- Email: [email protected]
Facile synthesis of flexible WO3 nanofibers as supercapacitor electrodes
Author’s Accepted Manuscript Facile synthesis of flexible WO3 nanofibers as supercapacitor electrodes Shunyu Yao, Xin Zheng, Xu Zhang, Huanhao Xiao, Fengyu Qu, Xiang Wu www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)31546-4 http://dx.doi.org/10.1016/j.matlet.2016.09.085 MLBLUE21523
To appear in: Materials Letters Received date: 14 August 2016 Revised date: 16 September 2016 Accepted date: 21 September 2016 Cite this article as: Shunyu Yao, Xin Zheng, Xu Zhang, Huanhao Xiao, Fengyu Qu and Xiang Wu, Facile synthesis of flexible WO 3 nanofibers as supercapacitor electrodes, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.09.085 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 galley proof before it is published in its final citable 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.
Facile synthesis of flexible WO3 nanofibers as supercapacitor electrodes Shunyu Yaoa, 1, Xin Zheng a, 1, Xu Zhang a, *, Huanhao Xiao a, Fengyu Qu a, Xiang Wu a, b* a
College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China b
School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China
*E-mail: [email protected] [email protected],
Abstract In this paper, hierarchical WO3 nanofibers are synthesized on a flexible carbon cloth by a facile hydrothermal route. The as-prepared products as supercapacitor electrodes show high discharge areal capacitance (1716.92 mF cm−2 at 2 mA cm−2) and cycling stability (20.9% loss after 6000 repetitive cycles at 10 mA cm−2). These superior performances of hierarchical WO3 structures indicate their promising applications in supercapacitor electrode materials. Keywords: Energy storage and conversion, particles, nanosize, nanocrystalline materials 1. Introduction Nowadays, an increase in fossil fuels use is still the main source of energy consumption. Considering the finite amounts of fossil fuels and the instability of some recycle resources, appropriate alternative energy is necessary to explore. Regarding electrochemical energy storage, incalculable strives have been put into lithium ion batteries (LIB) [1] as well as emergent techniques such as alkaline batteries [2,3] and supercapacitors (SCs) [4-6]. Typically, lithium-ion batteries possess 1
These authors contributed equally to this work 1
outstanding energy densities, low power densities and limited cycle life. On the contrary, with the characteristics of higher power density than batteries and higher energy density than conventional dielectric capacitors, supercapacitors have attracted much attention for next-generation power devices [7-11]. SCs are basically classified into electric double layer capacitors and pseudocapacitors according to charge storage mechanism [12]. The core of electrochemical property is the electrode material. Nanostructured semiconductor metal oxides have been considered as promising candidates for fabricating supercapacitors electrodes due to their electrochemical performances can be largely enhanced or modified [13]. Transition metal oxides have been widely studied in various pseudocapacitive materials [14]. As an n-type semiconductor, WO3 has attracted one’s interest due to its excellent electron transport properties and stabilities against corrosion [15, 16]. In the meantime, WO3 is also a promising electrode material due to its low cost, simple synthesis, outstanding conductivity and high theoretical capacity [17]. To date, many scientists have made tremendous efforts on WO3 as a negative pseudocapacitive electrode material.
Recently, Qiu’s group preapred flowerlike WO3 negative
electrode via an effective electrodeposition method for all-solid-state asymmetric supercapacitors, and showed an excellent electrochemical performance of 196 F g-1 at a scan rate of 10 mV s-1 [18]. Samu et al. reported nanoporous tungsten trioxide structures and studied their application as supercapacitor electrodes [19]. Herein, we reported the fabrication of flexible WO3 nanofibers through a facile hydrothermal approach. Electrochemical tests indicated the hierarchical WO3 nanofibers possess an excellent initial discharge capacitance and cycling stability. 2. Experimental details At first, a piece of carbon cloth (CC) (1 × 1 cm2 in square shape) were cleaned by acetone, deionized water and ethanol by ultrasonication in sequence and dried under atmospheric condition. In
2
a typical procedure, 2 mmol of Na2WO4 were dissolved into 40 mL of deionized water and stirred to form a clear white solution. Then an aqueous solution of nitric acid was added dropwise to the above mixture solution until pH value of the solution reached 2. The mixture was then transferred into a 50 mL autoclave and heated at 180 oC for 15 h. When the reaction was completed, the carbon cloth was taken out and washed with distilled water and absolute ethanol several times, and then dried in a vacuum. Finally, the carbon cloth coated WO3 product was obtained by annealing at 350 oC for 2 h. The microstructure of the samples was characterized by powder X-ray diffraction (XRD, Rigaku Dmax-2600/pc) equipped with Cu Kα radiation (λ = 0.1542 nm). The general and detailed morphologies of the as-prepared samples were examined by scanning electron microscopy (FESEM; Hitachi-S4800). The electrochemical properties of the samples were assessed under a typical three-electrode cell configuration on a CHI 660 electrochemical workstation with 0.5 M H2SO4 aqueous solution as the electrolyte at room temperature. WO3 product grown on carbon cloth (1 cm2 area) was directly used as working electrode. A platinum plate served as a counter electrode and an Ag/AgCl electrode served as the reference electrode. Cyclic voltammetric curves were measured in a potential range between -0.65 and 0 V at different scan rates and the charge-discharge process were performed by cycling the potential from -0.65 to 0 V at different current densities. The specific capacitance was calculated by C = IΔt/(SΔV), where I is the discharge current, Δt is the discharge time, S is the geometrical area of the electrode, and ΔV is the voltage drop upon discharging. The electrochemical impedance spectroscopy (EIS) measurements were performed by applying an AC voltage with 5 mV amplitude in a frequency range from 0.01 Hz to 100 kHz. 3. Results and discussion
3
The as-synthesized WO3 samples are first investigated by X-ray diffraction and the result is shown in Fig. 1a. Except for the peaks derived from carbon cloth, the other sharp peaks can be well indexed to WO3 (JCPDS card No. 33-1387). The strong and sharp peaks indicate that the as-prepared product is highly crystalline. Fig. 1b-d show SEM images of WO3 product at diverse magnifications. One can find that quantities of WO3 product grow tightly on flexible carbon nanofibers, as shown in Fig. 1b.
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Fig. 1(a) XRD pattern of WO3 product (b-d) SEM images of the as-synthesized product with different magnification. (e) EDS spectrum of the product (f-h) EDS mapping of the product
4
Further observation finds that the products are composed of many nanofibers, seen in Fig. 1c. Average diameter of the nanofibers is about 40 nm, which can benefit complete utilization of the active materials. To confirm the element composition of the as-synthesized sample, EDS element mapping is conducted. From the EDS spectra of the sample (Fig. e), the composition of W and O elements are evidently presented and well-distributed (Fig. 1f-f). The signal of C is caused by carbon cloth.
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Fig. 2 (a) CV curves of WO3 nanofiber and carbon cloth at a scan rate of 80 mV/s (b) CV curves of WO3 nanofiber at scan rates between 5 and 80 mV/s (c) WO3 nanofiber and carbon clothes discharge curves at 2 mA/cm2 (d) discharge curves at current densities from 2 to 10 mA/cm2
The electrochemical properties of WO3 product are tested to investigate their potential applications as supercapacitors electrode. Fig. 2a displays the cyclic voltammetry (CV) curves of WO3
5
nanofibers and CC at scan rates of 80 mV/s, revealing that the active materials possess higher capacitive current density than that of CC. CC contributes little to the total capacitance of WO 3 nanofibers electrode. CV curves of the working electrodes collected at diversify scan rates ranging from 5 to 80 mV/s are shown in (Fig. 2b). The charge storage mechanism of WO3 electrode may be explained by the following redox reaction. WO3 + xH+ + xe- ↔ HxWO3 [18]. Fig. 2c shows typical
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galvanostatic charge-discharge curves of WO3 nanofibers and CC at a current density of 2 mA cm−2.
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Fig. 3(a) Areal capacitances and specific capacitance of WO3 nanofibers at different current densities (b) the retention of areal capacitance after 6000 cycles at a current density of 10 mA cm−2 (c) EIS Nyquist plots of WO3 samples (d) Nitrogen adsorption/desorption isotherm and BJH pore size distribution plot of the as-synthesized product.
6
CC contributes little to the charge-discharge behavior and could be neglected in all cases. Besides, the charge-discharge performances of the WO3 electrode is also tested under different current densities of 2, 3, 4, 6, 8 and 10 mA cm−2 (Fig. 2d). The average areal capacitances of WO3 nanofibers are calculated to be 1716.92, 1656.92, 1470.76, 1384.61, 1292.31 and 1138.46 mF cm−2, respectively at current densities of 2, 3, 4, 6, 8 and 10 mA cm−2 (Fig. 3a). The specific capacitance (F g-1) of the electrode material was calculated from the galvanostatic discharge curves according to the following equation: C = I×t/(V×m). Where I is the discharge current (A), t is the discharge time (s), m is the mass of active material (g) and V is the voltage change (V) excluding IR drop in the discharge process. 0.0032 g of mass loading of WO3 product (grown on 1 cm2 carbon cloth) as working electrode was weight for electrochemical test. The inset of Fig. 3a demonstrates the calculated specific capacitance based on the discharge curves according to the above mentioned equation. They were 539.42, 516.34, 442.3, 409.61, 373.07 and 341.34 F g-1 at current densities of 2, 3, 4, 6, 8, and 10 A g-1, respectively, demonstrating that the specific capacitance decreases with increasing current density. Good cycling stability is very important to supercapacitor electrodes, which was assessed by repeating CV tests at 10 mV s-1 for 6000 cycles Fig. 3b. The specific capacitance of the product could retain 79.1% of the original areal capacitance after 6000 cycles, demonstrating excellent long-term electrochemical stability. To understand the conduction mechanisms, electrochemical impedance spectra of WO3 nanofiber electrodes were performed. Fig. 3c reveals Nyquist plot with two distinct regimes that include an arc in the high frequency regime and a sloped line in the low frequency regime. The arc corresponds to the charge transfer resistance (Rct) caused by Faradaic reactions at contact interface between the electrode and the electrolyte solution. Diameter of the semi-circular arc is Rct. However WO3 nanofiber electrode showed incomplete semicircles. Nonetheless it is clear from the inset of Fig. 3c, Rct for WO3 nanofiber electrode is the lowest, implying WO3 nanofibers exhibiting highest
7
conductivity (or lowest internal resistance)[7]. The diameter of semicircle presented the charge transfer resistance (Rct) of the WO3 nanofiber electrode is 0.38 Ω in the Nyquist plots (the inset in Fig. 3c). At a high frequency area, the intercept on the real axis represents the equivalent series resistance of the electrochemical system (Rs), which contain the inherent resistance of the electroactive material and electrolyte/electrode contact resistance between their interfaces. The real axis (Rs) in Nyquist plots is 2.61Ω. In the low frequency, the slope of the line corresponds to the Warburg impedance (Rw). The vertical shape in the spectrum indicates that WO3 nanofiber electrode presents a faster ion diffusion behavior [20]. According to this mechanism, the dynamic process can be represented by the equivalent circuit shown the inset in Fig. 3c. Rs is the series resistance. Fig. 3d shows the nitrogen adsorption and desorption isotherms. It can be seen that the isotherm of the as-synthesized WO3 nanofibers is a type IV isotherm with a distinct hysteresis loop in the range of 0.2-1.0 P/P0 according to the IUPAC classification, indicating the presence of mesoporous structure for WO3 nanofibers. The Brunauer-Emmett-Teller (BET) surface area of WO3 nanofibers is calculated to be 11.032 m2 g−1. The pore size distribution of the sample calculated by desorption isotherm is shown in the inset in Fig. 3d, indicating that a narrow pore size distribution peak centered at about 3.790 nm 4. Conclusions Hierarchical WO3 nanofibers grown on carbon clothes are prepared by facile hydrothermal process. The as-synthesized products as supercapacitor electrodes show prominent electrochemical performances, such as excellent specific capacity and outstanding cyclability. Such facile synthesis approach can be universal to design other flexible electrode materials for next-generation supercapacitors. References [1] L.N. Gao, F.Y. Qu, X. Wu, J. Mater. Chem. A. 2 (2014) 7367-7372.
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[2] R. Z. Li, Y. M. Wang, C. Zhou, C. Wang, X. Ba, Y.Y. Li, X.T. Huang, J. P. Liu, Adv. Funct. Mater. 25 (2015) 5384-5394. [3] J. Q. Liu, M.B. Zheng, X.Q. Shi, H.B. Zeng, H. Xia, Adv. Funct. Mater. 26 (2016) 919-930. [4] M. S. Zhu, W.J. Meng, Y. Huang, Y. Huang, C.Y. Zhi, ACS Appl. Mater. Interfaces 6 (2014) 18901-18910. [5] Y. Liu, Y. Jiao, Z.L. Zhang, F.Y. Qu, A. Umar, X. Wu, ACS Appl. Mater. Interfaces 6 (2014) 2174-2184. [6] C. Yang, D.G. Li, Mater. Lett. 155 (2015) 78-81. [7] Y. Jiao, Y. Liu, B.S. Yin, S.W. Zhang, F.Y. Qu, X. Wu, Nano Energy 10 (2014) 90-98. [8] Y. Liu, Y. Jiao, B.S. Yin, S.W. Zhang, F.Y. Qu, X. Wu, J. Mater. Chem. A, 3 (2015) 3676-3682. [9] M.S. Zhu, Y. Huang, Y. Huang, W.J. Meng, Q.C. Gong, G.M. Li, C.Y. Zhi, J. Mater. Chem. A, 3 (2015) 21321-21327. [10] X.H. Zhang, X.Y. Meng, S.L. Gong, P. Li, L. Jin, Q. Cao, Mater. Lett.179 (2016) 73-77. [11] X. Zheng, Z.C. Han, S.Y. Yao, F. Chai, F.Y. Qu, X. Wu, Dalton Trans. 45 (2016) 7094-7103. [12] P. H. Yang, W. J. Mai, Nano Energy 8 (2014) 274-290. [13] X. H. Xia, J. P. Tu, Y. Q. Zhang, C. D. Gu, X. B. Zhao, H. J. Fan, ACS Nano. 6 (2012) 55315538. [14] B. S. Yin, S.W. Zhang, H. Jiang, F. Y. Qu, X. Wu, J. Mater. Chem. A 3 (2015) 5722-5729. [15] C. J Yuan, H. B. Lin, H.Y. Lu, Y.S. Zhang, B. Y. Xie, Mater. Lett. 148 (2015) 167-170. [16] S. S. Thind, M. Tian, A. Chen, Electrochem. Commun. 43 (2014) 13-17. [17] H. D. Zheng, J. Z. Ou, M. S. Strano, K. Kalantar-zadeh, Adv. Funct. Mater. 21 (2011) 2175-2196. [18] M. J. Qiu, P. Sun, L. X. Shen, C X. Zhao, W. J. Mai, J. Mater. Chem. A, 4 (2016) 7266-7273. [19] G.F. Samu, K. Pencz, C. Janáky, K. Rajeshwar, J Solid State Electr. 19 (2015) 2741-2751.
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[20] M.S. Zhu, Y. Huang, Y. Huang, Z.X. Pei, Q. Xue, H.F. Li, H.Y. Geng, C.Y. Zhi, Adv. Funct. Mater. 26 (2016) 4481-4490.
Highlights
This work reports hydrothermal synthesis of flexible WO3 nanofibers.
The as-obtained products are directly used as supercapacitor electrode materials
The electrode possesses high areal capacitance and good cycling stability
10
PII: DOI: Reference:
S0167-577X(16)31546-4 http://dx.doi.org/10.1016/j.matlet.2016.09.085 MLBLUE21523
To appear in: Materials Letters Received date: 14 August 2016 Revised date: 16 September 2016 Accepted date: 21 September 2016 Cite this article as: Shunyu Yao, Xin Zheng, Xu Zhang, Huanhao Xiao, Fengyu Qu and Xiang Wu, Facile synthesis of flexible WO 3 nanofibers as supercapacitor electrodes, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.09.085 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 galley proof before it is published in its final citable 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.
Facile synthesis of flexible WO3 nanofibers as supercapacitor electrodes Shunyu Yaoa, 1, Xin Zheng a, 1, Xu Zhang a, *, Huanhao Xiao a, Fengyu Qu a, Xiang Wu a, b* a
College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China b
School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China
*E-mail: [email protected] [email protected],
Abstract In this paper, hierarchical WO3 nanofibers are synthesized on a flexible carbon cloth by a facile hydrothermal route. The as-prepared products as supercapacitor electrodes show high discharge areal capacitance (1716.92 mF cm−2 at 2 mA cm−2) and cycling stability (20.9% loss after 6000 repetitive cycles at 10 mA cm−2). These superior performances of hierarchical WO3 structures indicate their promising applications in supercapacitor electrode materials. Keywords: Energy storage and conversion, particles, nanosize, nanocrystalline materials 1. Introduction Nowadays, an increase in fossil fuels use is still the main source of energy consumption. Considering the finite amounts of fossil fuels and the instability of some recycle resources, appropriate alternative energy is necessary to explore. Regarding electrochemical energy storage, incalculable strives have been put into lithium ion batteries (LIB) [1] as well as emergent techniques such as alkaline batteries [2,3] and supercapacitors (SCs) [4-6]. Typically, lithium-ion batteries possess 1
These authors contributed equally to this work 1
outstanding energy densities, low power densities and limited cycle life. On the contrary, with the characteristics of higher power density than batteries and higher energy density than conventional dielectric capacitors, supercapacitors have attracted much attention for next-generation power devices [7-11]. SCs are basically classified into electric double layer capacitors and pseudocapacitors according to charge storage mechanism [12]. The core of electrochemical property is the electrode material. Nanostructured semiconductor metal oxides have been considered as promising candidates for fabricating supercapacitors electrodes due to their electrochemical performances can be largely enhanced or modified [13]. Transition metal oxides have been widely studied in various pseudocapacitive materials [14]. As an n-type semiconductor, WO3 has attracted one’s interest due to its excellent electron transport properties and stabilities against corrosion [15, 16]. In the meantime, WO3 is also a promising electrode material due to its low cost, simple synthesis, outstanding conductivity and high theoretical capacity [17]. To date, many scientists have made tremendous efforts on WO3 as a negative pseudocapacitive electrode material.
Recently, Qiu’s group preapred flowerlike WO3 negative
electrode via an effective electrodeposition method for all-solid-state asymmetric supercapacitors, and showed an excellent electrochemical performance of 196 F g-1 at a scan rate of 10 mV s-1 [18]. Samu et al. reported nanoporous tungsten trioxide structures and studied their application as supercapacitor electrodes [19]. Herein, we reported the fabrication of flexible WO3 nanofibers through a facile hydrothermal approach. Electrochemical tests indicated the hierarchical WO3 nanofibers possess an excellent initial discharge capacitance and cycling stability. 2. Experimental details At first, a piece of carbon cloth (CC) (1 × 1 cm2 in square shape) were cleaned by acetone, deionized water and ethanol by ultrasonication in sequence and dried under atmospheric condition. In
2
a typical procedure, 2 mmol of Na2WO4 were dissolved into 40 mL of deionized water and stirred to form a clear white solution. Then an aqueous solution of nitric acid was added dropwise to the above mixture solution until pH value of the solution reached 2. The mixture was then transferred into a 50 mL autoclave and heated at 180 oC for 15 h. When the reaction was completed, the carbon cloth was taken out and washed with distilled water and absolute ethanol several times, and then dried in a vacuum. Finally, the carbon cloth coated WO3 product was obtained by annealing at 350 oC for 2 h. The microstructure of the samples was characterized by powder X-ray diffraction (XRD, Rigaku Dmax-2600/pc) equipped with Cu Kα radiation (λ = 0.1542 nm). The general and detailed morphologies of the as-prepared samples were examined by scanning electron microscopy (FESEM; Hitachi-S4800). The electrochemical properties of the samples were assessed under a typical three-electrode cell configuration on a CHI 660 electrochemical workstation with 0.5 M H2SO4 aqueous solution as the electrolyte at room temperature. WO3 product grown on carbon cloth (1 cm2 area) was directly used as working electrode. A platinum plate served as a counter electrode and an Ag/AgCl electrode served as the reference electrode. Cyclic voltammetric curves were measured in a potential range between -0.65 and 0 V at different scan rates and the charge-discharge process were performed by cycling the potential from -0.65 to 0 V at different current densities. The specific capacitance was calculated by C = IΔt/(SΔV), where I is the discharge current, Δt is the discharge time, S is the geometrical area of the electrode, and ΔV is the voltage drop upon discharging. The electrochemical impedance spectroscopy (EIS) measurements were performed by applying an AC voltage with 5 mV amplitude in a frequency range from 0.01 Hz to 100 kHz. 3. Results and discussion
3
The as-synthesized WO3 samples are first investigated by X-ray diffraction and the result is shown in Fig. 1a. Except for the peaks derived from carbon cloth, the other sharp peaks can be well indexed to WO3 (JCPDS card No. 33-1387). The strong and sharp peaks indicate that the as-prepared product is highly crystalline. Fig. 1b-d show SEM images of WO3 product at diverse magnifications. One can find that quantities of WO3 product grow tightly on flexible carbon nanofibers, as shown in Fig. 1b.
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Fig. 1(a) XRD pattern of WO3 product (b-d) SEM images of the as-synthesized product with different magnification. (e) EDS spectrum of the product (f-h) EDS mapping of the product
4
Further observation finds that the products are composed of many nanofibers, seen in Fig. 1c. Average diameter of the nanofibers is about 40 nm, which can benefit complete utilization of the active materials. To confirm the element composition of the as-synthesized sample, EDS element mapping is conducted. From the EDS spectra of the sample (Fig. e), the composition of W and O elements are evidently presented and well-distributed (Fig. 1f-f). The signal of C is caused by carbon cloth.
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Fig. 2 (a) CV curves of WO3 nanofiber and carbon cloth at a scan rate of 80 mV/s (b) CV curves of WO3 nanofiber at scan rates between 5 and 80 mV/s (c) WO3 nanofiber and carbon clothes discharge curves at 2 mA/cm2 (d) discharge curves at current densities from 2 to 10 mA/cm2
The electrochemical properties of WO3 product are tested to investigate their potential applications as supercapacitors electrode. Fig. 2a displays the cyclic voltammetry (CV) curves of WO3
5
nanofibers and CC at scan rates of 80 mV/s, revealing that the active materials possess higher capacitive current density than that of CC. CC contributes little to the total capacitance of WO 3 nanofibers electrode. CV curves of the working electrodes collected at diversify scan rates ranging from 5 to 80 mV/s are shown in (Fig. 2b). The charge storage mechanism of WO3 electrode may be explained by the following redox reaction. WO3 + xH+ + xe- ↔ HxWO3 [18]. Fig. 2c shows typical
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galvanostatic charge-discharge curves of WO3 nanofibers and CC at a current density of 2 mA cm−2.
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20
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Z'(ohm)
Fig. 3(a) Areal capacitances and specific capacitance of WO3 nanofibers at different current densities (b) the retention of areal capacitance after 6000 cycles at a current density of 10 mA cm−2 (c) EIS Nyquist plots of WO3 samples (d) Nitrogen adsorption/desorption isotherm and BJH pore size distribution plot of the as-synthesized product.
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CC contributes little to the charge-discharge behavior and could be neglected in all cases. Besides, the charge-discharge performances of the WO3 electrode is also tested under different current densities of 2, 3, 4, 6, 8 and 10 mA cm−2 (Fig. 2d). The average areal capacitances of WO3 nanofibers are calculated to be 1716.92, 1656.92, 1470.76, 1384.61, 1292.31 and 1138.46 mF cm−2, respectively at current densities of 2, 3, 4, 6, 8 and 10 mA cm−2 (Fig. 3a). The specific capacitance (F g-1) of the electrode material was calculated from the galvanostatic discharge curves according to the following equation: C = I×t/(V×m). Where I is the discharge current (A), t is the discharge time (s), m is the mass of active material (g) and V is the voltage change (V) excluding IR drop in the discharge process. 0.0032 g of mass loading of WO3 product (grown on 1 cm2 carbon cloth) as working electrode was weight for electrochemical test. The inset of Fig. 3a demonstrates the calculated specific capacitance based on the discharge curves according to the above mentioned equation. They were 539.42, 516.34, 442.3, 409.61, 373.07 and 341.34 F g-1 at current densities of 2, 3, 4, 6, 8, and 10 A g-1, respectively, demonstrating that the specific capacitance decreases with increasing current density. Good cycling stability is very important to supercapacitor electrodes, which was assessed by repeating CV tests at 10 mV s-1 for 6000 cycles Fig. 3b. The specific capacitance of the product could retain 79.1% of the original areal capacitance after 6000 cycles, demonstrating excellent long-term electrochemical stability. To understand the conduction mechanisms, electrochemical impedance spectra of WO3 nanofiber electrodes were performed. Fig. 3c reveals Nyquist plot with two distinct regimes that include an arc in the high frequency regime and a sloped line in the low frequency regime. The arc corresponds to the charge transfer resistance (Rct) caused by Faradaic reactions at contact interface between the electrode and the electrolyte solution. Diameter of the semi-circular arc is Rct. However WO3 nanofiber electrode showed incomplete semicircles. Nonetheless it is clear from the inset of Fig. 3c, Rct for WO3 nanofiber electrode is the lowest, implying WO3 nanofibers exhibiting highest
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conductivity (or lowest internal resistance)[7]. The diameter of semicircle presented the charge transfer resistance (Rct) of the WO3 nanofiber electrode is 0.38 Ω in the Nyquist plots (the inset in Fig. 3c). At a high frequency area, the intercept on the real axis represents the equivalent series resistance of the electrochemical system (Rs), which contain the inherent resistance of the electroactive material and electrolyte/electrode contact resistance between their interfaces. The real axis (Rs) in Nyquist plots is 2.61Ω. In the low frequency, the slope of the line corresponds to the Warburg impedance (Rw). The vertical shape in the spectrum indicates that WO3 nanofiber electrode presents a faster ion diffusion behavior [20]. According to this mechanism, the dynamic process can be represented by the equivalent circuit shown the inset in Fig. 3c. Rs is the series resistance. Fig. 3d shows the nitrogen adsorption and desorption isotherms. It can be seen that the isotherm of the as-synthesized WO3 nanofibers is a type IV isotherm with a distinct hysteresis loop in the range of 0.2-1.0 P/P0 according to the IUPAC classification, indicating the presence of mesoporous structure for WO3 nanofibers. The Brunauer-Emmett-Teller (BET) surface area of WO3 nanofibers is calculated to be 11.032 m2 g−1. The pore size distribution of the sample calculated by desorption isotherm is shown in the inset in Fig. 3d, indicating that a narrow pore size distribution peak centered at about 3.790 nm 4. Conclusions Hierarchical WO3 nanofibers grown on carbon clothes are prepared by facile hydrothermal process. The as-synthesized products as supercapacitor electrodes show prominent electrochemical performances, such as excellent specific capacity and outstanding cyclability. Such facile synthesis approach can be universal to design other flexible electrode materials for next-generation supercapacitors. References [1] L.N. Gao, F.Y. Qu, X. Wu, J. Mater. Chem. A. 2 (2014) 7367-7372.
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[2] R. Z. Li, Y. M. Wang, C. Zhou, C. Wang, X. Ba, Y.Y. Li, X.T. Huang, J. P. Liu, Adv. Funct. Mater. 25 (2015) 5384-5394. [3] J. Q. Liu, M.B. Zheng, X.Q. Shi, H.B. Zeng, H. Xia, Adv. Funct. Mater. 26 (2016) 919-930. [4] M. S. Zhu, W.J. Meng, Y. Huang, Y. Huang, C.Y. Zhi, ACS Appl. Mater. Interfaces 6 (2014) 18901-18910. [5] Y. Liu, Y. Jiao, Z.L. Zhang, F.Y. Qu, A. Umar, X. Wu, ACS Appl. Mater. Interfaces 6 (2014) 2174-2184. [6] C. Yang, D.G. Li, Mater. Lett. 155 (2015) 78-81. [7] Y. Jiao, Y. Liu, B.S. Yin, S.W. Zhang, F.Y. Qu, X. Wu, Nano Energy 10 (2014) 90-98. [8] Y. Liu, Y. Jiao, B.S. Yin, S.W. Zhang, F.Y. Qu, X. Wu, J. Mater. Chem. A, 3 (2015) 3676-3682. [9] M.S. Zhu, Y. Huang, Y. Huang, W.J. Meng, Q.C. Gong, G.M. Li, C.Y. Zhi, J. Mater. Chem. A, 3 (2015) 21321-21327. [10] X.H. Zhang, X.Y. Meng, S.L. Gong, P. Li, L. Jin, Q. Cao, Mater. Lett.179 (2016) 73-77. [11] X. Zheng, Z.C. Han, S.Y. Yao, F. Chai, F.Y. Qu, X. Wu, Dalton Trans. 45 (2016) 7094-7103. [12] P. H. Yang, W. J. Mai, Nano Energy 8 (2014) 274-290. [13] X. H. Xia, J. P. Tu, Y. Q. Zhang, C. D. Gu, X. B. Zhao, H. J. Fan, ACS Nano. 6 (2012) 55315538. [14] B. S. Yin, S.W. Zhang, H. Jiang, F. Y. Qu, X. Wu, J. Mater. Chem. A 3 (2015) 5722-5729. [15] C. J Yuan, H. B. Lin, H.Y. Lu, Y.S. Zhang, B. Y. Xie, Mater. Lett. 148 (2015) 167-170. [16] S. S. Thind, M. Tian, A. Chen, Electrochem. Commun. 43 (2014) 13-17. [17] H. D. Zheng, J. Z. Ou, M. S. Strano, K. Kalantar-zadeh, Adv. Funct. Mater. 21 (2011) 2175-2196. [18] M. J. Qiu, P. Sun, L. X. Shen, C X. Zhao, W. J. Mai, J. Mater. Chem. A, 4 (2016) 7266-7273. [19] G.F. Samu, K. Pencz, C. Janáky, K. Rajeshwar, J Solid State Electr. 19 (2015) 2741-2751.
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Highlights
This work reports hydrothermal synthesis of flexible WO3 nanofibers.
The as-obtained products are directly used as supercapacitor electrode materials
The electrode possesses high areal capacitance and good cycling stability
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