Abstract
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.
1. Introduction
Supercapacitors with high power density, superior efficiency, and long cycle life have attracted increasing attention in energy storage devices [1, 2]. Compared to conventional carbon materials based on electrical double-layer capacitance, metal oxides/sulfides and conductive polymers possess preeminent specific capacitance due to faradaic pseudocapacitance [3, 4].
Among potential electrode materials for pseudocapacitors, transition metal sulfides, including CuS [5–9], Cu2S [10], SnS2 [3, 11–13] and SnS [14] have extensively been investigated owing to their earth abundance, low cost, environmental friendliness, and high theoretical capacitance. On the contrary, CTSs have particularly been applied in conversion and storage devices [15–18]. However, CTSs as electrode materials for supercapacitors still suffer from poor electrical conductivity and large volume change during redox reactions, leading to moderate electrochemical performances. Herein, CQDs were employed to decorate CTSs and yield CTSs@CQDs with enhanced electrochemical performances in terms of rate capability and cycling stability.
2. Experimental
2.1. Chemicals
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.
2.2. Synthesis of CTS Microspheres
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.
2.3. Synthesis of CTSs@CQDs Nanoparticles
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.
2.4. Characterization
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).
2.5. Electrochemical Measurements
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.
3. Results and Discussion
3.1. Structure and Morphology
Figure 1 shows the XRD patterns of CTSs and CTSs@CQDs. Obviously, CTSs are composited of predominant Cu2SnS3 (JCPDS # 89-2877) and partial Cu4SnS4 (JCPDS # 27-0196), where the sharp diffraction peaks at 2θ = 28.5°, 32.8°, 47.5°, 56.4°, 68.6°, and 76.3° were assigned to (111), (200), (220), (311), (400), and (331) of Cu2SnS3, respectively [19]. The composition of CTSs provided abundant valence states, which was beneficial to the redox reactions during charging/discharging processes. With the modification of CQDs, CTSs@CQDs displayed a similar XRD pattern to that of CTSs, demonstrating the CQDs were attached on the surface of CTSs without changing the crystalline nature of CTSs [20, 21].
TEM analysis was employed to investigate the micro/nanostructures of CTSs and CTSs@CQDs. As displayed in Figures 2(a) and 2(c), CTSs with an interesting flower structure were assembled by nanosheets. The typical HRTEM image of CTSs (Figure 2(e)) specified the lattice fringe spacing was estimated to be around 0.31 nm, in good accordance with the (111) plane for cubic CTSs [18]. With the modification of CQDs, CTSs@CQDs exhibited the nanoflower-like structure with a slight change (Figures 2(b) and 2(d)), further demonstrating the decoration of CQDs. As illustrated in Figure 2(f), the nanosheet was covered by CQDs (red rings), which was beneficial to fast electron transport and alleviate volume variation. In the TEM and corresponding EDX mapping results (Figure 2(g)), Cu, Sn, and S were homogeneously distributed. Besides, the CQDs were uniformly distributed throughout the nanoflower-like structures, which could considerably improve the electrochemical performance of supercapacitors [22, 23].
(a)
(b)
(c)
(d)
(e)
(f)
(g)
3.2. Electrochemical Performance
As shown in Figure 3(a), the CV curves of CTSs at a scan rate of 50 mV·s−1 exhibited two pairs of significant redox peaks, demonstrating obvious pseudocapacitive behaviour. The redox peaks at 0 V/−0.85 V may correspond to the redox reaction between Cu2SnS3 and Cu2SnS3OH. Another pair of redox peaks at −0.31 V/−0.43 V might be attributed to the redox reaction of Cu/Cu+ [5, 6]. As a fair comparison, the CV curves of CTSs@CQDs at various scan rates were depicted in Figure 3(b). The specific capacitances of the electrodes can be calculated using the following equation [24–27]:where Cm is the specific capacitance, i is the charge/discharge current, m is the mass of the active electrode material, is the scan rate, and ΔV is the voltage window.
(a)
(b)
(c)
(d)
Here, the calculated specific capacitances of CTSs and CTSs@CQDs were summarized in Figure 3(c). Interestingly, at a low scan rate of 2 and 5 mV·s−1, CTSs and CTSs@CQDs showed similar specific capacitances, which could be attributed to the nanoflower nanostructure, ensuring enough electrolyte penetration and fast ion transport. However, with the increasing scan rate, CTSs@CQDs demonstrated enhanced electrochemical performance than that of CTSs owing to the modification of CQDs. Even at a high scan rate of 50 mV·s−1, CTSs@CQDs delivered a specific capacitance of 474 F·g−1, which was over two times to that of CTSs (192 F·g−1), further confirming the important role of CQDs. The cycling stability of CTSs and CTSs@CQDs is depicted in Figure 3(d). Obviously, CTSs@CQDs displayed superior cycling performance to that of CTSs. It should be noted that the specific capacitance of CTSs@CQDs increased gradually in the initial 50 cycles. This phenomenon might be caused by the activation process of active materials owing to the decoration of CQDs, which not only improved the electrical conductivity of CTSs but also alleviated volume change during redox reactions [28].
4. Conclusions
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.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgments
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.