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Graphene quantum dots-induced morphological changes in CuCo2S4 nanocomposites for supercapacitor electrodes with enhanced performance
Accepted Manuscript Full Length Article Graphene quantum dots-induced morphological changes in CuCo2S4 nanocomposites for supercapacitor electrodes with enhanced performance Yuanyuan Huang, Liwei Lin, Tielin Shi, Siyi Cheng, Yan Zhong, Chen Chen, Zirong Tang PII: DOI: Reference:
S0169-4332(18)32387-0 https://doi.org/10.1016/j.apsusc.2018.08.247 APSUSC 40284
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
2 June 2018 16 August 2018 28 August 2018
Please cite this article as: Y. Huang, L. Lin, T. Shi, S. Cheng, Y. Zhong, C. Chen, Z. Tang, Graphene quantum dotsinduced morphological changes in CuCo2S4 nanocomposites for supercapacitor electrodes with enhanced performance, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.08.247
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Graphene quantum dots-induced morphological changes in CuCo2S4 nanocomposites for supercapacitor electrodes with enhanced performance Yuanyuan Huang1,2,3, Liwei Lin2,3 Tielin Shi1, Siyi Cheng1,2,3, Yan Zhong1, Chen Chen1 and Zirong Tang1* 1
State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China
2
Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA 3
Berkeley Sensor and Actuator, Berkeley, CA 94720, USA E-mail: [email protected]
*corresponding author, 1037 Luoyu Road, Wuhan 430074, China. Tel.: +86 27 87792241; fax: +86 27 87792413; E-mail address: [email protected] (Z. Tang)
Abstract Graphene quantum dots (GQDs) have been explored in recent years for electrochemical applications
with
considerable
potentials.
Here,
we
present
GQD-doped
CuCo2S4
nanocomposites through two-step hydrothermal process for supercapacitor electrodes. The surface of CuCo2S4 nanosheets changes from smooth to particles-accumulative shape, which assists the electrochemical cycling processes as well as the ion diffusion and charge transfer kinetics for improved supercapacitor performances. As a result, GQD/CuCo2S4 electrodes
demonstrate a specific capacitance of 1725 F g-1 under a current density of 0.5 A g-1 and a cycling life of 10000 cycles by retaining 90% of the energy storage capability. As such, this work extends the potential of GQDs in electrochemical applications by means of morphology change of CuCo2S4 nanosheets.
Highlight:
graphene quantum dots induce morphology changes of CuCo 2S4 by turning the smooth CuCo2S4 surface to particle-constituting hierarchical nanostructure.
The morphology change assists the electrochemical cycling processes as well as the ion diffusion and charge transfer kinetics for improved supercapacitor performances.
GQD/CuCo2S4 electrodes demonstrate a specific capacitance of 1725 F g-1 under a current density of 0.5 A g-1 and a cycling life of 10000 cycles by retaining 90% of the energy storage capability.
Key Words: GQDs, CuCo2S4, supercapacitors, morphology change, electrodes
1 Introduction Graphene quantum dots (GQDs) have drawn great attention due to their unique properties from both quantum dots and graphene [1]. The early works on GQDs mainly focus on bioimaging probes [2], photovoltaic [3] and light-emitting diodes [4] which utilize the physical properties of GQDs [5]. Recently, GQDs have been further applied in the area of electrochemical fields, such as GQDs-based electrochemical biosensors [6], edge-enriched GQDs supercapacitors [7] and GQDs coated VO2 for Li/Na-ion batteries [8, 9]. These results utilize the properties of GQDs in high specific surface areas, good electrical conductivity, good mobility, tunable bang gaps, and easy dispersion in various solvents. Supercapacitors have taken an increasing share of the energy devices market due to their high power density, long cycle life, and fast charge-discharge processes [10, 11]. However, the common electrode materials based on carbon [12], oxides [13], nitride [14], and polymer [15] have limited energy storage capacities, which can hardly meet the ever-growing energy demand for various electronic devices [16, 17]. In additional, comparing with rechargeable-batteries, supercapacitors deliver much low energy density [10]. As a result, abundant efforts have been concentrated on various nanomaterials to increase the energy storage capability [10, 18]. For example, transition metal oxides have advantages in their high theoretical capacity, relatively low cost, low toxicity, and great flexibility in morphology [19] which have been widely application such as: CuCo2O4 spinel studied as a candidate solid oxide fuel cell (SOFC) [20]. However, their practical application is hindered by the poor electron transport ability [21] and quick capacity decay at high charging-discharging rates [22]. Among these materials, transition metal sulfide such as CuCo2S4 has been well investigated [23, 24]. The two different metal cations in the system result in enhanced electrical/ionic conductivity and higher capacity due to the synergistic
effect of the two components for better electrochemical performance [23]. Whereas, the intrinsic defects, low electronic conductivity and unstable architecture result in low rate ability and poor cycling stability [25]. To overcome these, considerable researches have been focus on combining transition metal oxides or transition metal sulfides with carbon materials, such as NiCo 2S4 grown on graphene sheet to improve the specific capacitance [26], carbon coated CuCo2S4 exhibit improved capacity and cyclability [27], CuCo2S4 nanoparticles coated onto porous Scandia stabilized zirconia framework to provide more reactive sites [28], NiMn double hydroxide/CNT architecture with improved energy density [29]; and NiMn2O4/C arrays with stable cycling behavior [30]. Embellished these materials with carbon materials can effectively enhance their properties but it may also introduce some disadvantages, such as the reduction in the specific capacitance due to the low capacitance carbon materials. Considering the tiny size and excellent properties of GQDs, we present the design, fabrication, and testing of GQDs/CuCo 2S4 nanocomposites. It is found adding GQDs can induce morphology changes of CuCo 2S4 by turning the smooth CuCo2S4 surface to particle-constituting hierarchical nanostructure. The GQD/CuCo2S4 electrode exhibits an enhanced specific capacitance of 1725 F g -1 under a current density of 0.5 A g -1. Moreover, GQD/CuCo2S4 electrode has good cycling stability of 10,000 cycles with 10% capacitance losses. Another transition metal oxides material, NiMoO4, is also reported in this work with the synthesis of GQD/NiMoO4 nanocomposites for comparisons, but no obvious morphology changes can be identified. The morphology evolution mechanism needs further explore, nevertheless, the addition of GQDs is a potential way to increase the surface areas and conductivity of the electrodes for GQD/CuCo2S for electrochemical applications.
2 Experimental All the chemicals were used without further purification.
2.1 Synthesis of GQD/CuCo2S4 and CuCo2S4 precursor Firstly, Ni foam was carefully cleaned by HCL, ethanol and deionized (DI) water to remove the impurities. To synthesize GQD/CuCo 2S4 and CuCo2S4 precursor, 1 mmol Cu(NO3)2, 2 mmol Co(NO3)2, and 6 mmol urea were dissolved in a mixed solution of 20 ml DI water and 10 ml ethanediol under magnetic stirring for 10 minutes. Secondly, 10 mg GQDs powder (Xfnano) was added and dissolved in the solution (CuCo 2S4 precursor skipped this step). Thirdly, the mixture was transferred into a Teflon-lined autoclave and the cleaned Ni foam was also put into the autoclave as the substrate at 120 oC for 12 h. After the autoclave was cooled down to room temperature, the sample was clean several times with DI water. Finally, GQD/CuCo 2S4 and CuCo2S4 precursors grown on the Ni foam were obtained after drying at 60 oC for 12 h.
2.2 Synthesis of GQD/CuCo 2S4 and CuCo2S4 3 mmol Na2S was dissolved in 30 ml DI water and transferred into an autoclave. The Ni foam grown with the GQD/CuCo2S4 or CuCo2S4 precursors were put into the autoclave and kept at 160 oC for 6 h. After the autoclave was cooled down to room temperature, the GQD/CuCo 2S4 or CuCo2S4 was obtained and cleaned with DI water and the loading mass of active materials were respectively 10 mg/cm2 and 8 mg/cm2.
2.3 Synthesis of GQD/NiMoO4 and NiMoO4
Carbon cloth was used as the substrate and cleaned by 10% nitric acid and DI water. Firstly, 2.5 mmol Ni(NO3)2 6H2O, 2.5 mmol Na2MoO4 7H2O, and 10 mg GQD (synthesis of NiMoO4 without adding GQD) were dissolved in 50 ml DI water and transferred with cleaned carbon cloth into a Teflon-lined autoclave at 150 oC for 6 h. After the autoclave was cooled down to room temperature, the carbon cloth grown with NiMoO 4 precursor or GQD/NiMoO4 precursor was cleaned by DI water for several times. Next, the carbon cloth was dried in an oven at 60 oC for 12 h. Finally, the NiMoO4 or GQD/NiMoO4 nanowires grown on carbon cloth were obtained after annealing the precursor grown on carbon cloth at 400 oC for 1 h in Ar atmosphere.
2.4 Characterizations The morphologies of GQD/CuCo2S4 and CuCo2S4 were analysis by scanning electronic microscopy (SEM, FEI Nova NanoSEM 450), transmission electron microscopy (TEM, FEI Tecnai G2 S-TWIN). X-ray diffraction (XRD) patterns of GQD/CuCo2S4 and the CuCo2S4 nanosheets were collected through a Scanting XDS2000 diffractometer with Cu K radiation ( = 0.154 nm) at 40 kV with a step size of 0.01o.
2.5 Electrochemical test Electrochemical performance of the samples was carried out in a three-electrode configuration in the 3M KOH electrolyte. The GQD/CuCo2S4 and the CuCo2S4 electrodes served as the working electrodes, respectively. A Pt plate and saturated calomel electrode (SCE) were respectively used as the counter electrode and the reference electrode. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using an electrochemical
workstation (PGSTAT-302N, Eco Echemie B.V. Company). CV measurements were performed with increasing scan rates from 5 mV s -1 to 100 mV s-1. EIS measurements were carried out at open circuit potentials in a frequency range from 0.01 Hz to 100 KHz. A CT2001D tester (LAND electronics Co. Ltd., Wuhan, China) was employed to measure the galvanostatic chargedischarge (GCD) performance. All testing samples were cut into 1 cm × 1 cm in size and the active material mass loading of the GQD/CuCo2S4 electrode and the CuCo2S4 electrode were about 10 and 8 mg cm-2, respectively. The areal specific capacitance (Ca) and mass specific capacitance (Cm) were calculated from GCD curves by following the equations: [31] Cm=I×t/(m×ΔV)
(1)
Ca=I×t/(A ×ΔV)
(2)
Where I, t, m, ΔV, and A respectively are the discharge current (mA), the discharge time (s), the total mass of two electrode active materials (g), the potential window of the electrode(V), and the surface area of the electrode (cm2).
3 Results and discussion
Fig. 1. (A) Schematic diagrams of the synthesis processes of the CuCo 2S4 nanosheets and the GQD/CuCo2S4 nanocomposites grown on the Ni foam. (B) The schematic diagram showing the morphology evolutions of the GQD/CuCo2S4 nanocomposites during the sulfuration processes. (C) SEM image of the CuCo 2S4 nanosheets. (D) SEM image of the GQD/CuCo 2S4 nanocomposites. (E) Nitrogen absorption and desorption isotherm of the CuCo 2S4 nanosheets and the GQD/CuCo2S4 nanocomposites.
The CuCo2S4 nanosheets and GQD/CuCo2S4 nanocomposites reported in this study are prepared by a two-step hydrothermal process as shown in Fig. 1A. First, the Cu/Co precursor and GQD/Cu/Co precursor grown on the Ni foam substrate are synthesized by the first hydrothermal process. The CuCo2S4 nanosheeets and GQD/CuCo2S4 nanocomposites are obtained through a following second-step sulfuration reaction. Comparing the SEM images of CuCo2S4 (Fig. 1C) and GQD/CuCo2S4 (Fig. 1D), it is found the addition of GQDs induces the morphology change
of CuCo2S4. In Fig. 1C, the CuCo2S4 nanosheets cover the Ni foam to form an interconnected structure, which looks like the coral thicket. Whereas, the GQD/CuCo2S4 nanocomposites consist of dense and rough-surface with particles-accumulative characteristics, which result in larger specific surface area (SSA) and redox reaction sites in favor of electrochemical reactions. To investigate the surface properties of the as-prepared GQD/CuCo2S4 and CuCo2S4, BrunauerEmmett-Teller (BET) nitrogen adsorption-desorption measurements were carried out (Fig. 1E). The specific surface areas of the CuCo 2S4 nanosheets and GQD/CuCo 2S4 nanocomposites are 6.207, 8.398 m2 g-1, respectively, which increased by 35%. This result is also better than the similar structure CuCo 2S4 electrodes, such as CuCo 2S4 nanosheets of 6.163 m2 g-1 [32]. This larger surface area of the coral-thicket–like GQD/CuCo2S4 nanocomposites offers rich electroactive sites and short diffusion paths for charge transports resulting in enhanced electrochemical performance. Further study will be done to optimize the morphology of the GQD/CuCo2S4 nanocomposites thorough adjusting the addition mass of the GQD or the other reaction conditions. Furthermore, adjusting the morphology evolution by the addition of GQD presents a new way for the improvement of the electrochemical performance. The morphology change could be explained by the anion exchange reaction during the second-step reaction process (Fig. 1B) [33]. During the first-step process, the Cu/Co precursor grows on the Ni foam via a hydrothermal process and GQDs are incorporate into the precursor. Subsequently, the GQD/Cu/Co precursor is transformed into GQD/CuCo2S4 nanocomposites through an anion exchange reaction during the second-step process. Divalent sulfur ions S2reacts with the Cu/Co precursor forming CuCo2S4 nanoparticles on the surface, which combine with GQDs nanoparticles to prevent the anisotropic growth of CuCo2S4 nanoparticles [18, 34], The new-formed CuCo2S4 crystal containing large amount of GQD on the surface, which stop
growing in all directions because of the tiny-sized GQDs block the CuCo 2S4 growth units [35]. As the reaction continues, numerous CuCo2S4 nanoparticles agglomerate to form GQD/CuCo2S4 nanosheets.
Fig. 2. The SEM images of NiMoO4 nanowires (A), and GQD/NiMoO4 nanowires (B) grown on the carbon cloth substrate.
We also added GQDs to the NiMoO4 fabrication process. However, the introduction of GQDs doesn’t induce considerable morphology change of NiMoO4. Scanning electron microscopy (SEM) images of NiMoO4 (Figure. 2A) and GQD/NiMoO4 (Figure. 2B) show that the morphology of these nanowires are similar. This result demonstrates the introduction of GQDs can only induce the morphology change under certain conditions. For example, the synthesis process and material parameters are not suitable to change the morphology of NiMoO 4 nanostructures.
Fig. 3. The XRD patterns of CuCo2S4 nanosheets and GQD/CuCo2S4 nanocomposites grown on the Ni foam substrates.
The crystal structure of GQD/CuCo 2S4 has been characterized by XRD (Figure. 3). The three diffraction peaks at 44.7o, 52.1o and 76.5o respectively correspond to the (111), (200) and (220) crystal planes of the Ni foam. The resulting diffraction peaks at 31.5o, 38.1o, 50o, 55o collected from CuCo2S4 (JCPDS card No.42-1450) were indexed respectively as the (113), (004), (115) and (044) [23] planes of CuCo2S4. Meanwhile, no obvious peak at 25o exits for the presence of GQDs, which corresponds to the low amount and low diffraction intensity of GQDs. Compared to the full-width at half-maximum (FWHM), the GQDs/CuCo2S4 nanocomposites have a smaller value than CuCo2S4, which means a decrease in crystallite size as the crystal growth process of CuCo 2S4 is affected by GQDs.
Fig. 4. (A) The TEM image of CuCo2S4 and (B) GQD/CuCo2S4 nanocomposites. (C) EDX elemental mapping of Cu, Co, S and C. (D) HRTEM image of the GQD/CuCo2S4 nanocomposites.
Figure 4A shows a typical low-magnification TEM image of the CuCo2S4 nanosheets, in which a piece of nanosheet forms a fiber-like morphology. As expected, the TEM image of GQD/CuCo2S4 nanocomposites (in Fig. 4B) show obvious different crystal state from CuCo2S4 nanosheet. The EDX mapping images of GQD/CuCo2S4 are shown in Fig. 4C, which describes the distribution of Cu, Co, S, and C elements to indicate the existence and distribution of GQDs
in the GQD/CuCo2S4 nanocomposites. Furthermore, high-resolution transmission electron microscopy (HRTEM) is used to characterize GQD/CuCo2S4 (Fig. 4D). The measured fringe lattice spacing of 0.28 nm is the inter-planar distance of CuCo 2S4 which corresponds to the (113) [23].
Fig. 5. (A) The CV curves of electrodes made of NiMoO4 and GQD/NiMoO4 nanocomposites at a scan rate of 5 mVs-1. (B) The CV curves of electrodes made of CuCo2S4 and GQD/CuCo2S4
nanocomposites at a scan rate of 5 mVs -1. (C) The galvanostatic charge-discharge curves of GQDs/CuCo2S4 nanocomposite electrodes at different current densities. (D) The discharge mass capacitance of CuCo2S4 and GQDs/CuCo2S4 nanocomposites at different current densities. (E) Charging-discharging properties of GQDs/CuCo2S4 nanocomposites under a current density of 20 m A cm-2 for 10000 cycles. (F) EIS Nyquist plots of CuCo2S4 and GQDs/CuCo2S4 nanocomposites electrode.
The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) are recorded for electrodes made of CuCo2S4, GQD/CuCo2S4, NiMoO4 and GQD/NiMoO4 using 3M KOH as the electrolyte. First, the CV curves of electrodes made of NiMoO4 and GQD/NiMoO4 (Fig. 5A) show inconsiderable differences, implying the introduction of GQD doesn’t attribute to the NiMoO4 and GQD/NiMoO4 electrode electrochemical performance. Figure 5B shows the CV curves of electrodes made of CuCo 2S4 and GQD/CuCo2S4 nanocomposites at a scan rate of 5 mV s -1 have similar redox peaks corresponding to the reversible Faradaic processes of Co 4+/Co3+ and Cu2+/Cu+ [36]. The CV curve of the GQD/CuCo2S4 electrode has higher peak current and larger enclosed area as the introduction of GQDs can induce the morphological changes and bring more electrochemical reactions. The galvanostatic charge-discharge tests are carried out at various current densities (0.5 A g-1 to 5 A g-1) with an electrochemical window ranging from 0 to 0.5 V in 3 M KOH aqueous as shown in Fig. 5C. The specific capacitance of the CuCo2S4 electrode and GQD/CuCo2S4 electrode are calculated from the charge-discharge curves (Fig. 5D). It is found that the GQD/CuCo2S4 electrode has about twice the specific capacitances of those from the bare CuCo2S4 electrodes. The highest specific capacitance of the nanocomposite electrode is 1725 F
g-1 at 0.1 A g-1, which is comparable to the values reported in the literature based on CuCo2S4 electrode, such as the flower-like CuCo2S4 nanosheet at 908.9 F g-1 [37], the CuCo2S4-CNT hybrid electrode at 504 F g-1 [32]; and the mesoporous CuCo 2S4 nanoparticles at 752 F g-1 [38]. Even at a current density of 5 A g-1, the hybrid electrode still shows good rate capability. In Fig. 5E, the cycling stability of CuCo 2S4 nanosheets electrode and GQD/CuCo 2S4 nanocomposite electrode are investigated. The GQD/CuCo 2S4 nanocomposite electrode owns 90% of the initial value retention after 10,000 cycles, which is much better than those of the electrodes made of bare CuCo2S4. Furthermore, this cycling stability result is better than most other reported results based on CuCo2S4 electrode. The Nyquist plot of the CuCo 2S4 and GDQ/CuCo2S4 electrodes exhibits a straight line in the low-frequency region and an arc shape in the high frequency region as shown in Fig. 5F. The EIS Nyquist plots are fitted by an equivalent circuit, which is composed of an internal resistance, Rs, an interfacial charge transfer resistance, Rct, a Warburg resistance, W, and a double layer capacitance, C. At high-frequency, the GQD/CuCo 2S4 electrode shows a semicircle with a small diameter, which means the GQD/CuCo 2S4 electrode has a lower Rct than that of the CuCo 2S4 electrode. The vertical shape at low-frequency is related to the Warburg resistance, which corresponds to the ion diffusion process in the electrode. The slop of the GQD/CuCo 2S4 electrode in the EIS plot is clearly larger than that of the CuCo 2S4 electrode, implying better ion diffusion process. These results manifest that GQDs/CuCo2S4 nanocomposite electrodes have lower resistance and faster electron-transfer rate due to the high conductivity of GQDs. These improved electrochemical results are achieved with the introduction of GQDs forming 3D nanostructures which exhibit large surface areas and reduced resistance between the electrode/electrolyte as well as the alleviations on the volume expansion issues. Specifically,
GQDs are 0D material with large surface areas to be processed with vast variety of organic groups to form strong contacts with CuCo 2S4 and suppress the dissolution and agglomeration of the CuCo2S4 during the redox reaction. The base properties of GQDs derived from graphene and quantum dots are in favor of the ion diffusion and charge transfer kinetics for enhanced electrochemical performances.
4 Conclusion In conclusion, the introduction of GQDs induces the morphology change of CuCo2S4, forming a unique nanostructure with rough surface. The addition of GQD and the morphology evolution result in improved conductivity and enhanced capacitance with improved long-term cycling stability. These results show that GQDs hold great promise in electrochemical applications and are beneficial for further improvements in the electrodes for electrochemical reactions. It is also an effective and feasible approach to adjust the nanostructure and morphology through adding 0D quantum dots, such as GQDs.
Acknowledgments This work is supported by the National Science Foundation of China (No. 51775218), the Program for Changjiang Scholars and the Innovative Research Team in University (grant no. IRT13017). We would like to thank the Analytical and Testing Center of Huazhong University of Science and Technology. Y.H. and S.C. acknowledge additional support though the China Scholarship Council.
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S0169-4332(18)32387-0 https://doi.org/10.1016/j.apsusc.2018.08.247 APSUSC 40284
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
2 June 2018 16 August 2018 28 August 2018
Please cite this article as: Y. Huang, L. Lin, T. Shi, S. Cheng, Y. Zhong, C. Chen, Z. Tang, Graphene quantum dotsinduced morphological changes in CuCo2S4 nanocomposites for supercapacitor electrodes with enhanced performance, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.08.247
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Graphene quantum dots-induced morphological changes in CuCo2S4 nanocomposites for supercapacitor electrodes with enhanced performance Yuanyuan Huang1,2,3, Liwei Lin2,3 Tielin Shi1, Siyi Cheng1,2,3, Yan Zhong1, Chen Chen1 and Zirong Tang1* 1
State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China
2
Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA 3
Berkeley Sensor and Actuator, Berkeley, CA 94720, USA E-mail: [email protected]
*corresponding author, 1037 Luoyu Road, Wuhan 430074, China. Tel.: +86 27 87792241; fax: +86 27 87792413; E-mail address: [email protected] (Z. Tang)
Abstract Graphene quantum dots (GQDs) have been explored in recent years for electrochemical applications
with
considerable
potentials.
Here,
we
present
GQD-doped
CuCo2S4
nanocomposites through two-step hydrothermal process for supercapacitor electrodes. The surface of CuCo2S4 nanosheets changes from smooth to particles-accumulative shape, which assists the electrochemical cycling processes as well as the ion diffusion and charge transfer kinetics for improved supercapacitor performances. As a result, GQD/CuCo2S4 electrodes
demonstrate a specific capacitance of 1725 F g-1 under a current density of 0.5 A g-1 and a cycling life of 10000 cycles by retaining 90% of the energy storage capability. As such, this work extends the potential of GQDs in electrochemical applications by means of morphology change of CuCo2S4 nanosheets.
Highlight:
graphene quantum dots induce morphology changes of CuCo 2S4 by turning the smooth CuCo2S4 surface to particle-constituting hierarchical nanostructure.
The morphology change assists the electrochemical cycling processes as well as the ion diffusion and charge transfer kinetics for improved supercapacitor performances.
GQD/CuCo2S4 electrodes demonstrate a specific capacitance of 1725 F g-1 under a current density of 0.5 A g-1 and a cycling life of 10000 cycles by retaining 90% of the energy storage capability.
Key Words: GQDs, CuCo2S4, supercapacitors, morphology change, electrodes
1 Introduction Graphene quantum dots (GQDs) have drawn great attention due to their unique properties from both quantum dots and graphene [1]. The early works on GQDs mainly focus on bioimaging probes [2], photovoltaic [3] and light-emitting diodes [4] which utilize the physical properties of GQDs [5]. Recently, GQDs have been further applied in the area of electrochemical fields, such as GQDs-based electrochemical biosensors [6], edge-enriched GQDs supercapacitors [7] and GQDs coated VO2 for Li/Na-ion batteries [8, 9]. These results utilize the properties of GQDs in high specific surface areas, good electrical conductivity, good mobility, tunable bang gaps, and easy dispersion in various solvents. Supercapacitors have taken an increasing share of the energy devices market due to their high power density, long cycle life, and fast charge-discharge processes [10, 11]. However, the common electrode materials based on carbon [12], oxides [13], nitride [14], and polymer [15] have limited energy storage capacities, which can hardly meet the ever-growing energy demand for various electronic devices [16, 17]. In additional, comparing with rechargeable-batteries, supercapacitors deliver much low energy density [10]. As a result, abundant efforts have been concentrated on various nanomaterials to increase the energy storage capability [10, 18]. For example, transition metal oxides have advantages in their high theoretical capacity, relatively low cost, low toxicity, and great flexibility in morphology [19] which have been widely application such as: CuCo2O4 spinel studied as a candidate solid oxide fuel cell (SOFC) [20]. However, their practical application is hindered by the poor electron transport ability [21] and quick capacity decay at high charging-discharging rates [22]. Among these materials, transition metal sulfide such as CuCo2S4 has been well investigated [23, 24]. The two different metal cations in the system result in enhanced electrical/ionic conductivity and higher capacity due to the synergistic
effect of the two components for better electrochemical performance [23]. Whereas, the intrinsic defects, low electronic conductivity and unstable architecture result in low rate ability and poor cycling stability [25]. To overcome these, considerable researches have been focus on combining transition metal oxides or transition metal sulfides with carbon materials, such as NiCo 2S4 grown on graphene sheet to improve the specific capacitance [26], carbon coated CuCo2S4 exhibit improved capacity and cyclability [27], CuCo2S4 nanoparticles coated onto porous Scandia stabilized zirconia framework to provide more reactive sites [28], NiMn double hydroxide/CNT architecture with improved energy density [29]; and NiMn2O4/C arrays with stable cycling behavior [30]. Embellished these materials with carbon materials can effectively enhance their properties but it may also introduce some disadvantages, such as the reduction in the specific capacitance due to the low capacitance carbon materials. Considering the tiny size and excellent properties of GQDs, we present the design, fabrication, and testing of GQDs/CuCo 2S4 nanocomposites. It is found adding GQDs can induce morphology changes of CuCo 2S4 by turning the smooth CuCo2S4 surface to particle-constituting hierarchical nanostructure. The GQD/CuCo2S4 electrode exhibits an enhanced specific capacitance of 1725 F g -1 under a current density of 0.5 A g -1. Moreover, GQD/CuCo2S4 electrode has good cycling stability of 10,000 cycles with 10% capacitance losses. Another transition metal oxides material, NiMoO4, is also reported in this work with the synthesis of GQD/NiMoO4 nanocomposites for comparisons, but no obvious morphology changes can be identified. The morphology evolution mechanism needs further explore, nevertheless, the addition of GQDs is a potential way to increase the surface areas and conductivity of the electrodes for GQD/CuCo2S for electrochemical applications.
2 Experimental All the chemicals were used without further purification.
2.1 Synthesis of GQD/CuCo2S4 and CuCo2S4 precursor Firstly, Ni foam was carefully cleaned by HCL, ethanol and deionized (DI) water to remove the impurities. To synthesize GQD/CuCo 2S4 and CuCo2S4 precursor, 1 mmol Cu(NO3)2, 2 mmol Co(NO3)2, and 6 mmol urea were dissolved in a mixed solution of 20 ml DI water and 10 ml ethanediol under magnetic stirring for 10 minutes. Secondly, 10 mg GQDs powder (Xfnano) was added and dissolved in the solution (CuCo 2S4 precursor skipped this step). Thirdly, the mixture was transferred into a Teflon-lined autoclave and the cleaned Ni foam was also put into the autoclave as the substrate at 120 oC for 12 h. After the autoclave was cooled down to room temperature, the sample was clean several times with DI water. Finally, GQD/CuCo 2S4 and CuCo2S4 precursors grown on the Ni foam were obtained after drying at 60 oC for 12 h.
2.2 Synthesis of GQD/CuCo 2S4 and CuCo2S4 3 mmol Na2S was dissolved in 30 ml DI water and transferred into an autoclave. The Ni foam grown with the GQD/CuCo2S4 or CuCo2S4 precursors were put into the autoclave and kept at 160 oC for 6 h. After the autoclave was cooled down to room temperature, the GQD/CuCo 2S4 or CuCo2S4 was obtained and cleaned with DI water and the loading mass of active materials were respectively 10 mg/cm2 and 8 mg/cm2.
2.3 Synthesis of GQD/NiMoO4 and NiMoO4
Carbon cloth was used as the substrate and cleaned by 10% nitric acid and DI water. Firstly, 2.5 mmol Ni(NO3)2 6H2O, 2.5 mmol Na2MoO4 7H2O, and 10 mg GQD (synthesis of NiMoO4 without adding GQD) were dissolved in 50 ml DI water and transferred with cleaned carbon cloth into a Teflon-lined autoclave at 150 oC for 6 h. After the autoclave was cooled down to room temperature, the carbon cloth grown with NiMoO 4 precursor or GQD/NiMoO4 precursor was cleaned by DI water for several times. Next, the carbon cloth was dried in an oven at 60 oC for 12 h. Finally, the NiMoO4 or GQD/NiMoO4 nanowires grown on carbon cloth were obtained after annealing the precursor grown on carbon cloth at 400 oC for 1 h in Ar atmosphere.
2.4 Characterizations The morphologies of GQD/CuCo2S4 and CuCo2S4 were analysis by scanning electronic microscopy (SEM, FEI Nova NanoSEM 450), transmission electron microscopy (TEM, FEI Tecnai G2 S-TWIN). X-ray diffraction (XRD) patterns of GQD/CuCo2S4 and the CuCo2S4 nanosheets were collected through a Scanting XDS2000 diffractometer with Cu K radiation ( = 0.154 nm) at 40 kV with a step size of 0.01o.
2.5 Electrochemical test Electrochemical performance of the samples was carried out in a three-electrode configuration in the 3M KOH electrolyte. The GQD/CuCo2S4 and the CuCo2S4 electrodes served as the working electrodes, respectively. A Pt plate and saturated calomel electrode (SCE) were respectively used as the counter electrode and the reference electrode. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using an electrochemical
workstation (PGSTAT-302N, Eco Echemie B.V. Company). CV measurements were performed with increasing scan rates from 5 mV s -1 to 100 mV s-1. EIS measurements were carried out at open circuit potentials in a frequency range from 0.01 Hz to 100 KHz. A CT2001D tester (LAND electronics Co. Ltd., Wuhan, China) was employed to measure the galvanostatic chargedischarge (GCD) performance. All testing samples were cut into 1 cm × 1 cm in size and the active material mass loading of the GQD/CuCo2S4 electrode and the CuCo2S4 electrode were about 10 and 8 mg cm-2, respectively. The areal specific capacitance (Ca) and mass specific capacitance (Cm) were calculated from GCD curves by following the equations: [31] Cm=I×t/(m×ΔV)
(1)
Ca=I×t/(A ×ΔV)
(2)
Where I, t, m, ΔV, and A respectively are the discharge current (mA), the discharge time (s), the total mass of two electrode active materials (g), the potential window of the electrode(V), and the surface area of the electrode (cm2).
3 Results and discussion
Fig. 1. (A) Schematic diagrams of the synthesis processes of the CuCo 2S4 nanosheets and the GQD/CuCo2S4 nanocomposites grown on the Ni foam. (B) The schematic diagram showing the morphology evolutions of the GQD/CuCo2S4 nanocomposites during the sulfuration processes. (C) SEM image of the CuCo 2S4 nanosheets. (D) SEM image of the GQD/CuCo 2S4 nanocomposites. (E) Nitrogen absorption and desorption isotherm of the CuCo 2S4 nanosheets and the GQD/CuCo2S4 nanocomposites.
The CuCo2S4 nanosheets and GQD/CuCo2S4 nanocomposites reported in this study are prepared by a two-step hydrothermal process as shown in Fig. 1A. First, the Cu/Co precursor and GQD/Cu/Co precursor grown on the Ni foam substrate are synthesized by the first hydrothermal process. The CuCo2S4 nanosheeets and GQD/CuCo2S4 nanocomposites are obtained through a following second-step sulfuration reaction. Comparing the SEM images of CuCo2S4 (Fig. 1C) and GQD/CuCo2S4 (Fig. 1D), it is found the addition of GQDs induces the morphology change
of CuCo2S4. In Fig. 1C, the CuCo2S4 nanosheets cover the Ni foam to form an interconnected structure, which looks like the coral thicket. Whereas, the GQD/CuCo2S4 nanocomposites consist of dense and rough-surface with particles-accumulative characteristics, which result in larger specific surface area (SSA) and redox reaction sites in favor of electrochemical reactions. To investigate the surface properties of the as-prepared GQD/CuCo2S4 and CuCo2S4, BrunauerEmmett-Teller (BET) nitrogen adsorption-desorption measurements were carried out (Fig. 1E). The specific surface areas of the CuCo 2S4 nanosheets and GQD/CuCo 2S4 nanocomposites are 6.207, 8.398 m2 g-1, respectively, which increased by 35%. This result is also better than the similar structure CuCo 2S4 electrodes, such as CuCo 2S4 nanosheets of 6.163 m2 g-1 [32]. This larger surface area of the coral-thicket–like GQD/CuCo2S4 nanocomposites offers rich electroactive sites and short diffusion paths for charge transports resulting in enhanced electrochemical performance. Further study will be done to optimize the morphology of the GQD/CuCo2S4 nanocomposites thorough adjusting the addition mass of the GQD or the other reaction conditions. Furthermore, adjusting the morphology evolution by the addition of GQD presents a new way for the improvement of the electrochemical performance. The morphology change could be explained by the anion exchange reaction during the second-step reaction process (Fig. 1B) [33]. During the first-step process, the Cu/Co precursor grows on the Ni foam via a hydrothermal process and GQDs are incorporate into the precursor. Subsequently, the GQD/Cu/Co precursor is transformed into GQD/CuCo2S4 nanocomposites through an anion exchange reaction during the second-step process. Divalent sulfur ions S2reacts with the Cu/Co precursor forming CuCo2S4 nanoparticles on the surface, which combine with GQDs nanoparticles to prevent the anisotropic growth of CuCo2S4 nanoparticles [18, 34], The new-formed CuCo2S4 crystal containing large amount of GQD on the surface, which stop
growing in all directions because of the tiny-sized GQDs block the CuCo 2S4 growth units [35]. As the reaction continues, numerous CuCo2S4 nanoparticles agglomerate to form GQD/CuCo2S4 nanosheets.
Fig. 2. The SEM images of NiMoO4 nanowires (A), and GQD/NiMoO4 nanowires (B) grown on the carbon cloth substrate.
We also added GQDs to the NiMoO4 fabrication process. However, the introduction of GQDs doesn’t induce considerable morphology change of NiMoO4. Scanning electron microscopy (SEM) images of NiMoO4 (Figure. 2A) and GQD/NiMoO4 (Figure. 2B) show that the morphology of these nanowires are similar. This result demonstrates the introduction of GQDs can only induce the morphology change under certain conditions. For example, the synthesis process and material parameters are not suitable to change the morphology of NiMoO 4 nanostructures.
Fig. 3. The XRD patterns of CuCo2S4 nanosheets and GQD/CuCo2S4 nanocomposites grown on the Ni foam substrates.
The crystal structure of GQD/CuCo 2S4 has been characterized by XRD (Figure. 3). The three diffraction peaks at 44.7o, 52.1o and 76.5o respectively correspond to the (111), (200) and (220) crystal planes of the Ni foam. The resulting diffraction peaks at 31.5o, 38.1o, 50o, 55o collected from CuCo2S4 (JCPDS card No.42-1450) were indexed respectively as the (113), (004), (115) and (044) [23] planes of CuCo2S4. Meanwhile, no obvious peak at 25o exits for the presence of GQDs, which corresponds to the low amount and low diffraction intensity of GQDs. Compared to the full-width at half-maximum (FWHM), the GQDs/CuCo2S4 nanocomposites have a smaller value than CuCo2S4, which means a decrease in crystallite size as the crystal growth process of CuCo 2S4 is affected by GQDs.
Fig. 4. (A) The TEM image of CuCo2S4 and (B) GQD/CuCo2S4 nanocomposites. (C) EDX elemental mapping of Cu, Co, S and C. (D) HRTEM image of the GQD/CuCo2S4 nanocomposites.
Figure 4A shows a typical low-magnification TEM image of the CuCo2S4 nanosheets, in which a piece of nanosheet forms a fiber-like morphology. As expected, the TEM image of GQD/CuCo2S4 nanocomposites (in Fig. 4B) show obvious different crystal state from CuCo2S4 nanosheet. The EDX mapping images of GQD/CuCo2S4 are shown in Fig. 4C, which describes the distribution of Cu, Co, S, and C elements to indicate the existence and distribution of GQDs
in the GQD/CuCo2S4 nanocomposites. Furthermore, high-resolution transmission electron microscopy (HRTEM) is used to characterize GQD/CuCo2S4 (Fig. 4D). The measured fringe lattice spacing of 0.28 nm is the inter-planar distance of CuCo 2S4 which corresponds to the (113) [23].
Fig. 5. (A) The CV curves of electrodes made of NiMoO4 and GQD/NiMoO4 nanocomposites at a scan rate of 5 mVs-1. (B) The CV curves of electrodes made of CuCo2S4 and GQD/CuCo2S4
nanocomposites at a scan rate of 5 mVs -1. (C) The galvanostatic charge-discharge curves of GQDs/CuCo2S4 nanocomposite electrodes at different current densities. (D) The discharge mass capacitance of CuCo2S4 and GQDs/CuCo2S4 nanocomposites at different current densities. (E) Charging-discharging properties of GQDs/CuCo2S4 nanocomposites under a current density of 20 m A cm-2 for 10000 cycles. (F) EIS Nyquist plots of CuCo2S4 and GQDs/CuCo2S4 nanocomposites electrode.
The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) are recorded for electrodes made of CuCo2S4, GQD/CuCo2S4, NiMoO4 and GQD/NiMoO4 using 3M KOH as the electrolyte. First, the CV curves of electrodes made of NiMoO4 and GQD/NiMoO4 (Fig. 5A) show inconsiderable differences, implying the introduction of GQD doesn’t attribute to the NiMoO4 and GQD/NiMoO4 electrode electrochemical performance. Figure 5B shows the CV curves of electrodes made of CuCo 2S4 and GQD/CuCo2S4 nanocomposites at a scan rate of 5 mV s -1 have similar redox peaks corresponding to the reversible Faradaic processes of Co 4+/Co3+ and Cu2+/Cu+ [36]. The CV curve of the GQD/CuCo2S4 electrode has higher peak current and larger enclosed area as the introduction of GQDs can induce the morphological changes and bring more electrochemical reactions. The galvanostatic charge-discharge tests are carried out at various current densities (0.5 A g-1 to 5 A g-1) with an electrochemical window ranging from 0 to 0.5 V in 3 M KOH aqueous as shown in Fig. 5C. The specific capacitance of the CuCo2S4 electrode and GQD/CuCo2S4 electrode are calculated from the charge-discharge curves (Fig. 5D). It is found that the GQD/CuCo2S4 electrode has about twice the specific capacitances of those from the bare CuCo2S4 electrodes. The highest specific capacitance of the nanocomposite electrode is 1725 F
g-1 at 0.1 A g-1, which is comparable to the values reported in the literature based on CuCo2S4 electrode, such as the flower-like CuCo2S4 nanosheet at 908.9 F g-1 [37], the CuCo2S4-CNT hybrid electrode at 504 F g-1 [32]; and the mesoporous CuCo 2S4 nanoparticles at 752 F g-1 [38]. Even at a current density of 5 A g-1, the hybrid electrode still shows good rate capability. In Fig. 5E, the cycling stability of CuCo 2S4 nanosheets electrode and GQD/CuCo 2S4 nanocomposite electrode are investigated. The GQD/CuCo 2S4 nanocomposite electrode owns 90% of the initial value retention after 10,000 cycles, which is much better than those of the electrodes made of bare CuCo2S4. Furthermore, this cycling stability result is better than most other reported results based on CuCo2S4 electrode. The Nyquist plot of the CuCo 2S4 and GDQ/CuCo2S4 electrodes exhibits a straight line in the low-frequency region and an arc shape in the high frequency region as shown in Fig. 5F. The EIS Nyquist plots are fitted by an equivalent circuit, which is composed of an internal resistance, Rs, an interfacial charge transfer resistance, Rct, a Warburg resistance, W, and a double layer capacitance, C. At high-frequency, the GQD/CuCo 2S4 electrode shows a semicircle with a small diameter, which means the GQD/CuCo 2S4 electrode has a lower Rct than that of the CuCo 2S4 electrode. The vertical shape at low-frequency is related to the Warburg resistance, which corresponds to the ion diffusion process in the electrode. The slop of the GQD/CuCo 2S4 electrode in the EIS plot is clearly larger than that of the CuCo 2S4 electrode, implying better ion diffusion process. These results manifest that GQDs/CuCo2S4 nanocomposite electrodes have lower resistance and faster electron-transfer rate due to the high conductivity of GQDs. These improved electrochemical results are achieved with the introduction of GQDs forming 3D nanostructures which exhibit large surface areas and reduced resistance between the electrode/electrolyte as well as the alleviations on the volume expansion issues. Specifically,
GQDs are 0D material with large surface areas to be processed with vast variety of organic groups to form strong contacts with CuCo 2S4 and suppress the dissolution and agglomeration of the CuCo2S4 during the redox reaction. The base properties of GQDs derived from graphene and quantum dots are in favor of the ion diffusion and charge transfer kinetics for enhanced electrochemical performances.
4 Conclusion In conclusion, the introduction of GQDs induces the morphology change of CuCo2S4, forming a unique nanostructure with rough surface. The addition of GQD and the morphology evolution result in improved conductivity and enhanced capacitance with improved long-term cycling stability. These results show that GQDs hold great promise in electrochemical applications and are beneficial for further improvements in the electrodes for electrochemical reactions. It is also an effective and feasible approach to adjust the nanostructure and morphology through adding 0D quantum dots, such as GQDs.
Acknowledgments This work is supported by the National Science Foundation of China (No. 51775218), the Program for Changjiang Scholars and the Innovative Research Team in University (grant no. IRT13017). We would like to thank the Analytical and Testing Center of Huazhong University of Science and Technology. Y.H. and S.C. acknowledge additional support though the China Scholarship Council.
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