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Bi2S3 nanoparticles anchored on graphene nanosheets with superior electrochemical performance for supercapacitors
Accepted Manuscript Title: Bi2 S3 Nanoparticles Anchored on Graphene Nanosheets with Superior Electrochemical Performance for Supercapacitors Authors: Haochen Lu, Qiubo Guo, Feng Zan, Hui Xia PII: DOI: Reference:
S0025-5408(17)31277-1 http://dx.doi.org/doi:10.1016/j.materresbull.2017.05.047 MRB 9366
To appear in:
MRB
Received date: Revised date: Accepted date:
2-4-2017 5-5-2017 25-5-2017
Please cite this article as: Haochen Lu, Qiubo Guo, Feng Zan, Hui Xia, Bi2S3 Nanoparticles Anchored on Graphene Nanosheets with Superior Electrochemical Performance for Supercapacitors, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.05.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bi2S3 Nanoparticles Anchored on Graphene Nanosheets with Superior Electrochemical Performance for Supercapacitors Haochen Lua,b†, Qiubo Guoa,b†, Feng Zana,b*, Hui Xiaa,b a
School of Materials Science and Engineering, Nanjing University of Science and
Technology, Nanjing 210094, China. b
Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and
Technology, Nanjing 210094, China. († Both authors contributed equally to this work) Graphical Abstract
Corresponding author, e-mail: [email protected]
Corresponding author, e-mail: [email protected] Tel: (86) 25 84303408 , Fax: (86) 25 84303408
Research highlights
A facile hydrothermal method is developed to prepare Bi2S3/functionalized graphene nanosheets (FGS) composites.
The Bi2S3/FGS composites are first time investigated as electrode materials for supercapacitors in neutral aqueous electrolyte.
The Bi2S3/FGS composite electrodes exhibit good rate capability and cycle performance, owing to the synergistic effect between Bi2S3 and FGS.
Abstract Great progress has been made in developing nanostructured metal oxides and metal sulfides as electrode materials for supercapacitors. Poor electrical conductivities of these materials, however, limit their supercapacitive performance. In this work, a facile synthesis strategy is developed to prepare Bi2S3/graphene composites with
Bi2S3 nanoparticles anchored on graphene nanosheets. The Bi2S3/graphene composite electrodes exhibit promising electrochemical performance in a negative potential window of -0.9-0 V (vs. Ag/AgCl) in 1 M Na2SO4 electrolyte. A large specific capacitance of about 400 F/g can be achieved by the Bi2S3/graphene composite electrode with good rate capability and cycle performance. The present work indicates that the Bi2S3/graphene composites are promising anode materials for developing high-performance asymmetric supercapacitors. Keywords: Supercapacitors; Composites; Bi2S3; Graphene; Nanoparticles 1. Introduction As a bridge connecting traditional capacitors and batteries, supercapacitors are becoming a new type of energy storage devices, which are getting more and more attention [1]. In order to meet the power requirements of fast-developing electrical devices, advanced supercapacitors with large capacitance, high power density, high energy density, and long cycle life are imminently needed. As the key component for supercapacitors, electrode materials play an important role in determining electrochemical performance. Among the electrode materials, transition metal oxides and sulfides have attracted more and more attention due to their large pseudocapacitance from reversible faradic redox reactions. As a member of metal oxides, Bi2O3 is a promising electrode material for supercapacitors owing to its large capacitance, appropriate potential window, good electrochemical stability, and simple preparation process [2-4]. Compared with Bi2O3, Bi2S3 could be more attractive as
electrode material for supercapcitors because of its improved electronic conductivity [5]. Many metal sulfides exhibit enhanced electronic conductivities than their oxide counterparts because the electronegativity of sulfur is lower than that of oxygen, making it easier for electrons to transport in the structure. The study of Bi2S3 as electrode material for supercapcitors, however, is very rare in literature. Therefore, it is imperative to investigate the electrochemical performance of Bi2S3 as electrode materials for supercapacitors. As charge storage occurs at the electrode surface for supercapacitors, it is a rational way to develop the electrode materials with large surface areas. Therefore, various nanostructures, including nanoparticles, nanowires, nanosheets, and etc., have been developed for improving the supercapacitive performance [6]. Although nanoparticles with large surface area are promising to obtain large specific capacitance, they tend to aggregate during the charge and discharge process due to the large surface energy, thus losing their advantage during cycling. To solve this problem, these metal oxide or sulfide nanoparticles are usually mixed with conductive additives to make composites, which can suppress aggregation of nanoparticles and further improve their electrical conductivity [7,8]. Specifically, decorating metal oxide or sulfide nanoparticles on graphene nanosheets has been demonstrated to be an effective strategy to improve the supercapacitive performance because graphene can provide a large conductive matrix, offering large surface area and fast electron transport [9-11]. In this work, a facile hydrothermal method was developed to prepare Bi2S3/functionalized graphene nanosheets (FGS) composites. In the composite, Bi2S3
nanoparticles of 20-50 nm in size were well distributed and tightly anchored on the FGS without severe aggregation. The Bi2S3/FGS composite electrode exhibited a large specific capacitance of about 396 F/g when tested as anode in 1M Na2SO4 electrolyte between -0.9-0 V (vs. Ag/AgCl) for supercapcitors. In addition to large specific capacitance, the Bi2S3/FGS composite electrode also presented good rate performance and good cycling stability. The present study indicated that the Bi2S3/FGS composite could be promising anode material for developing highperformance asymmetric supercapcitors. 2. Experimental 2.1. Preparation of FGS Graphene oxide (GO) was synthesized by Hummers method from purified natural graphite. After that, the dried GO was thermally exfoliated at 300℃ for 5 min in a tube furnace in air and then heated at 900℃ in Ar for 3 h with a heating rate of 2℃/min to obtain the FGS. 2.2. Preparation of Bi2S3/FGS composites As illustrated in Fig. 1a, the Bi2S3/FGS composites were synthesized by a simple hydrothermal method. In a typical synthesis, 0.05g FGS and 0.185g thioacetamide (TAA) was added to 20 mL deionized water with ultrasonication and magnetic stirring for 30min, respectively, and then 2 mL 0.4 M HNO3 and 0.2530 g Bi(NO3)3• 5H2O were added into the solution. After stirring for 1 h, 20 mL of dimethylformamide was added into the above solution and the well mixed solution
was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 150℃ for 2 h. After cooling down to room temperature, the products were collected by centrifugation and washed several times with distilled water and ethanol. After that, the samples were dried at 60 ℃ in a vacuum oven for further characterization. By changing the quantities of TAA and Bi(NO3)3•5H2O, the mass ratios of Bi2S3/FGS in the composites can be tuned as 1:1, 2:1, and 3:1. In the following text, the samples with different Bi2S3 contents are referred as 1:1, 2:1, and 3:1 samples. 2.3. Characterizations and electrochemical measurements The X-ray diffraction (XRD, Bruker-AXS D8 Advance with monochromatized Cu Kα radiation), Raman spectroscopy (Jobin-Yvon T6400 Micro-Raman system), and X-ray photoelectron spectroscopy (XPS, PhiQuantera SXM spectromenter using Al KαX-ray as the excitation source) were performed to characterize structural features and phase purity of the samples. The morphology and microstructure of the samples were investigated by using field emission scanning electron microscope (FESEM, Quant 250 FEG) and transmission electron microscope (TEM, FEI Tecnai 20). Electrochemical properties of the samples were investigated by using three-electrode cells with a Pt foil as counter electrode, Ag/AgCl as reference electrode, and Bi2S3/FGS composite as working electrode. To prepare the working electrode, active material, conductive agent (Super P), and polyvinylidene fluoride (PVDF) binder with a mass ratio of 8: 1: 1 were dissolved in N-Methyl-2-pyrrolidone (NMP) to form a slurry, and then the slurry was pasted on a Ti foil and dried in a vacuum oven at 80℃ for 12 h. CHI760C electrochemical workstation (Chenhua, Shanghai) and battery test
system (5V 1mA LANDCT2001A) were used to carry out cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge measurements in 1M Na2SO4 aqueous electrolyte. 3. Results and discussions The XRD patterns of the Bi2S3/FGS composites with different Bi2S3 contents (1:1, 2:1, and 3:1) are shown in Fig. 2. Standard XRD patterns of Bi2S3 and graphene are also presented at the bottom in Fig. 2. Fig. 2a shows the XRD pattern of the 1:1 Bi2S3/FGS sample. Except for the diffraction peak of graphene at 26.6° (JCPDS card No. 26-1079), another two diffraction peaks at 24.9° and 28.6° can be observed, which correspond to the (130) and (211) planes of Bi2S3, respectively. As shown in Fig. 2b and 2c, all diffraction peaks corresponding to Bi2S3 can be clearly observed for the 2:1 and 3:1 samples due to the large Bi2S3 contents in the composites. Based on the XRD analysis, it is clear to see that the Bi2S3/FGS composites can be successfully synthesized without the trace of impurity phase by the facile hydrothermal method [12, 13]. Fig. 3 shows the Raman spectra of the pristine Bi2S3, FGS, and the Bi2S3/FGS composites with different Bi2S3 contents. The D band at 1325 cm-1 and the G band at 1598 cm-1 of graphene can be clearly observed in the Raman spectra of the FGS and Bi2S3/FGS composites [14]. As shown in Fig. 3d and the enlarged image inserted in Fig. 3, six Raman bands located at 129, 226, 251, 420, 610, and 965 cm-1 can be observed, agreeing well with the Raman feature of Bi2S3 in literature [15]. These
Raman bands attributed to Bi2S3 can also be seen in Fig. 3a-3c. Agreeing well with XRD results, there is no impurity phase detected from the Raman spectra, further confirming the successful preparation of the Bi2S3/FGS composites. To investigate the chemical states of various bonded elements at the surface of the Bi2S3/FGS composite, XPS measurements were carried out on the 2:1 sample. As shown in Fig.4a, the full survey scan XPS spectrum demonstrates the presence of C, Bi, S, and O elements in the composite. Fig. 4b shows the C 1s core-level XPS spectrum, which can be deconvoluted into four components. The peak at 284.6 eV corresponds to graphitic carbon in graphene (C-C), while other three peaks can be attributed to C-OH (285.5 eV), C-O (286.7 eV), and C=O (289 eV), respectively, because of the existence of functional groups on the graphene surface. The Bi 4f corelevel XPS spectrum (Fig.4c) shows two distinct peaks located at 158 eV for Bi 4f7/2 and 163 eV for Bi 4f5/2 [16, 17], agreeing well with literature reported for Bi2S3. Fig. 4d shows the S 2s core-level XPS spectrum, and the peak at 225 eV can be assigned to S 2s1/2 of Bi2S3 [18, 19]. The XRD, Raman, and XPS results agree well with each other, confirming the successful synthesis of Bi2S3/FGS composites. Fig.5 shows the morphology and microstructure of the Bi2S3/FGS composites characterized by FESEM and TEM. As shown in Fig.5a and 5b, the 1:1 Bi2S3/FGS composite well resembles the morphology of graphene with crumpled nanosheets being observed. No obvious Bi2S3 nanoparticles can be seen from the FESEM images, which can be attributed to the small particles size and low Bi2S3 content in the 1:1 sample. As the Bi2S3 content in the composite increases, the Bi2S3 nanoparticles can
be clearly observed in the FESEM images in Fig.5c-f for the 2:1 and 3:1 samples. The composites still retain the two-dimensional nanosheet morphology with Bi2S3 nanoparticles uniformly distributed and anchored on the FGS surface. To further investigate the microstructure of the Bi2S3/FGS composite, TEM measurements were carried out on the 2:1 sample. Fig. 5g-i shows the TEM images of the Bi2S3/FGS composite at different magnifications. It can be seen that Bi2S3 nanoparticles of 20-50 nm in size are well distributed on the transparent graphene nanosheets, agreeing well with the FESEM images. The selected area electron diffraction (SAED) pattern (inset in Figure 5i) of the Bi2S3/FGS composite presents three diffraction rings, corresponding to (130), (221), and (002) crystal planes of Bi2S3, respectively. The electrochemical properties of the Bi2S3/FGS composites were investigated by CV and galvanostatic charge-discharge measurements using three-electrode cells with 1 M Na2SO4 aqueous electrolyte. Fig. 6a shows the CV curves of the Bi2S3/FGS composite (2:1 sample) electrode at different scan rates from 5 to 1600 mV/s in a potential window of -0.9-0 V (vs. Ag/AgCl). The CV curves retain rectangular shape even at high scan rate of 1600 mV/s, indicating ideal capacitive behavior and high reversibility. The specific capacitance of the Bi2S3/FGS composite electrode can reach 396 F/g at a scan rate of 5 mV/s. Obvious redox peaks can be observed in the CV curves, indicating pseudocapacitance contribution from the Bi2S3/FGS composite electrode due to faradic redox reactions at the surface. The redox peaks can be attributed to Na+ intercalation and deintercalation, and the reaction can be expressed by the following equation:
Bi2S3 + xNa+ + xe-→ NaxBi2S3
(1)
Fig. 6b shows the charge-discharge curves of the Bi2S3/FGS composite (2:1 sample) electrode between -0.9 and 0 V (vs. Ag/AgCl ) at different current densities from 1 to 40 A/g. The Bi2S3/FGS composite electrode can deliver a large specific capacitance of about 292 F/g at a current density of 1 A/g. Small voltage plateaus can be observed in the charge-discharge curves, corresponding to the redox peaks observed in the CV curves. The specific capacitances as a function of scan rate of different Bi2S3/FGS composite (1:1, 2:1, and 3:1) electrodes are compared in Fig. 6c. The Bi2S3/FGS composite (2:1) electrode can still deliver a large specific capacitance of about 100 F/g even at a high scan rate of 1600 mV/s, indicating good rate capability. To further understand the electrochemical behavior for the Bi2S3/FGS composites, EIS measurements were carried out on different composite electrodes and the obtained Nyquist plots are shown in Fig. 6d. It can be seen that the graphene content in the composite plays an important role in determining the resistance of the electrodes. The 1:1 sample exhibits the smallest ohmic resistance and charge transfer resistance due to its large graphene content, favoring fast electron transport and fast faradic redox reactions. Fig. 6e compares the cycle performances of the different Bi2S3/FGScomposite electrodes. Among the three electrodes, the 2:1 sample exhibits the best cycle performance with a capacitance retention of 75% after 5000 cycles. The superior cycle performance of the 2:1 sample can be attributed to its appropriate graphene content, which can effectively suppress the volume change of Bi2S3 during charge and discharge, providing improved structural stability.
The promising supercapacitive performance of the Bi2S3/FGS composite electrodes can be attributed to its unique heterostructure with Bi2S3 nanoparticles anchored on the FGS surface [20]. First, the FGS has a large specific surface area and excellent electrical conductivity, offering a stubborn conductive matrix for fast electron and ion transports. Second, Bi2S3 nanoparticles are well separated and dispersed on the FGS, guaranteeing large charge storage sites and high utilization of Bi2S3 with large pseudocapacitance contribution. Finally, the soft FGS can also function as buffer layer, which can reduce the stain associated with volume change of Bi2S3 during charge and discharge, thus resulting in improved structural stability and good cycle performance. 4. Conclusions In summary, a facile hydrothermal method was developed to prepare the Bi 2S3/FGS composites with various Bi2S3 contents. The Bi2S3/FGS composites retain the nanosheet morphology of graphene with Bi2S3 nanoparticles uniformly anchored on the surface of FGS. The Bi2S3/FGS composites were first time investigated as electrode materials for supercapacitors in neutral aqueous electrolyte, and they exhibit promising supercapacitive performance. The specific capacitance of the 2:1 Bi2S3/FGS composite electrode can reach 396 F/g at a scan rat of 5 mV/s between -0.9 and 0 V (vs. Ag/AgCl). In addition to large specific capacitance, the Bi2S3/FGS composite electrode also exhibit good rate capability and cycle performance, owing to the synergistic effect between Bi2S3 and FGS. The present results indicate that the
Bi2S3/FGS composites could be promising anode materials for developing highperformance asymmetric supercapacitors. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51572129 and U1407106), International S&T Cooperation Program of China (No. 2016YFE0111500), QingLan Project of Jiangsu Province, A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Fundamental Research Funds for the Central Universities (No. 30915011204). References [1] J. Liu, M. Zheng, X. Shi, H. Zeng, H. Xia, Amorphous FeOOH Quantum Dots Assembled Mesoporous Film Anchored on Graphene Nanosheets with Superior Electrochemical Performance for Supercapacitors, Adv. Funct. Mater. 26 (2016) 919930. [2] H. Xu, X. Hu, H. Yang, Y. Sun, C. Hu, Y. Huang, Flexible Asymmetric MicroSupercapacitors Based on Bi2O3and MnO2Nanoflowers: Larger Areal Mass Promises Higher Energy Density, Adv. Energy Mater. 5 (2015) 1401882. [3] D. Qu, L. Wang, D. Zheng, L. Xiao, B. Deng, D. Qu, An asymmetric supercapacitor with highly dispersed nano-Bi2O3 and active carbon electrodes, J. Power Sources 269 (2014) 129-135. [4] S.X. Wang, C.C. Jin, W.J. Qian, Bi2O3 with activated carbon composite as a
supercapacitor electrode, J. Alloys and Compounds 615 (2014) 12-17. [5] J. Shi, X. Li, G. He, L. Zhang, M. Li, Electrodeposition of high-capacitance 3D CoS/graphene nanosheets on nickel foam for high-performance aqueous asymmetric supercapacitors, J. Mater. Chem. A 3 (2015) 20619-20626. [6] D. Kong, J. Luo, Y. Wang, W. Ren, T. Yu, Y. Luo, Y. Yang, C. Cheng, ThreeDimensional Co3O4@MnO2Hierarchical Nanoneedle Arrays: Morphology Control and Electrochemical Energy Storage, Adv. Funct. Mater. 24 (2014) 3815-3826. [7] X. Xie, Z. Ao, D. Su, J. Zhang, G. Wang, MoS2/Graphene Composite Anodes with Enhanced Performance for Sodium-Ion Batteries: The Role of the Two-Dimensional Heterointerface, Adv. Funct. Mater. 25 (2015) 1393-1403. [8] Y. Jiang, Y. Feng, B. Xi, S. Kai, K. Mi, J. Feng, J. Zhang, S. Xiong, Ultrasmall SnS2nanoparticles anchored on well-distributed nitrogen-doped graphene sheets for Liion and Na-ion batteries, J. Mater. Chem. A 4 (2016) 10719-10726. [9] H. Wang, Z. Xu, H. Yi, H. Wei, Z. Guo, X. Wang, One-step preparation of singlecrystalline Fe2O3 particles/graphene composite hydrogels as high performance anode materials for supercapacitors, Nano Energy 7 (2014) 86-96. [10] K.K. Lee, S. Deng, H.M. Fan, S. Mhaisalkar, H.R. Tan, E.S. Tok, K.P. Loh, W.S. Chin, C.H. Sow, alpha-Fe2O3 nanotubes-reduced graphene oxide composites as synergistic electrochemical capacitor materials, Nanoscale 4 (2012) 2958-61. [11] S. Yang, X. Song, P. Zhang, J. Sun, L. Gao, Self-Assembled alpha-Fe2O3 mesocrystals/graphene nanohybrid for enhanced electrochemical capacitors, Small 10 (2014) 2270-9.
[12] J. Ni, Y. Zhao, T. Liu, H. Zheng, L. Gao, C. Yan, L. Li, Strongly Coupled Bi2S3@CNT Hybrids for Robust Lithium Storage, Adv. Energy Mater. 4(2014) 1400798. [13] W. Sun, X. Rui, D. Zhang, Y. Jiang, Z. Sun, H. Liu, S. Dou, Bismuth sulfide: A high-capacity anode for sodium-ion batteries, J. Power Sources 309 (2016) 135-140. [14] W.J. Li, C. Han, S.L. Chou, J.Z. Wang, Z. Li, Y.M. Kang, H.K. Liu, S.X. Dou, Graphite-Nanoplate-Coated Bi2S3 Composite with High-Volume Energy Density and Excellent Cycle Life for Room-Temperature Sodium-Sulfide Batteries, Chemistry 22 (2016) 590-7. [15] Y. Xiao, H. Cao, K. Liu, S. Zhang, V. Chernow, The synthesis of superhydrophobic Bi2S3 complex nanostructures, Nanotechnology 21 (2010) 145601. [16] Y.W. Koh, C.S. Lai, A.Y. Du, E.R.T. Tiekink, K.P. Loh, Growth of Bismuth Sulfide Nanowire Using Bismuth Trisxanthate Single Source Precursors, Cheminform 15 (2003) 4544-4554. [17] L. Li, X. Zhang, Z. Zhang, M. Zhang, L. Cong, Y. Pan, S.Y. Lin, Bismuth oxide nanosheets coated electrospun carbon nanofibers film: a free-standing negative electrode for flexible asymmetric supercapacitors, J. Mater. Chem. A (2016). [18] R. Malakooti, L. Cademartiri, Y. Akçakir, S. Petrov, A. Migliori, G.A. Ozin, ShapeControlled Bi2S3 Nanocrystals and Their Plasma Polymerization into Flexible Films, Adv.Mater. 18 (2006) 2189-2194. [19] Q. Han, J. Chen, X. Yang, A. Lude Lu, X. Wang, Preparation of Uniform Bi2S3 Nanorods Using Xanthate Complexes of Bismuth (III), Journal of Phys. Chem. C 111
(2007) 14072-14077. [20] T. Liu, Y. Zhao, L. Gao, J. Ni, Engineering Bi2O3-Bi2S3 heterostructure for superior lithium storage, Sci Rep 5 (2015) 9307.
Fig. 1. Schematic diagram of the preparation process of the Bi2S3/FGS composites.
Fig. 2. XRD patterns of the Bi2S3/FGS composites with different mass ratios: (a) 1: 1, (b) 2: 1, and (c) 3: 1.
Fig. 3.Raman spectra of the Bi2S3/FGS composites with different mass ratios of (a) 3:1, (b) 2:1, and (c) 1:1, (d) pristine Bi2S3, and (e) pristine FGS.
Fig. 4. (a) Full survey scan XPS spectrum, (b) core-level C 1s XPS spectrum, (c) core-level Bi 4f XPS spectrum, and (d) core-level S 2s XPS spectrum of the Bi2S3/ FGS composites (2:1 sample).
Fig. 5.FESEM images of the Bi2S3/FGS composites with different Bi2S3 contents: (a, b) 1: 1 sample, (c, d) 2:1 sample, and (e, f) 3: 1 sample. (g-i) TEM images of the Bi2S3 /FGS composite at different magnifications (2:1 sample).
Fig. 6.(a) CV curves of the 2:1 Bi2S3/FGS composite electrode at different scan rates from 5 to 1600 mV/s. (b) Charge and discharge curves of the 2:1 Bi2S3/FGS composite electrode at different current densities from 1 to 40 A/g. (c) The specific capacitances as a function of the scan rate of different Bi2S3/FGS composite electrodes. (d) Nyquist plots and (e) charge/discharge cycle performances of different Bi2S3/FGS composite electrodes.
S0025-5408(17)31277-1 http://dx.doi.org/doi:10.1016/j.materresbull.2017.05.047 MRB 9366
To appear in:
MRB
Received date: Revised date: Accepted date:
2-4-2017 5-5-2017 25-5-2017
Please cite this article as: Haochen Lu, Qiubo Guo, Feng Zan, Hui Xia, Bi2S3 Nanoparticles Anchored on Graphene Nanosheets with Superior Electrochemical Performance for Supercapacitors, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.05.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bi2S3 Nanoparticles Anchored on Graphene Nanosheets with Superior Electrochemical Performance for Supercapacitors Haochen Lua,b†, Qiubo Guoa,b†, Feng Zana,b*, Hui Xiaa,b a
School of Materials Science and Engineering, Nanjing University of Science and
Technology, Nanjing 210094, China. b
Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and
Technology, Nanjing 210094, China. († Both authors contributed equally to this work) Graphical Abstract
Corresponding author, e-mail: [email protected]
Corresponding author, e-mail: [email protected] Tel: (86) 25 84303408 , Fax: (86) 25 84303408
Research highlights
A facile hydrothermal method is developed to prepare Bi2S3/functionalized graphene nanosheets (FGS) composites.
The Bi2S3/FGS composites are first time investigated as electrode materials for supercapacitors in neutral aqueous electrolyte.
The Bi2S3/FGS composite electrodes exhibit good rate capability and cycle performance, owing to the synergistic effect between Bi2S3 and FGS.
Abstract Great progress has been made in developing nanostructured metal oxides and metal sulfides as electrode materials for supercapacitors. Poor electrical conductivities of these materials, however, limit their supercapacitive performance. In this work, a facile synthesis strategy is developed to prepare Bi2S3/graphene composites with
Bi2S3 nanoparticles anchored on graphene nanosheets. The Bi2S3/graphene composite electrodes exhibit promising electrochemical performance in a negative potential window of -0.9-0 V (vs. Ag/AgCl) in 1 M Na2SO4 electrolyte. A large specific capacitance of about 400 F/g can be achieved by the Bi2S3/graphene composite electrode with good rate capability and cycle performance. The present work indicates that the Bi2S3/graphene composites are promising anode materials for developing high-performance asymmetric supercapacitors. Keywords: Supercapacitors; Composites; Bi2S3; Graphene; Nanoparticles 1. Introduction As a bridge connecting traditional capacitors and batteries, supercapacitors are becoming a new type of energy storage devices, which are getting more and more attention [1]. In order to meet the power requirements of fast-developing electrical devices, advanced supercapacitors with large capacitance, high power density, high energy density, and long cycle life are imminently needed. As the key component for supercapacitors, electrode materials play an important role in determining electrochemical performance. Among the electrode materials, transition metal oxides and sulfides have attracted more and more attention due to their large pseudocapacitance from reversible faradic redox reactions. As a member of metal oxides, Bi2O3 is a promising electrode material for supercapacitors owing to its large capacitance, appropriate potential window, good electrochemical stability, and simple preparation process [2-4]. Compared with Bi2O3, Bi2S3 could be more attractive as
electrode material for supercapcitors because of its improved electronic conductivity [5]. Many metal sulfides exhibit enhanced electronic conductivities than their oxide counterparts because the electronegativity of sulfur is lower than that of oxygen, making it easier for electrons to transport in the structure. The study of Bi2S3 as electrode material for supercapcitors, however, is very rare in literature. Therefore, it is imperative to investigate the electrochemical performance of Bi2S3 as electrode materials for supercapacitors. As charge storage occurs at the electrode surface for supercapacitors, it is a rational way to develop the electrode materials with large surface areas. Therefore, various nanostructures, including nanoparticles, nanowires, nanosheets, and etc., have been developed for improving the supercapacitive performance [6]. Although nanoparticles with large surface area are promising to obtain large specific capacitance, they tend to aggregate during the charge and discharge process due to the large surface energy, thus losing their advantage during cycling. To solve this problem, these metal oxide or sulfide nanoparticles are usually mixed with conductive additives to make composites, which can suppress aggregation of nanoparticles and further improve their electrical conductivity [7,8]. Specifically, decorating metal oxide or sulfide nanoparticles on graphene nanosheets has been demonstrated to be an effective strategy to improve the supercapacitive performance because graphene can provide a large conductive matrix, offering large surface area and fast electron transport [9-11]. In this work, a facile hydrothermal method was developed to prepare Bi2S3/functionalized graphene nanosheets (FGS) composites. In the composite, Bi2S3
nanoparticles of 20-50 nm in size were well distributed and tightly anchored on the FGS without severe aggregation. The Bi2S3/FGS composite electrode exhibited a large specific capacitance of about 396 F/g when tested as anode in 1M Na2SO4 electrolyte between -0.9-0 V (vs. Ag/AgCl) for supercapcitors. In addition to large specific capacitance, the Bi2S3/FGS composite electrode also presented good rate performance and good cycling stability. The present study indicated that the Bi2S3/FGS composite could be promising anode material for developing highperformance asymmetric supercapcitors. 2. Experimental 2.1. Preparation of FGS Graphene oxide (GO) was synthesized by Hummers method from purified natural graphite. After that, the dried GO was thermally exfoliated at 300℃ for 5 min in a tube furnace in air and then heated at 900℃ in Ar for 3 h with a heating rate of 2℃/min to obtain the FGS. 2.2. Preparation of Bi2S3/FGS composites As illustrated in Fig. 1a, the Bi2S3/FGS composites were synthesized by a simple hydrothermal method. In a typical synthesis, 0.05g FGS and 0.185g thioacetamide (TAA) was added to 20 mL deionized water with ultrasonication and magnetic stirring for 30min, respectively, and then 2 mL 0.4 M HNO3 and 0.2530 g Bi(NO3)3• 5H2O were added into the solution. After stirring for 1 h, 20 mL of dimethylformamide was added into the above solution and the well mixed solution
was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 150℃ for 2 h. After cooling down to room temperature, the products were collected by centrifugation and washed several times with distilled water and ethanol. After that, the samples were dried at 60 ℃ in a vacuum oven for further characterization. By changing the quantities of TAA and Bi(NO3)3•5H2O, the mass ratios of Bi2S3/FGS in the composites can be tuned as 1:1, 2:1, and 3:1. In the following text, the samples with different Bi2S3 contents are referred as 1:1, 2:1, and 3:1 samples. 2.3. Characterizations and electrochemical measurements The X-ray diffraction (XRD, Bruker-AXS D8 Advance with monochromatized Cu Kα radiation), Raman spectroscopy (Jobin-Yvon T6400 Micro-Raman system), and X-ray photoelectron spectroscopy (XPS, PhiQuantera SXM spectromenter using Al KαX-ray as the excitation source) were performed to characterize structural features and phase purity of the samples. The morphology and microstructure of the samples were investigated by using field emission scanning electron microscope (FESEM, Quant 250 FEG) and transmission electron microscope (TEM, FEI Tecnai 20). Electrochemical properties of the samples were investigated by using three-electrode cells with a Pt foil as counter electrode, Ag/AgCl as reference electrode, and Bi2S3/FGS composite as working electrode. To prepare the working electrode, active material, conductive agent (Super P), and polyvinylidene fluoride (PVDF) binder with a mass ratio of 8: 1: 1 were dissolved in N-Methyl-2-pyrrolidone (NMP) to form a slurry, and then the slurry was pasted on a Ti foil and dried in a vacuum oven at 80℃ for 12 h. CHI760C electrochemical workstation (Chenhua, Shanghai) and battery test
system (5V 1mA LANDCT2001A) were used to carry out cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge measurements in 1M Na2SO4 aqueous electrolyte. 3. Results and discussions The XRD patterns of the Bi2S3/FGS composites with different Bi2S3 contents (1:1, 2:1, and 3:1) are shown in Fig. 2. Standard XRD patterns of Bi2S3 and graphene are also presented at the bottom in Fig. 2. Fig. 2a shows the XRD pattern of the 1:1 Bi2S3/FGS sample. Except for the diffraction peak of graphene at 26.6° (JCPDS card No. 26-1079), another two diffraction peaks at 24.9° and 28.6° can be observed, which correspond to the (130) and (211) planes of Bi2S3, respectively. As shown in Fig. 2b and 2c, all diffraction peaks corresponding to Bi2S3 can be clearly observed for the 2:1 and 3:1 samples due to the large Bi2S3 contents in the composites. Based on the XRD analysis, it is clear to see that the Bi2S3/FGS composites can be successfully synthesized without the trace of impurity phase by the facile hydrothermal method [12, 13]. Fig. 3 shows the Raman spectra of the pristine Bi2S3, FGS, and the Bi2S3/FGS composites with different Bi2S3 contents. The D band at 1325 cm-1 and the G band at 1598 cm-1 of graphene can be clearly observed in the Raman spectra of the FGS and Bi2S3/FGS composites [14]. As shown in Fig. 3d and the enlarged image inserted in Fig. 3, six Raman bands located at 129, 226, 251, 420, 610, and 965 cm-1 can be observed, agreeing well with the Raman feature of Bi2S3 in literature [15]. These
Raman bands attributed to Bi2S3 can also be seen in Fig. 3a-3c. Agreeing well with XRD results, there is no impurity phase detected from the Raman spectra, further confirming the successful preparation of the Bi2S3/FGS composites. To investigate the chemical states of various bonded elements at the surface of the Bi2S3/FGS composite, XPS measurements were carried out on the 2:1 sample. As shown in Fig.4a, the full survey scan XPS spectrum demonstrates the presence of C, Bi, S, and O elements in the composite. Fig. 4b shows the C 1s core-level XPS spectrum, which can be deconvoluted into four components. The peak at 284.6 eV corresponds to graphitic carbon in graphene (C-C), while other three peaks can be attributed to C-OH (285.5 eV), C-O (286.7 eV), and C=O (289 eV), respectively, because of the existence of functional groups on the graphene surface. The Bi 4f corelevel XPS spectrum (Fig.4c) shows two distinct peaks located at 158 eV for Bi 4f7/2 and 163 eV for Bi 4f5/2 [16, 17], agreeing well with literature reported for Bi2S3. Fig. 4d shows the S 2s core-level XPS spectrum, and the peak at 225 eV can be assigned to S 2s1/2 of Bi2S3 [18, 19]. The XRD, Raman, and XPS results agree well with each other, confirming the successful synthesis of Bi2S3/FGS composites. Fig.5 shows the morphology and microstructure of the Bi2S3/FGS composites characterized by FESEM and TEM. As shown in Fig.5a and 5b, the 1:1 Bi2S3/FGS composite well resembles the morphology of graphene with crumpled nanosheets being observed. No obvious Bi2S3 nanoparticles can be seen from the FESEM images, which can be attributed to the small particles size and low Bi2S3 content in the 1:1 sample. As the Bi2S3 content in the composite increases, the Bi2S3 nanoparticles can
be clearly observed in the FESEM images in Fig.5c-f for the 2:1 and 3:1 samples. The composites still retain the two-dimensional nanosheet morphology with Bi2S3 nanoparticles uniformly distributed and anchored on the FGS surface. To further investigate the microstructure of the Bi2S3/FGS composite, TEM measurements were carried out on the 2:1 sample. Fig. 5g-i shows the TEM images of the Bi2S3/FGS composite at different magnifications. It can be seen that Bi2S3 nanoparticles of 20-50 nm in size are well distributed on the transparent graphene nanosheets, agreeing well with the FESEM images. The selected area electron diffraction (SAED) pattern (inset in Figure 5i) of the Bi2S3/FGS composite presents three diffraction rings, corresponding to (130), (221), and (002) crystal planes of Bi2S3, respectively. The electrochemical properties of the Bi2S3/FGS composites were investigated by CV and galvanostatic charge-discharge measurements using three-electrode cells with 1 M Na2SO4 aqueous electrolyte. Fig. 6a shows the CV curves of the Bi2S3/FGS composite (2:1 sample) electrode at different scan rates from 5 to 1600 mV/s in a potential window of -0.9-0 V (vs. Ag/AgCl). The CV curves retain rectangular shape even at high scan rate of 1600 mV/s, indicating ideal capacitive behavior and high reversibility. The specific capacitance of the Bi2S3/FGS composite electrode can reach 396 F/g at a scan rate of 5 mV/s. Obvious redox peaks can be observed in the CV curves, indicating pseudocapacitance contribution from the Bi2S3/FGS composite electrode due to faradic redox reactions at the surface. The redox peaks can be attributed to Na+ intercalation and deintercalation, and the reaction can be expressed by the following equation:
Bi2S3 + xNa+ + xe-→ NaxBi2S3
(1)
Fig. 6b shows the charge-discharge curves of the Bi2S3/FGS composite (2:1 sample) electrode between -0.9 and 0 V (vs. Ag/AgCl ) at different current densities from 1 to 40 A/g. The Bi2S3/FGS composite electrode can deliver a large specific capacitance of about 292 F/g at a current density of 1 A/g. Small voltage plateaus can be observed in the charge-discharge curves, corresponding to the redox peaks observed in the CV curves. The specific capacitances as a function of scan rate of different Bi2S3/FGS composite (1:1, 2:1, and 3:1) electrodes are compared in Fig. 6c. The Bi2S3/FGS composite (2:1) electrode can still deliver a large specific capacitance of about 100 F/g even at a high scan rate of 1600 mV/s, indicating good rate capability. To further understand the electrochemical behavior for the Bi2S3/FGS composites, EIS measurements were carried out on different composite electrodes and the obtained Nyquist plots are shown in Fig. 6d. It can be seen that the graphene content in the composite plays an important role in determining the resistance of the electrodes. The 1:1 sample exhibits the smallest ohmic resistance and charge transfer resistance due to its large graphene content, favoring fast electron transport and fast faradic redox reactions. Fig. 6e compares the cycle performances of the different Bi2S3/FGScomposite electrodes. Among the three electrodes, the 2:1 sample exhibits the best cycle performance with a capacitance retention of 75% after 5000 cycles. The superior cycle performance of the 2:1 sample can be attributed to its appropriate graphene content, which can effectively suppress the volume change of Bi2S3 during charge and discharge, providing improved structural stability.
The promising supercapacitive performance of the Bi2S3/FGS composite electrodes can be attributed to its unique heterostructure with Bi2S3 nanoparticles anchored on the FGS surface [20]. First, the FGS has a large specific surface area and excellent electrical conductivity, offering a stubborn conductive matrix for fast electron and ion transports. Second, Bi2S3 nanoparticles are well separated and dispersed on the FGS, guaranteeing large charge storage sites and high utilization of Bi2S3 with large pseudocapacitance contribution. Finally, the soft FGS can also function as buffer layer, which can reduce the stain associated with volume change of Bi2S3 during charge and discharge, thus resulting in improved structural stability and good cycle performance. 4. Conclusions In summary, a facile hydrothermal method was developed to prepare the Bi 2S3/FGS composites with various Bi2S3 contents. The Bi2S3/FGS composites retain the nanosheet morphology of graphene with Bi2S3 nanoparticles uniformly anchored on the surface of FGS. The Bi2S3/FGS composites were first time investigated as electrode materials for supercapacitors in neutral aqueous electrolyte, and they exhibit promising supercapacitive performance. The specific capacitance of the 2:1 Bi2S3/FGS composite electrode can reach 396 F/g at a scan rat of 5 mV/s between -0.9 and 0 V (vs. Ag/AgCl). In addition to large specific capacitance, the Bi2S3/FGS composite electrode also exhibit good rate capability and cycle performance, owing to the synergistic effect between Bi2S3 and FGS. The present results indicate that the
Bi2S3/FGS composites could be promising anode materials for developing highperformance asymmetric supercapacitors. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51572129 and U1407106), International S&T Cooperation Program of China (No. 2016YFE0111500), QingLan Project of Jiangsu Province, A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Fundamental Research Funds for the Central Universities (No. 30915011204). References [1] J. Liu, M. Zheng, X. Shi, H. Zeng, H. Xia, Amorphous FeOOH Quantum Dots Assembled Mesoporous Film Anchored on Graphene Nanosheets with Superior Electrochemical Performance for Supercapacitors, Adv. Funct. Mater. 26 (2016) 919930. [2] H. Xu, X. Hu, H. Yang, Y. Sun, C. Hu, Y. Huang, Flexible Asymmetric MicroSupercapacitors Based on Bi2O3and MnO2Nanoflowers: Larger Areal Mass Promises Higher Energy Density, Adv. Energy Mater. 5 (2015) 1401882. [3] D. Qu, L. Wang, D. Zheng, L. Xiao, B. Deng, D. Qu, An asymmetric supercapacitor with highly dispersed nano-Bi2O3 and active carbon electrodes, J. Power Sources 269 (2014) 129-135. [4] S.X. Wang, C.C. Jin, W.J. Qian, Bi2O3 with activated carbon composite as a
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Fig. 1. Schematic diagram of the preparation process of the Bi2S3/FGS composites.
Fig. 2. XRD patterns of the Bi2S3/FGS composites with different mass ratios: (a) 1: 1, (b) 2: 1, and (c) 3: 1.
Fig. 3.Raman spectra of the Bi2S3/FGS composites with different mass ratios of (a) 3:1, (b) 2:1, and (c) 1:1, (d) pristine Bi2S3, and (e) pristine FGS.
Fig. 4. (a) Full survey scan XPS spectrum, (b) core-level C 1s XPS spectrum, (c) core-level Bi 4f XPS spectrum, and (d) core-level S 2s XPS spectrum of the Bi2S3/ FGS composites (2:1 sample).
Fig. 5.FESEM images of the Bi2S3/FGS composites with different Bi2S3 contents: (a, b) 1: 1 sample, (c, d) 2:1 sample, and (e, f) 3: 1 sample. (g-i) TEM images of the Bi2S3 /FGS composite at different magnifications (2:1 sample).
Fig. 6.(a) CV curves of the 2:1 Bi2S3/FGS composite electrode at different scan rates from 5 to 1600 mV/s. (b) Charge and discharge curves of the 2:1 Bi2S3/FGS composite electrode at different current densities from 1 to 40 A/g. (c) The specific capacitances as a function of the scan rate of different Bi2S3/FGS composite electrodes. (d) Nyquist plots and (e) charge/discharge cycle performances of different Bi2S3/FGS composite electrodes.