The Comparative Study of Electrochemical Capacitance Performance between Sulphur-Doped Co 3 O 4 and CoS Anodes

Anode materials with high capacitance performance are highly desirable for supercapacitors (SCs). In this work, nanomaterials cobalt sulfide (CoS), sulphur-doped Co 3 O 4 (S-Co 3 O 4 ), and Co 3 O 4 were fabricated on a carbon cloth substrate by hydrothermal method. The composition and morphology of the material were characterized by X-ray diffraction (XRD) patterns and Scanning ElectronMicroscope (SEM).The electrochemical measurements were performed in a three-electrode system.The result shows that CoS nanomaterial as anode is of the best electrochemical performance, achieving areal capacitance of 1.98 F/cm at 2mA/cm in a 5M LiCl solution. Moreover, the CoS anode has long-term cycling stability with more than 85.7% capacitance retention after 10000 cycles, confirming its larger capacitance, good redox activity, and electrochemical stability.


Introduction
With the ever increasing energy depletion and aggravating environmental issue, considerable attention was paid to energy storage and conversion and intense research has been concentrated on this field [1,2].Of them, supercapacitors, with their advantages of high-power density, excellent reversibility, fast recharge ability, and long cycle life and their potential application in the field of highpower electronic devices, emergency power supplies, and hybrid electric vehicles, have attracted significant research attention [1][2][3][4][5][6].However, the relatively low energy density of SCs impeded their further development and application.Therefore, based on not sacrificing the power density and cycle life, it is highly desirable to increase the energy density of SCs.According to the equation of energy density,  = 0.5 CV 2 , the energy density of a SC can be improved if the output voltage and/or the capacitance is increased [7].An effective approach to increase the voltage is to use organic and ionic liquid electrolytes [8][9][10], such as tetraethylammonium tetrafluoroborate in acetonitrile [9] or lithium perchlorate in propylene carbonate [10].Although the voltage window of up to 3 V can be reached, the disadvantages such as high cost, poor ionic conductivity, and high toxicity prevent SCs from further applications.A hopeful alternative is to develop asymmetric supercapacitors (ASCs) based on the environmentally friendly aqueous electrolyte.As compared with symmetric supercapacitors (SSCs), ASCs combined two electrodes with different voltage to increase the operating voltage in aqueous electrolyte which is up to 2 V [11,12].Great efforts have been made to research various materials, such as graphene-MnO 2 //activated carbon nanofiber, Ni(OH) 2 -graphene and graphene, V 2 O 5 //active carbon, and MnO 2 //FeOOH [11,13,14], for their application on ASCs.However, previous studies have been mainly focused on the development of high-performance cathode materials [15].By comparison, explorations for anode materials have been very few as yet.The most common anode materials are carbonbased nanomaterials, yet they suffer from the relatively low capacitance which prevents ASCs from further enhancement in energy density.Therefore, it is necessary to explore novel anode materials to advance the performance of ASCs.
As promising, environmentally friendly, and low-cost candidates, cobalt sulfides can also be grown on a flexible conductive substrate, but they have not been systematically studied as anode materials yet [16].In this paper, we reported the fabrication of Co 3 O 4 , sulphur-doped Co 3 O 4 , and CoS and compared their electrochemical performance as anodes of ASCs.The result shows that CoS nanomaterial is of the best electrochemical performance when used as anode, achieving areal capacitance of 1.98 F/cm 2 at 2 mA/cm 2 and excellent long-term cycling stability in a 5 M LiCl solution.

Experimental
All reagents used in the experiment are up to the standard of analytical grade that were used without any further purification.Co 3 O 4 were fabricated on a carbon cloth substrate by using hydrothermal method.First, 5 mmol of Co(NO 3 ) 2 6H 2 O, 10 mmol of NH 4 F, and 25 mmol of CO(NH 2 ) 2 were dissolved in 50 mL of DI water under stirring, respectively.After 10 minutes of slight stirring, a Teflon-lined stainless-steel autoclave of 25 mL capacity was filled with 20 mL of homogeneous solution prepared above.Then, carbon cloth (2 cm × 3 cm) was cleaned by ethanol, deionized water, and using ultrasonic treatment in turn and then immersed into the reaction solution.Subsequently, the autoclave was sealed and heated at 120 ∘ C for 5 h and allowed to cool down to room temperature.After the reaction, a layer of Co 3 O 4 film was grown on the carbon cloth substrate and then washed with DI water and dried at 60 ∘ C. The S-Co 3 O 4 was prepared by immersing a piece of Co 3 O 4 (2 cm × 3 cm) into a 25 mL autoclave containing 0.1 M of C 2 H 5 NS, which was then sealed and heated at 80 ∘ C for 3 h.After cooling down to room temperature, the sample was washed and then dried at 60 ∘ C. The CoS was prepared by immersing a piece of Co 3 O 4 (2 cm × 3 cm) into a 25 mL autoclave containing 0.3 M of NaS, which was then sealed and heated at 120 ∘ C for 36 h.After cooling down to room temperature, the sample was washed and then dried at 60 ∘ C.

Material Characterization and Electrochemical Measurement
The composition morphology and structure of the electrode materials were characterized by X-ray diffraction patterns, field-emission SEM (FE-SEM, JSM-6330F), and CV, and galvanostatic charge/discharge measurements were conducted using an electrochemical workstation (CHI 760D).The electrochemical measurements were performed in a threeelectrode cell, with the individual sample used as working electrode, a Pt mesh as counter-electrode, and a SCE as reference electrode.

Results and Discussion
The SEM images of Co  Figure 3(c) shows the areal capacitances of the CoS anode at the scan rates from 10 mV/s to 400 mV/s calculated according to its CV curves.The highest areal capacitance of 0.46 F/cm 2 obtained at the scan rate of 10 mV/s is much better than the value (0.27 F/cm 2 ) obtained for N-Fe 2 O 3 negative electrode and comparable to the excellent value for other negative electrodes, such as Co 3 O 4 @RuO 2 (0.6 F/cm 2 ) and VOS (0.56 F/cm 2 ), demonstrating the excellent capacitive performance and a good electrical conductivity of CoS anode [3,15,16].
The galvanostatic charge-discharge curves of the S-Co 3 O 4 and CoS electrodes at a current density of 2 mA/cm 2 are shown in Figure 3(d).Compared with the S-Co 3 O 4 electrode, the CoS exhibits slower and more symmetrical charge and discharge curves and a more linear variation of potential versus time, which is also supported by the galvanostatic chargedischarge curves collected for CoS electrode at different current densities from 2 to 3, 4, 5, 6, 8, and 10 mA/cm 2 (Figure 3(e)), confirming the enhanced capacitance, superior Coulombic efficiency, and excellent reversibility of the CoS electrode at a high current density.The areal capacitance of 0.68 F/cm 2 at current density of 10 mA/cm 2 (shown in Figure 3(e)) is also comparable to Co 3 O 4 @RuO 2 electrode (0.67 F/cm 2 ), the best value for cobalt sulfides, proving that CoS can be a suitable candidate for anode [16].
Figure 4 demonstrates the long-term cycling performance of the S-Co 3 O 4 and CoS as anodes, respectively.Although the capacitance retention CoS is a little lower than that of S-Co 3 O 4 , more than 85.7% of the capacitance retention after 10000 cycles at a scan rate of 200 mV/s still indicates that CoS is of excellent long cycle life.
To further find the electrochemical reason for the phenomenon that the fully converted CoS can significantly improve capacitance (Figure 3(a)), the electrochemical impedance spectroscopy (EIS) measurements on the Co 3 O 4 , S-Co 3 O 4 , and CoS electrodes were performed (as shown in Figure 5).It is qualitatively demonstrated that the Nyquist plots of the EIS spectra show a similar semicircle in a high frequency range and a spike in a low frequency range.In a high frequency area, the intercept to the -axis represents the bulk resistance (  ), while the semicircle corresponds to the double-layer capacitance and charge-transfer resistance ( ct ).The   resistances of CoS and S-Co 3 O 4 electrodes are smaller than that of pristine Co 3 O 4 electrodes, especially for CoS electrode, indicating the enhanced electrical conductivity and the significantly improved capacitance of CoS electrodes.

Conclusion
In summary, high-performance CoS anode was demonstrated on woven carbon fabrics.The electrochemical performances of CoS were enhanced dramatically compared with that of Co 3 O 4 and were better than that of S-Co 3 O 4 .Moreover, stable rate capability and cycle capability were also achieved.These encouraging performances have access to its applications in high-performance asymmetric supercapacitors, opening up a novel approach to advancing the performance of energy storage devices.

Figure 3 :Figure 4 :
Figure 3: (a) CV curves collected for CoS and S-Co 3 O 4 anodes at a scan rate of 100 mV/s.(b) CV curves collected for CoS anode at various scan rates.(c) Areal capacitance of the CoS anode as a function of the scan rate.(d) Galvanostatic charge-discharge curves of the S-Co 3 O 4 and CoS anodes at a current density of 2 mA/cm 2 .(e) Galvanostatic charge-discharge curves of the CoS anodes collected at different current densities.(f) Areal capacitance of the CoS anode collected from galvanostatic charge-discharge curves as a function of current density.
It can be seen that the individual carbon fiber was uniformly wrapped by many nanowires with a diameter between 300 and 500 nm and length up to 4 m.The obvious difference in morphology between the SEM images of Co 3 O 4 , S-Co 3 O 4 , and CoS can be observed.The SEM images of the S-Co 3 O 4 and CoS are much rougher compared with that of Co 3 O 4 ; furthermore, the SEM image of CoS is rougher than that of S-Co 3 O 4 (Figures 1(b) and 1(c)).Figure 2 shows the X-ray diffraction (XRD) patterns of the Co 3 O 4 , S-Co 3 O 4 , and CoS samples.The peaks of Co 3 O 4 and S-Co 3 O 4 samples can be indexed to the standard card (JCPDS Card no.42-1467), which are in good accordance with cubic spinel Co 3 O 4 phase.In addition, the peak shift can be observed in S-Co 3 O 4 as compared to the peak of Co 3 O 4 , especially for (311) crystal face, confirming the S doping in the material.Energy dispersive spectrometry (EDS) elemental spectrum was performed to provide the doping amount of S (26.08 wt%).The peaks of CoS sample can be indexed to the standard card (JCPDS Card no.65-3418), which are in good agreement with spinel CoS structure.