f exhibits a superior cycle performance, which maintained 78.7% after 10 000 cycles at 10 A·g^-1. The superiority of specific capacitance and capacitance retention may be contrib- uted to the special nanostructure. The unique structure can provide large space for reaction between electrode and electrolyte by large interface which may supply more active sites.

  图 6

  

  Figure 6.  Electrochemical measurements of the resultant CoNi₂S₄@NiS//rGO: (a) CV curves of the resultant CoNi₂S₄@NiS and rGO at 10 mV·s^-1; (b) CV curves of the device at different current densities; (c) GCD curves of the ASC device; (d) Specific capacitance at various current densities; (e) EIS Nyquist plots of the device; (f) Cycling performance of the device at 10 A·g^-1 for 10 000 cycles

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  The energy and power density calculated to evalu- ated the properties of the CoNi₂S₄@NiS//rGO ASC device. Fig. 7 exhibits the Ragone plot of the CoNi₂S₄ @NiS//rGO ASC. The CoNi₂S₄@NiS//rGO ASC device exhibited a high energy density of 36.6 Wh·kg^-1 at 800 W·kg^-1 and the energy density maintained 21.7 Wh· kg^-1 even at 8 000 W·kg^-1. The CoNi₂S₄@NiS//rGO ASC device have an advantage over some other report- ed devices, such as CoNi₂S₄//YS-CS (yolk-shell carbon spheres) (35 Wh·kg^-1 at 640 W·kg^-1), Ni₃S₂/MWCNT (multiwalled carbon nanotube)-NC//AC (19.8 Wh·kg^-1 at 798 W·kg^-1), NiCo₂S₄//rGO (16.6 Wh·kg^-1 at 2 348 W·kg^-1) and NiS/rGO//AC (18.7 Wh·kg^-1 at 1 240 W·kg^-1)^[37-40].

  图 7

  

  Figure 7.  Ragone plot of the ASC device

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  3.   Conclusions

  In conclusion, the CoNi₂S₄@NiS was successfully synthesized by combining the hydrothermal and electrodeposition methods. The as-obtained samples exhibited an excellent specific capacitance (1 433 F·g^-1 at 1 A·g^-1) and superior rate performance (998 F·g-1 at 10 A·g^-1). The flexible solid-state asymmetric supercapacitor assembled with CoNi₂S₄ as the positive electrode and the reduced rGO as the negative electrode showed superior energy density of 36.6 Wh·kg^-1 at a power density of 800 W·kg^-1, remarkable rate performance, and excellent cycle performance (78.7% at a high current density of 10 A·g^-1 after 10 000 cycles). The results indicate that the CoNi₂S₄@NiS would be a promising electrode material for the flexible solid-state asymmetric supercapacitors.

  
  
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  Figure 1  XRD patterns of the NiS, CoNi₂S₄@CC and CoNi₂S₄@NiS

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  Figure 2  (a) XPS survey spectrum of CoNi₂S₄@NiS; (b~d) XPS spectra of Co2p, Ni2p and S2p

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  Figure 3  SEM images of (a, b) CoNi₂S₄@CC, (c, d) NiS and (e, f) CoNi₂S₄@NiS

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  Figure 4  (a, b) TEM images of CoNi₂S₄@NiS; (c~f) Element mappings of the Co, Ni, Co/Ni and S

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  Figure 5  Electrochemical performance of CoNi₂S₄, NiS and CoNi₂S₄@NiS: (a) CV curves of the CoNi₂S₄, NiS and CoNi₂S₄@NiS samples at a scan rate of 10 mV·s^-1; (b) CV curves of the CoNi₂S₄@NiS sample at various scan rates; (c) GCD curves of the CoNi ₂S₄CC, NiS and CoNi₂S₄@NiS samples at a current density of 1 A·g^-1; (d) GCD curves of the CoNi₂S₄@NiS at various current densities; (e) Comparison of specific capacitance; (f) EIS Nyquist plots of the CoNi₂S₄, NiS and CoNi₂S₄@NiS samples

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  Figure 6  Electrochemical measurements of the resultant CoNi₂S₄@NiS//rGO: (a) CV curves of the resultant CoNi₂S₄@NiS and rGO at 10 mV·s^-1; (b) CV curves of the device at different current densities; (c) GCD curves of the ASC device; (d) Specific capacitance at various current densities; (e) EIS Nyquist plots of the device; (f) Cycling performance of the device at 10 A·g^-1 for 10 000 cycles

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  Figure 7  Ragone plot of the ASC device

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