Copper tin sulfides (CTSs) have widely been investigated as electrode materials for supercapacitors owing to their high theoretical pseudocapacitances. However, the poor intrinsic conductivity and volume change during redox reactions hindered their electrochemical performances and broad applications. In this study, carbon quantum dots (CQDs) were employed to modify CTSs. The structures and morphologies of obtained materials were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD revealed CTSs were composed of Cu2SnS3 and Cu4SnS4, and TEM suggested the decoration of CQDs on the surface of CTSs. With the decoration of CQDs, CTSs@CQDs showed a remarkable specific capacitance of 856 F·g−1 at 2 mV·s−1 and a high rate capability of 474 F·g−1 at 50 mV·s−1, which were superior to those of CTSs (851 F·g−1 at 2 mV·s−1 and 192 F·g−1 at 50 mV·s−1, respectively). This was mainly ascribed to incorporation of carbon quantum dots, which improved the electrical conductivity and alleviated volume change of CTSs during charge/discharge processes.
Supercapacitors with high power density, superior efficiency, and long cycle life have attracted increasing attention in energy storage devices [
Among potential electrode materials for pseudocapacitors, transition metal sulfides, including CuS [
Tin chloride pentahydrate (SnCl4·5H2O), copric chloride dihydrate (CuCl2·2H2O), and polyethylene glycol (average Mn 200) were purchased from Aladdin (Shanghai, China). Thioacetamide were purchased from Sinopharm Chemical Regent Co., Ltd. (Shanghai, China). All chemicals were of analytical grade.
All reactants and solvents were used directly as received without further purification. In the typical procedure, 1 mmol SnCl4·5H2O, 2 mmol CuCl2·2H2O, and 3 mmol thioacetamide were successively added into a 50 mL beaker containing 30 mL PEG-200. The mixture was then heated at 60°C for 40 min under constant magnetic stirring to form a black solution. The obtained mixture was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 180°C for 16 h. After cooling to room temperature, the mixture was centrifuged, washed three times with deionized water and ethanol, and dried in a vacuum oven at 60°C for 12 h to yield the precursor. To obtain CTS microspheres, the precursor was annealed at 400°C for 4 h under Ar atmosphere.
CTSs@CQDs nanoparticles were prepared by adding 20 mg of as-synthesized CTSs microspheres and 0.2 g carbon quantum dots solution into 1.8 g acetone under constant stirring at room temperature for 1 h. The CTSs@CQDs were obtained after drying at 80°C to evaporate remaining acetone.
The crystallinity and composition of the materials were identified by XRD. The morphologies were viewed by TEM, high-resolution TEM (HRTEM), and energy dispersive X-ray (EDX).
The electrochemical performances of CTSs@CQDs were measured in a three-electrode system connected to a CHI660E electrochemical workstation. Pt was used as counter, Hg/HgO as reference, and CTSs@CQDs as the working electrode. The working electrodes were prepared by mixing CTSs@CQDs, carbon black, and PTFE at a weight ratio of 85 : 5 : 10. The mixture was ground in a mortar to form slurry, which was then thin sliced with a glass rod. The working electrodes were harvested by pressing the thin slice (∼1 mg) on the nickel foam current collector.
Figure
XRD patterns of CTSs and CTSs@CQDs.
TEM analysis was employed to investigate the micro/nanostructures of CTSs and CTSs@CQDs. As displayed in Figures
Characteristic TEM images of (a, c) CTSs and (b, d) CTSs@CQDs at low and high magnification. HRTEM images of (e) CTSs and (f) CTSs@CQDs. (g) TEM image of CTSs@CQDs and corresponding elemental mapping results.
As shown in Figure
(a, b) CV curves of CTSs and CTSs@CQDs. (c) Rate capability of CTSs@CQDs and CTSs. (d) Cycling performance of CTSs@CQDs and CTSs at the current density of 10 A·g−1.
Here, the calculated specific capacitances of CTSs and CTSs@CQDs were summarized in Figure
CQDs were employed to modify CTS and yield CTSs@CQDs with enhanced electrochemical performances. CTSs@CQDs possessed remarkable specific capacitance reaching 856 F·g−1 at 2 mV·s−1 and a high rate capability of 474 F·g−1 at 50 mV·s−1. These values were superior to those of CTS (851 F·g−1 at 2 mV·s−1 and 192 F·g−1 at 50 mV·s−1, respectively). The improved electrochemical performances of CTSs@CQDs were mainly ascribed to modification of CQDs, which improved the electrical conductivity and alleviated the volume change during redox reactions.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors acknowledge the financial support from the National Natural Science Foundation of China Program (no. 51602111), Natural Science Foundation of Guangdong Province (2018A030313739), Cultivation Project of National Engineering Technology Center (2017B090903008), Xijiang R&D Team (X.W.), Guangdong Provincial Grant (2017A050506009), Special Fund Project of Science and Technology Application in Guangdong (2017B020240002), and 111 Project.