Full Length ArticleStannous sulfide nanoparticles for supercapacitor application
Introduction
There is a growing interest among the scientists and engineers for the development of pollution free energy storage devices in recent years due to the rapid depletion of fossil fuels [1]. Electrochemical supercapacitors offer superior power density, fast charge-discharge rate and long-cycle stability, which are expected to bridge with secondary batteries with a high energy density [2], [3]. Supercapacitor devices occupy a significant position in automotive and electronic industry and according to the energy storage mechanism, the device can be divided into two classes, such as, electrical double layer capacitor and pseudocapacitor. The first one store electrical energy electrostatically from the reversible adsorption of ions onto their surfaces, leading to high power delivery at the cost of low energy density [4], whereas, for the pseudocapacitors the process can be realized by fast and reversible redox reactions at the surface or near-surface regions of the electrode to store electric charge [2].
The electrode materials play a crucial role in the development of high performance capacitors and three types of electrode material has been reported so far for the supercapacitors application, such as, carbonaceous materials [5], metallic materials [6] and conjugated organo-polymers [7]. Carbon-based materials, with high electrical conductivity and long cycling stability, falls under the category of electrical double layer capacitor, whereas, the other two types of materials store energy in a Faradaic or redox-type process similar to batteries, which gives high energy density and under the category of pseudocapacitor. It has been reported that the metallic materials such as oxides of ruthenium, manganese, nickel and cobalt [3], [8], [9], [10] and also the conjugated polymers [11] exhibit superior capacitive behaviour as compared with carbonaceous materials [12]. However, the polymers exhibit inferior recyclability during the charge-discharge process due to change of oxidation states which, in turn, lowers the electrical conductivity of the working electrode. To increase the performance of electrochemical capacitors, metal oxides in combination with carbon were investigated to receive the duel advantages of double-layer-capacitor and pseudocapacitor [13], [14]. For the past few years, metal sulfides have emerged as one of the most potential candidates for energy storage application. Both physical (electrical conductivity, mechanical and thermal stability) and chemical (redox chemistry) properties of the metal sulfides contribute to high specific capacitance and make them suitable as the electrode material for lithium-ion battery and supercapacitor applications [15].
There are several reports have been published on the energy storage applications using various metal sulfides, such as, iron, nickel, copper, cobalt, manganese, molybdenum etc [15], [16]. Apart from the pure metal sulfides, hybrid system of metal sulfide in combination with carbonaceous materials showed excellent performance in supercapacitor and battery applications [16]. A one-pot spray pyrolysis method has been reported for the synthesis of stannous sulfide-carbon (SnS-C) composite and employed as anode materials for Na-ion batteries with high reversible capacity and good cycling performance [17]. A report has been published for the preparation of SnS-C composite by carbonization method at an elevated temperature of 650 °C, where the hybrid system showed the specific capacitance value of 36.16 F g−1 in galvanostatic charge-discharge process [18]. A surfactant-free solvothermal route was employed for the fabrication of SnS nanorods and showed better electrochemical properties towards supercapacitor application [19] than the previous reference [18]. It is also important to mention that three dimensional porous SnS-sulphur doped graphene hybrid system exhibited very high capacitance with good cycling performance, excellent flexibility and mechanical stability [20].
In this current work, we have synthesized citrate stabilized stannous sulfide by a simple, and eco-friendly hydrothermal method. The as synthesised material was characterized using different analytical techniques. To explore the electrochemical properties of the synthesized material, we have added carbon black to the stannous sulfide in the ratio of 1:4, respectively, and that substantially improve the capacitive performance for the hybrid system.
Section snippets
Materials
All the chemicals, stannous chloride, sodium sulfide, carbon black and citric acid, were of analytical grade and used as received for synthesizing the sample.
Synthesis of stannous sulfide (SnS) nanoparticles
In a typical synthesis process, 10 mL of citric acid (10−3 mol dm−3) was added with 40 mL of SnCl2.2H2O (10−3 mol dm−3). To the above solution, 50 mL of sodium sulfide (10−3 mol dm−3) was added dropwise under continuous stirring condition. The precipitated material was transferred to a Teflon-lined stainless steel autoclave and kept under
Results and discussion
Fig. 1A displays the TEM image of the synthesized material. The figure shows the high-density distribution of dark spots. The EDX spectrum from the dark area is shown in Fig. 1B where the presence of elements, such as, tin, sulphur and copper are clearly indicated. The copper peak originates from the TEM copper mesh support grid, whereas, tin and sulphur are from the stannous sulfide. The X-ray diffraction (XRD) technique was used to study the phase purity of the as-synthesized SnS. All of the
Conclusion
In this current work we report a facile and potentially scalable technique to fabricate citric acid stabilized stannous sulfide nanoparticles using a hydrothermal synthesis method. The synthesized material was characterized using optical and microscopic techniques that delivers the information about the size, phase and optoelectronic properties of the material. We have further confirmed the phase purity of the stannous sulfide by using various surface characterizing techniques. The
Acknowledgements
The authors acknowledge financial support from the University of Johannesburg and the National Research Foundation, South Africa. RB, ND and VKP further acknowledge financial support from the Global Excellence and Stature fellowship from the University of Johannesburg.
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