Mechanical Alloying Synthesis of Co₉S₈ Particles as Materials for Supercapacitors

Abstract

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.

1. 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 CoS₂-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, Co₉S₈ 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.

2. Materials and Methods

2.1. Materials Synthesis

The synthesis of Co₉S₈ 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 Co₉S₈ 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 Co₉S₈ precursor was calcined at 500 °C for 4 h to get pure Co₉S₈ phase.

2.2. Structure Characterization and Electrochemical Measurement

Microstructure and morphology were investigated by X-ray diffraction (XRD, Rigaku, Beijing, China), field emission scanning electron microscopy (SEM, JSM-6701F, JEOL, Tokyo, Japan) and transmission electron microscope (TEM, JEM-2010, JEOL, Toyko, Japan). The elemental surface composition was analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5700, ESCA, ULVAC NINGBO Co., Ltd, Ningbo, China). DSC-TGA (SDT Q600, Perkin Elmer, Shanghai, China) analysis was measured under a N₂ atmosphere. Electrochemical tests were measured by electrochemical working station (CHI 660D, CH instruments, Inc., Austin, TX, USA) with three electrodes system in 2 moles/liter KOH solution electrolyte, a platinum sheet as counter-electrode and a saturated calomel electrode as reference electrode.

3. Results and Discussion

The Co₉S₈ precursor is characterized by XRD to assure its components (in Figure 1a); most of the peaks are coherent with the standard pattern of Co₉S₈ (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 Co₉S₈, 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 Co₉S₈, and the further crystallization of the Co₉S₈. 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 Co₉S₈. 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, Co₉S₈ precursor is calcined at 500 °C to obtain pure Co₉S₈. Figure 1c shows the XRD pattern of the Co₉S₈. The peaks are coherent with the standard pattern of Co₉S₈ (JCPDS card No. 86-2273); eight strong diffraction peaks are observed at 2θ values of 15.454°, 29.838°, 31.198°, 39.554°, 47.576°, 68.08°, 73.205°, and 76.779°, and they can be ascribed to the (111), (311), (222), (331), (511), (440), (731), and (800) crystal planes. The results indicate the successful synthesis of pure Co₉S₈ phase after calcination. XPS is used to further analyze the chemical composition and valence state of Co₉S₈. Figure 1d illustrates that the complete survey XPS spectrum of the Co₉S₈ 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 Co³⁺ and Co²⁺ 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 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]:

$${\mathrm{Co}}_9{\mathrm{S}}_8+{\mathrm{OH}}^{-}\leftrightarrow {\mathrm{Co}}_9{\mathrm{S}}_8\mathrm{OH}+{\mathrm{e}}^{-}$$

(1)

$${\mathrm{Co}}_9{\mathrm{S}}_8\mathrm{OH}+{\mathrm{OH}}^{-}\leftrightarrow {\mathrm{Co}}_9{\mathrm{S}}_8\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$

(2)

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:

$$C=\frac{I \times \Delta t}{M \times \Delta t}$$

(3)

where C (F·g⁻¹) is the specific capacitance, I (A) is the discharging current and Δv (V) is the potential window. The calculated specific capacitance of the Co₉S₈ 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 Co₉S₈. 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 Co₉S₈, an AC//Co₉S₈ 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//Co₉S₈. The galvanostatic charge-discharge curves are also very close to the zig-zag (Figure 2f), showing that the 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.

4. Conclusions

Co₉S₈ 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 Co₉S₈ is a promising pseudocapacitive material and mechanical alloying is an excellent approach to fabricating metal sulfide materials with excellent pseudocapacitive properties.