Mechanical Alloying Synthesis of Co 9 S 8 Particles as Materials for Supercapacitors

Cobalt sulfide (Co9S8) particles are compounded as the electrode materials of supercapacitors by a mechanical alloying method. They show excellent properties including good cycling stability and high specific capacitance. A supercapacitor is assembled using Co9S8 as the anode and activated carbon (AC) as the cathode. It gains a maximum specific capacitance of 55 F ̈g ́1 at a current density of 0.5 A ̈g ́1, and also an energy density of 15 Wh ̈kg ́1. Those results show that the novel and facile synthetic route may be able to offer a new way to synthesize alloy compounds with excellent supercapacitive properties.


Introduction
As industry and society develop rapidly, it is difficult for natural energy resources to satisfy the requirements.Therefore, we need to jump into action immediately to change this situation.What comforts us is that the invention of a supercapacitor possessing an outstanding performance, including good cycling stability and high power density, may be able to handle the energy problems [1,2].Supercapacitors can be divided into electrical double-layer capacitors and pseudocapacitors [3,4].Due to the higher specific capacitance and larger energy density of pseudocapacitors compared to electrical double-layer capacitors, researchers have paid more attention to the materials of pseudocapacitors [5,6].
There are many pseudocapacitive materials, including conducting polymers [7,8], metal oxides and their composites [9,10].Metal oxides exhibit high specific capacitance, but their electrical conductivity and rate capability are too poor to satisfy the requirements for commercialization.Therefore, transition metal sulfides with higher electrical conductivity than metal oxides are employed as pseudocapacitive materials.Facing this situation, some researchers use transition metal sulfides as pseudocapacitive materials.Cobalt sulfides have good electrical conductivity, high specific capacitance and excellent cycling stability, and will be an outstanding material for pseudocapacitors [11].Tang et al. prepared CoS 2 -reduced graphene oxide nanocomposite using thesolvothermal method which suggests a specific capacitance of 331 F¨g ´1 at a current density of 0.5 A¨g ´1 [12].Cobalt sulfide nanoparticles were also synthesized through a hydrothermal route and demonstrated a specific capacitance of 478.75 F¨g ´1 at a scan rate of 5 mV¨s ´1 [13].Meanwhile, cobalt sulfide nanotubes were prepared by a hydrothermal method and exhibited a specific capacitance of 285 F¨g ´1 at a current density of 0.5 A¨g ´1 [14].This research indicates that cobalt sulfides possess excellent pseudocapacitive properties.However, the above methods are complex and harmful to the environment, and still also exhibit deficient properties such as low capacitance as well as bad cycling stability.This inspires us to search further for the synthesis and pseudocapacitive performance of cobalt sulfide.Herein, Co 9 S 8 particles are compounded by a mechanical alloying method; it shows high specific capacitance and excellent cycling stability, so cobalt sulfide could serve as an outstanding material for pseudocapacitors.

Materials Synthesis
The synthesis of Co 9 S 8 contains two processes.Firstly, Co powder (purity 99.9%, 300 mesh) and excess sulphur powder (purity 99.5%) were use as raw material to synthesize Co 9 S 8 precursor by the mechanical alloying method using planetary ball mill (QM-3SP04, Nanjing NanDa Instrument Plant, Nanjing, China) and stainless steel milling balls, this process was conducted under an Ar atmosphere.The ball-to-powder weight ratio is 20:1, milling 8 h at temperature of 25 ˝C and the rotating speed is 450 r¨min ´1.Secondly, the Co 9 S 8 precursor was calcined at 500 ˝C for 4 h to get pure Co 9 S 8 phase.

Results and Discussion
The Co 9 S 8 precursor is characterized by XRD to assure its components (in Figure 1a); most of the peaks are coherent with the standard pattern of Co 9 S 8 (JCPDS card No. 86-2273).However, several impurity peaks of CoS (JCPDS card No. 75-0605) appeared, indicating the existence of the CoS impurity.To obtain pure Co 9 S 8 , the precursor is calcined at a high temperature.The optimum calcination temperature is determined by DSC-TGA (as shown in Figure 1b).It shows one main weight loss interval and two heat exchange peaks between 0 and 400 ˝C.The exothermic peak observed around 110 ˝C is ascribed to the crystal polymorphic transformation of CoS to Co 9 S 8 , and the further crystallization of the Co 9 S 8 .The main weight loss at about 300 ˝C is mainly due to the evaporation of surplus sulfur which comes from the transformation of CoS to Co 9 S 8 .This could be further identified by the obvious endothermic peak around 320 ˝C in the DSC curve.When the temperature exceeds 400 ˝C, the weight loss of the material can hardly be seen, suggesting that the crystallization process is nearly complete.Therefore, Co 9 S 8 precursor is calcined at 500 ˝C to obtain pure Co 9 S 8 .Figure 1c shows the XRD pattern of the Co 9 S 8 .The peaks are coherent with the standard pattern of Co  1d illustrates that the complete survey XPS spectrum of the Co 9 S 8 contains O 1s, C 1s, Co 2p, and S 2p peaks.The O peaks can be ascribed to the physically and chemically adsorbed oxygen from the air.The high resolution Co 2p XPS spectrum (Figure 1e) shows Co 2p 3/2 , Co 2p 1/2 , and two satellites peaks.The fitting for the Co 2p 3/2 peak reveals the presence of two peaks at 779.1 eV and 781.9 eV, and that of the Co 2p 1/2 peak also reveals two peaks at 794.1 eV and 798.1 eV, which are ascribed to the existence of the Co 3+ and Co 2+ state, respectively.The S 2p spectrum (Figure 1f) is fitted into three peaks; the peaks at 161.8 eV and 163.8 eV are correspond to S 2p 2/3 and S 3p 1/2 , respectively.Another is the peak at 168.5 eV which is generated by O adsorbed on the surface of S. The results suggest that Co 9 S 8 is successfully prepared.SEM images of Co 9 S 8 precursor and Co 9 S 8 particles are shown in Figure 1g, which illustrate that the Co 9 S 8 precursor and Co 9 S 8 have granular shapes, and no obvious aggregation is found after calcination.Combined with the results of TEM (Figure 1i), there is a large amount of voids between the particles, which can serve as a buffer for the electrolyte so that the electrode materials can be maximally infiltrated by the electrolyte, leading to excellent pseudocapacitive properties.respectively.The S 2p spectrum (Figure 1f) is fitted into three peaks; the peaks at 161.8 eV and 163.8 eV are correspond to S 2p2/3 and S 3p1/2, respectively.Another is the peak at 168.5 eV which is generated by O adsorbed on the surface of S. The results suggest that Co9S8 is successfully prepared.SEM images of Co9S8 precursor and Co9S8 particles are shown in Figure 1g, which illustrate that the Co9S8 precursor and Co9S8 have granular shapes, and no obvious aggregation is found after calcination.Combined with the results of TEM (Figure 1i), there is a large amount of voids between the particles, which can serve as a buffer for the electrolyte so that the electrode materials can be maximally infiltrated by the electrolyte, leading to excellent pseudocapacitive properties.Figure 2a shows the cyclic voltammetry (CV) curves of the Co9S8 electrode.Obviously, they are almost symmetrically rectangular, indicating its ideal capacitive properties.With the increase of the scanning rate, the area of the CV curves became larger but its shape almost did not change, which indicates the excellent reversibility of Co9S8 particle electrodes.The mechanism for the charge-storage of Co9S8-based electrodes in alkaline solution can be explained by the following reversible process [15]: The nearly symmetrical triangular shape in the galvanostatic charge-discharge (Figure 2b) curves further indicates the ideal pseudocapacitive performance of Co9S8.The specific capacitance is calculated by the equation: Figure 2a shows the cyclic voltammetry (CV) curves of the Co 9 S 8 electrode.Obviously, they are almost symmetrically rectangular, indicating its ideal capacitive properties.With the increase of the scanning rate, the area of the CV curves became larger but its shape almost did not change, which indicates the excellent reversibility of Co 9 S 8 particle electrodes.The mechanism for the charge-storage of Co 9 S 8 -based electrodes in alkaline solution can be explained by the following reversible process [15]: Faraday reaction of AC//Co9S8 is highly reversible.The specific capacitance of the AC//Co9S8 capacitor is 55 F g −1 at 0.5 A g −1 , and 80% of this value is retained when the current density is increased to 8 A g −1 , exhibiting an excellent rate capability.The energy density reached 15 Wh kg −1 (Figure 2h) and is higher than that of the AC//AC supercapacitors.The cycle stability (Figure 2i) remains at 100% after 3000 cycles, suggesting the excellent cycling stability of AC//Co9S8.The above results indicate that the Co9S8 exhibits an ideal pseudocapacitive performance and the mechanical alloying is an excellent approach to fabricating metal sulfide materials for supercapacitors.The nearly symmetrical triangular shape in the galvanostatic charge-discharge (Figure 2b) curves further indicates the ideal pseudocapacitive performance of Co 9 S 8 .The specific capacitance is calculated by the equation:

Conclusions
where C (F¨g ´1) is the specific capacitance, I (A) is the discharging current and ∆v (V) is the potential window.The calculated specific capacitance of the Co 9 S 8 electrode (as shown in Figure 2c) is 520 F¨g ´1 at a current density of 0.5 A¨g ´1, and 56% of that is reserved when the current density is increased to 8 A¨g ´1.The results indicate the high specific capacitances and good rate capability of the Co 9 S 8 .The cycling stability is shown in Figure 2d, where 100% of the specific capacitance remains after cycling 1500 cycles, indicating excellent cycling stability.Ascribed to the excellent pseudocapacitive properties of Co 9 S 8 , an AC//Co 9 S 8 asymmetric capacitor is assembled.In Figure 2e, the CV curves are nearly symmetrically rectangular and the shape changed very little with the increase of the scanning rate, indicating an excellent and ideal pseudocapacitive performance of the AC//Co 9 S 8 .The galvanostatic charge-discharge curves are also very close to the zig-zag (Figure 2f), showing that the Faraday reaction of AC//Co 9 S 8 is highly reversible.The specific capacitance of the AC//Co 9 S 8 capacitor is 55 F¨g ´1 at 0.5 A¨g ´1, and 80% of this value is retained when the current density is increased to 8 A¨g ´1, exhibiting an excellent rate capability.The energy density reached 15 Wh¨kg ´1 (Figure 2h) and is higher than that of the AC//AC supercapacitors.The cycle stability (Figure 2i) remains at 100% after 3000 cycles, suggesting the excellent cycling stability of AC//Co 9 S 8 .The above results indicate that the Co 9 S 8 exhibits an ideal pseudocapacitive performance and the alloying is an excellent approach to fabricating metal sulfide materials for supercapacitors.

Conclusions
Co 9 S 8 particles are prepared by a mechanical alloying method and used as a material for asymmetric supercapacitors.It shows excellent and ideal pseudocapacitive performance, including excellent rate capability with 80% of the specific capacitance retention when the current density increases from 0.5 to 8 A¨g ´1 and also outstanding cycling stability with the specific capacitance remaining at 100% after 1500 cycles.The excellent electrochemical properties mean that Co 9 S 8 is a promising pseudocapacitive material and mechanical alloying is an excellent approach to fabricating metal sulfide materials with excellent pseudocapacitive properties.

Figure 1 .
Figure 1.(a) XRD pattern and (b) DSC-TGA curve of the Co 9 S 8 precursor.(c) XRD pattern of Co 9 S 8 .(d) Overall XPS, (e) Co 2p, and (f) S 2p XPS spectra of Co 9 S 8 .(g) SEM image of Co 9 S 8 precursor.(h) SEM image and (i) TEM image of the Co 9 S 8 .