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activated carbon composites for high cycle performance supercapacitor electrode
Journal of Alloys and Compounds 767 (2018) 419e423
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Zn2SnO4/activated carbon composites for high cycle performance supercapacitor electrode Kaile Jin a, Qiyue Wang a, Luye Chen a, Zixuan Lv a, Jingcai Xu a, b, *, Bo Hong a, Xinqing Wang a, ** a b
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2018 Received in revised form 10 July 2018 Accepted 11 July 2018
Owing to the advantages of the conductive activated carbon (AC) and capacitive Zn2SnO4, a series of Zn2SnO4/AC composites are successfully prepared by a co-precipitation method. The textural structure of Zn2SnO4/AC composites illustrate that the Zn2SnO4 nanoparticles are anchored into the micropores of AC. The systematical electrochemical investigation demonstrates that the specific capacitance of the Z-150 sample (including 25% Zn2SnO4) is 322.6 F g1 at a current density of 1 A g1. The specific capacitance values are retained 96.5%, 91.1% and 81.6% at the current density of 1 A g1, 8 A g1 and 16 A g1 after 3000 cycles, respectively. This confirms the excellent electrochemical performance and good long-term cycling stability of Zn2SnO4/AC composites as supercapacitor electrodes at high current densities. © 2018 Elsevier B.V. All rights reserved.
Keywords: Activated carbon Zn2SnO4 Cycle performance Supercapacitor
1. Introduction With the deterioration of environment and depletion of traditional non-renewable energy (such as coal, oil and nature gas), the flexible, lightweight and environmentally friendly renewable energy storage devices are promoted [1e3]. Supercapacitor, a promising candidates for next-generation power devices, has attracted widespread attention because of its high power density, fast charging capability, excellent cycle stability and long cycle life [4,5]. Carbon based materials [6], metal oxides [7], metal sulfides [8] and conducting polymers [9] are used as supercapacitor electrode materials. The ternary metal oxides, such as MCo2O4 (M ¼ Zn [10], Ni [11]), MFe2O4 (M ¼ Zn [12], Co [13], Ni [14]) and M2SnO4 (M ¼ Mg [15], Zn [16], Co [17]), play an important role in supercapacitor electrode because of the high theoretical specific capacitance. Among them, the cobalt oxides are viewed as the most excellent electrochemical materials. However, due to the rarity, costly and toxicity of Co element, many researchers try to use the cheaper and lower toxic elements instead of Co element [18]. Zn and Sn with low
* Corresponding author. College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China. ** Corresponding author. E-mail addresses: [email protected] (J. Xu), [email protected] (X. Wang). https://doi.org/10.1016/j.jallcom.2018.07.140 0925-8388/© 2018 Elsevier B.V. All rights reserved.
price and toxicity are one of the ideal elements. Therefore, Zn2SnO4 is considered to be an important ternary electrode material due to its high electron mobility (10e15 cm2 V1 S1), excellent adsorption and chemical stability [16]. However, Zn2SnO4 as electrode materials will cause huge volume expansion (>200%) in the process of electrochemical reaction, resulting in the rapid attenuation of the specific capacity, which prevent the commercialization process of Zn2SnO4 electrode materials [19]. Recently, the researchers discover that the nanocrystallization, doping and compound of Zn2SnO4 could avoid volume expansion and improve the electrochemical performances. C.T. Cherian et al. [20] demonstrated that the Zn2SnO4 nanowires showed much more stable capacity than the Zn2SnO4 nanoplates. K. Wang et al. [21] exploited polypyrrole doped hollow Zn2SnO4 to overcome the expansion problem and improve cycling performance. L. Bao et al. [16] fabricated flexible Zn2SnO4/MnO2 Core/Shell nanocable-carbon microfiber hybrid composites to improve the electrochemical performance supercapacitor electrodes. In addition, the comparative worse conductivity of bare Zn2SnO4 also causes the unsatisfactory electrochemical performances. Tremendous efforts have been devoted to combine carbon based materials and Zn2SnO4 as composites to improve the electrochemical performances. The combined materials could take the advantages of both components: high conductivity and buffer huge volume changes of carbon based materials and high specific capacity of Zn2SnO4. The nanocarbon
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based materials combined Zn2SnO4, such as, Zn2SnO4/graphene [22,23], Zn2SnO4/CNTs [24], Zn2SnO4/MnO2/carbon microfiber [16], Zn2SnO4/C [25], etc. have been successfully synthesized and the specific capacity and cycle stability were proven to be higher than the bare Zn2SnO4. Though the nanocarbon based materials like CNTs and graphene combined Zn2SnO4 composites exhibit high specific capacity and cycle stability, these nanocarbon materials are costly and difficult for preparation and commercial production [26]. For the application, activated carbon based supercapacitor is more useful due to its high surface area, electrical conductivity, chemical stability and low cost. In this study, combining the advantages of Zn2SnO4 and AC, a series of Zn2SnO4 with higher specific capacity and cycle stability are anchored into the micropores of AC to form Zn2SnO4/AC composites by a co-precipitation method. The textural structure of asprepared samples are characterized and discussed by x-ray diffraction (XRD), thermal gravity analysis (TGA) and surface area and porosity analyzer (ASAP). The electrochemical performances such as charge-discharge behaviors (CP), cycle stability and cycle life are investigated in a three-electrode aqueous configuration. These Zn2SnO4/AC composites electrodes for supercapacitor exhibit high special capacitances and excellent cycling stabilities at high current densities.
2. Experimental 2.1. Materials Commercial activated carbon (100 meshes) is provided by Changnan Activated Carbon Co., LTD. (Zhejiang, China). All other chemicals, including Tin(IV) chloride [SnCl4], Zinc chloride [ZnCl2], Seignette salt [C4O6H4KNa] and Sodium hydroxide (NaOH) purchased from Hangzhou Chemical Co., LTD. (Zhejiang, China) are analytic grade. All solutions are prepared with deionized water.
2.2. Preparation of Zn2SnO4/AC composites A series of Zn2SnO4/AC composites are prepared by coprecipitation method. For details: 0.005 mol SnCl4, 0.01 mol ZnCl2 and 0.002 mol C4O6H4KNa are dissolved in 100 ml deionized water to prepare 4 parts of the same solution. Then 1 mol, 0.750 mol, 0.500 mol and 0.250 mol of activated carbon are added into the solution respectively. The as-prepared samples are labeled as Z200, Z-150, Z-100 and Z-50. After vigorous stirring for 12 h, 100 ml of 2.0 mol/L NaOH solution is added dropwise and continue stirred for 2 h. The mixed liquids are allowed to react in a 100 C water bath for 24 h. After centrifugal separation, the mixture are washed repeatedly with distilled water and anhydrous ethanol, then the materials are placed in a muffle furnace and heated at 400 C for 2 h with a ramping rate of 1 C/min in air atmosphere. The resulting samples are the Zn2SnO4/AC composites.
2.4. Electrodes preparation and measurements The prepared Zn2SnO4/AC composites materials are prepared as the supercapacitor electrode materials according to the following steps: Concretely, the Zn2SnO4/AC composites materials, conductive carbon black, 60% (W/W) PTFE emulsion are mixed according to a mass ratio of 80:10:10. Evenly, a certain amount of absolute ethanol are added under a magnetic stirrer to stir into a certain viscosity slurry. The above slurry are coated on the foamed nickel and pressed into a supercapacitor electrode with a pressure of 10 MPa. The constant current charge-discharge and cycle life test of the obtained supercapacitor electrode materials are measured using three-electrode mode with the electrode materials as a working electrode, the platinum electrode (1 cm2) as the counter electrode, the saturated calomel electrode as reference electrode and 6 M KOH as electrolyte by an electrochemical workstation (CHI 660E, Shanghai Chenhua, China). 3. Results and discussion Fig. 1 shows the XRD patterns of Zn2SnO4/AC composites and pure AC. The diffraction peaks of XRD curves of all samples are normalized according to the strongest peaks. The AC curve shows two broad peaks at about 2q ¼ 25 and 43 can be assigned to the characteristic peaks of amorphous AC. The diffraction peaks of Zn2SnO4 are marked with “◊”. It can be seen from the XRD pattern that the characteristic diffraction peaks of Zn2SnO4 appear at the 2q of 17, 29 , 34 , 35 , 41, 55 and 60 , which representing the (111), (220), (311), (222), (400), (511) and (440) crystal planes of Zn2SnO4 (JCPDF NO:24-1470) and belong to the Fd-3m (227) space group. Moreover, the diffraction peaks of Zn2SnO4 gradually increase and the amorphous peaks of AC gradually decrease with increasing the amount of Zn2SnO4, which indicating that the composites materials contain Zn2SnO4 and AC phases. Fig. 2 shows the TGA curves of Zn2SnO4/AC composites and pure AC. It can be seen from the figure that the steep slope of AC observes at the temperature range from 400 C to 560 C due to decomposition of the amorphous carbon. The pyrolytic temperature of Zn2SnO4/AC composites has almost no change compare to AC, which indicating that the Zn2SnO4/AC composites don't reduce its thermal stability. In addition, from the remaining ratio of Zn2SnO4/ AC composites and pure AC after 600 C, it can be obtained that Z-
2.3. Characterization of Zn2SnO4/AC composites The phase structure of Zn2SnO4/AC composites are characterized by an XRD diffractometer (DX2700 China) with the Cu-Ka radiation (l ¼ 1.54051 Å, step 0.02 ) at 40 kV and 30 mA. Pyrolysis process (TGA) of the prepared composites are tested by a thermal balance instrument (SDT Q600, TA) in air with a heating rate of 5 C/min. BET surface area, pore diameter, and pore volume are carried out by a surface analyzer (ASAP 2020 USA) with N2 as the adsorbate at 196 C.
Fig. 1. XRD patterns of pure AC and Zn2SnO4/AC composites.
K. Jin et al. / Journal of Alloys and Compounds 767 (2018) 419e423
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Table 1 The textural of pure AC and Zn2SnO4/AC composites. Samples
AC
Z-200
Z-150
Z-100
Z-50
SBET (m2$g1) Vtotal (cm3$g1) daverage (nm)
948.75 0.51 3.70
808.33 0.44 3.76
696.26 0.38 3.84
572.91 0.32 4.18
402.07 0.25 4.25
SBET: Brunauer-Emmett-Teller surface area. Vtotal: Adsorption total pore volume. daverage: BJH Desorption average pore diameter.
Fig. 2. TGA curves of pure AC and Zn2SnO4/AC composites.
200, Z-150, Z-100 and Z-50 composites contain 14%, 25%, 36% and 47% of Zn2SnO4 respectively. Fig. 3 depicts the N2 adsorption/desorption isotherms of Zn2SnO4/AC composites and pure AC. It can be clearly seen from the N2 adsorption/desorption isotherms that all Zn2SnO4/AC isotherms exhibit a similar feature of the H2-type hysteresis loop (according to the IUPAC classification) with AC, which illustrates that all of Zn2SnO4/AC retain unique multi-channel structure of AC. Meanwhile, a rising slope of the adsorption isotherm are detected under the low relative pressure, indicating the presence of mesopores. Specific structural parameters of each samples (The BET specific surface area, total pore volume, average pore diameter) calculated according to the corresponding N2 isotherms are listed in Table 1. The specific surface area and total pore volume of AC are 948.75 m2 g1 and 0.51 cm3 g1, which are larger than the Zn2SnO4/ AC composites. The content of Zn2SnO4 has a significant influence on the specific surface area and total pore volume of Zn2SnO4/AC composites. With the increasing content of Zn2SnO4 in composites, the specific surface area of Zn2SnO4/AC decreases continuously. The specific surface area of Z-200, Z-150, Z-100 and Z-50 are 808.33 m2 g1, 696.26 m2 g1, 572.91 m2 g1 and 402.07 m2 g1, respectively. The total pore volume also has a tendency to gradually becomes smaller which reduces from 0.51 cm3 g1 of pure AC and
Fig. 3. N2 adsorptionedesorption isotherms of pure AC and Zn2SnO4/AC composites.
0.44 cm3 g1 of Z-200 to 0.25 cm3 g1 of Z-50. It can be seen from Fig. 4 that the average pore size distribution of Zn2SnO4/AC composites ranges from 3 to 4 nm and part of the micropore distribution decreases, resulting in the average pore diameter of the Zn2SnO4/AC increase slightly. Therefore, it can be concluded that the Zn2SnO4 nanoparticles should enter the mesopores of the AC or block some of the micropores, which resulting in a decrease in specific surface area and total pore volume. In order to investigate the electrochemical performances of Zn2SnO4/AC composites as active supercapacitor electrodes, the specific capacitance values of pure AC and four kinds of Zn2SnO4/AC composites electrode materials are tested. The galvanostatic charge-discharge (GCD) measurements of four kinds of Zn2SnO4/AC composites electrode materials are tested at a current density of 1 A g1. The results are showed in Fig. 5a and Fig. 5b, pure AC shows a symmetrical isosceles triangle with no obvious internal voltage drops, indicating a typical supercapacitor double-layer energy storage. While the potential-time curves of Zn2SnO4/AC composites electrode materials exhibit the nonlinear voltage platform during the charging and discharging process. This mainly associate with a reversible redox reaction between Sn2þ and Sn4þ ions, which is the principle of the typical characteristic of pseudocapacitance. This can be based on the following equations:
Zn2 SnO4 þ OH þ H2 O4SnOOH þ 2ZnOOH þ e
(1)
Meanwhile, the charge and discharge curves are tested at different current densities of 1, 2, 4, 8, and 16 A g1. The specific capacitance values of the electrode are calculated from the GCD curves according to the formula:
CS ¼
I*Dt m*Dv
(2)
Fig. 4. BJH adsorption pore size distribution of pure AC and Zn2SnO4/AC composites.
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K. Jin et al. / Journal of Alloys and Compounds 767 (2018) 419e423
Fig. 5. (a) Galvanostatic chargeedischarge curves of AC at 1 A g1, (b) Galvanostatic chargeedischarge curves of Zn2SnO4/AC composites at 1 A g1, (c) The specific capacitance values calculated from various discharge curves, (d) The cycling performance of Z-150 at varying current densities.
where CS (F g1) is the specific capacity, I (A) is the charge-discharge current, △t (s) is the discharge time, m (g) is the electroactive material, △v (V) is the potential window. The results are shown in Fig. 5c. The specific capacitances of pure AC at the current density of 1, 2, 4, 8, 16 A g1 are 69.1, 66, 62.8, 59.2, 54.4 F g1, respectively, which are lower than those of the Z-200 (264.5, 253, 237.6, 212.5, and 178.4 F g1), Z-150 (322.6, 306.4, 280.1, 245.9, and 195.8 F g1), Z-100 (397.2, 377, 340.2, 275.1, and 188.7 F g1) and Z-50 (469.4, 429.7, 356.8, 269, and 180.4 F g1), respectively. By comparison, it is not difficult to find that with the increasing of the current density, the specific capacitance values of the electrode material decrease. Whereas, the decline of different samples exists obvious differences. The specific capacitance values of pure AC, Z-200, Z-150, Z100 and Z-50 are reduced by 21.3%, 32.5%, 39.3%, 52.5% and 61.5% when the current density increase to 16 A g1. This illustrates that with the increasing content of the Zn2SnO4 in the composite electrode, the specific capacitance value increases at a relatively small current density, but greatly decreases at a large current density. This is because of the reduction of the AC content in the composite, which resulting in a decline of the composite conductivity. As seen from Fig. 5c, the specific capacitance value of Z-100 is larger than that of Z-50 at the current density of 8 A g1. Especially at 16 A g1, the specific capacitance value of Z-150 is larger than that of Z-100 and Z-50. Therefore, considering the specific capacitance value at different current densities, we consider that the Z-150 sample (including 25% Zn2SnO4) is optimized for electrochemical performance. In order to further illustrate the excellent electrochemical performance of the Z-150 electrode material, the long-term cycling stability of Z-150 is investigated by continuous
chargingedischarging measurements at different current densities. The results are shown in Fig. 5d. The specific capacitance values of Z-150 electrode material are retained 96.5%, 91.1% and 81.6% at the current density of 1 A g1, 8 A g1 and 16 A g1 after 3000 cycles, respectively. This further confirms the excellent electrochemical performance and good long-term cycling stability of Zn2SnO4/AC composites as supercapacitor electrodes at high current densities. 4. Conclusions In summary, combining the advantages of the good conductive AC and high capacitive Zn2SnO4, a series of Zn2SnO4/AC composites are successfully prepared by a co-precipitation method. The textural structures of Zn2SnO4/AC composites illustrate the Zn2SnO4 nanoparticles are anchored into the micropores of AC. The systematical electrochemical investigation demonstrates that the specific capacitance of the Z-150 sample (including 25% Zn2SnO4) is 322.6 F g1 at a current density of 1 A g1 and retained 96.5% after 3000 cycles. Especially, the specific capacitance values still remains 91.1% at 8 A g1and 81.6% at 16 A g1 after 3000 cycles. This confirms the excellent electrochemical performance and good longterm cycling stability of Zn2SnO4/AC composites as supercapacitor electrodes at high current densities. Acknowledgments The research was funded by the Foundation of Science and Technology Department of Zhejiang Province (No. 2017C37067, 2017C33078, 2016C31113).
K. Jin et al. / Journal of Alloys and Compounds 767 (2018) 419e423
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[15] V.K. Premkumar, G. Sivakumar, Hydrothermally synthesized cubic magnesium stannate (Mg2SnO4) nanoparticles and its electrochemical performances, J. Mater. Sci. Mater. Electron. 28 (2017) 14226. [16] L. Bao, J. Zang, X. Li, Flexible Zn2SnO4/MnO2 core/shell nanocable-carbon microfiber hybrid composites for high-performance supercapacitor electrodes, Nano Lett. 11 (2011) 1215. [17] V.K. Premkumar, G. Sivakumar, S. Dinesh, S. Barathan, Facile hydrothermal synthesis of cobalt stannate (Co2SnO4) nano particles for electrochemical properties, J. Mater. Sci. Mater. Electron. 28 (2017) 4780. [18] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797. [19] L. Qin, S. Liang, A. Pan, X. Tan, Facile solvothermal synthesis of Zn2SnO4 nanoparticles as anode materials for lithium-ion batteries, Mater. Lett. 141 (2015) 255. [20] C.T. Cherian, M. Zheng, M.V. Reddy, et al., Zn2SnO4 nanowires versus nanoplates: electrochemical performance and morphological evolution during LiCycling, ACS Appl. Mater. Interfaces 5 (2013) 6054. [21] K. Wang, Y. Huang, T. Han, et al., Facile synthesis and performance of polypyrrole coated hollow Zn2SnO4 boxes as anode materials for lithium-ion batteries, Ceram. Int. 40 (2014) 2359. [22] L. Qin, S. Liang, X. Tan, A. Pan, Zn2SnO4/graphene composites as anode materials for high performance lithium-ion batteries, J. Alloys Compd. 692 (2017) 124. [23] M. Jing, Z. Hou, H. Yang, et al., Zn2SnO4 coated reduced graphene oxide nanoribbons with enhanced electrochemical performance for lithium-ion batteries, J. Mater. Res. 31 (2016) 3666. [24] L. Qin, S. Liang, A. Pan, X. Tan, Zn2SnO4/carbon nanotubes composite with enhanced electrochemical performance as anode materials for lithium-ion batteries, Mater. Lett. 164 (2016) 44. [25] T. Jiang, X. Tian, H. Gu, et al., Zn2SnO4@C coreeshell nanorods with enhanced anodic performance for lithium-ion batteries, J. Alloys Compd. 639 (2015) 239. [26] J. Xu, F. Liu, X. Peng, et al., Hydrothermal synthesis of NiCo2O4/activated carbon composites for supercapacitor with enhanced cycle performance, ChemistrySelect 2 (2017) 5189.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Zn2SnO4/activated carbon composites for high cycle performance supercapacitor electrode Kaile Jin a, Qiyue Wang a, Luye Chen a, Zixuan Lv a, Jingcai Xu a, b, *, Bo Hong a, Xinqing Wang a, ** a b
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2018 Received in revised form 10 July 2018 Accepted 11 July 2018
Owing to the advantages of the conductive activated carbon (AC) and capacitive Zn2SnO4, a series of Zn2SnO4/AC composites are successfully prepared by a co-precipitation method. The textural structure of Zn2SnO4/AC composites illustrate that the Zn2SnO4 nanoparticles are anchored into the micropores of AC. The systematical electrochemical investigation demonstrates that the specific capacitance of the Z-150 sample (including 25% Zn2SnO4) is 322.6 F g1 at a current density of 1 A g1. The specific capacitance values are retained 96.5%, 91.1% and 81.6% at the current density of 1 A g1, 8 A g1 and 16 A g1 after 3000 cycles, respectively. This confirms the excellent electrochemical performance and good long-term cycling stability of Zn2SnO4/AC composites as supercapacitor electrodes at high current densities. © 2018 Elsevier B.V. All rights reserved.
Keywords: Activated carbon Zn2SnO4 Cycle performance Supercapacitor
1. Introduction With the deterioration of environment and depletion of traditional non-renewable energy (such as coal, oil and nature gas), the flexible, lightweight and environmentally friendly renewable energy storage devices are promoted [1e3]. Supercapacitor, a promising candidates for next-generation power devices, has attracted widespread attention because of its high power density, fast charging capability, excellent cycle stability and long cycle life [4,5]. Carbon based materials [6], metal oxides [7], metal sulfides [8] and conducting polymers [9] are used as supercapacitor electrode materials. The ternary metal oxides, such as MCo2O4 (M ¼ Zn [10], Ni [11]), MFe2O4 (M ¼ Zn [12], Co [13], Ni [14]) and M2SnO4 (M ¼ Mg [15], Zn [16], Co [17]), play an important role in supercapacitor electrode because of the high theoretical specific capacitance. Among them, the cobalt oxides are viewed as the most excellent electrochemical materials. However, due to the rarity, costly and toxicity of Co element, many researchers try to use the cheaper and lower toxic elements instead of Co element [18]. Zn and Sn with low
* Corresponding author. College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China. ** Corresponding author. E-mail addresses: [email protected] (J. Xu), [email protected] (X. Wang). https://doi.org/10.1016/j.jallcom.2018.07.140 0925-8388/© 2018 Elsevier B.V. All rights reserved.
price and toxicity are one of the ideal elements. Therefore, Zn2SnO4 is considered to be an important ternary electrode material due to its high electron mobility (10e15 cm2 V1 S1), excellent adsorption and chemical stability [16]. However, Zn2SnO4 as electrode materials will cause huge volume expansion (>200%) in the process of electrochemical reaction, resulting in the rapid attenuation of the specific capacity, which prevent the commercialization process of Zn2SnO4 electrode materials [19]. Recently, the researchers discover that the nanocrystallization, doping and compound of Zn2SnO4 could avoid volume expansion and improve the electrochemical performances. C.T. Cherian et al. [20] demonstrated that the Zn2SnO4 nanowires showed much more stable capacity than the Zn2SnO4 nanoplates. K. Wang et al. [21] exploited polypyrrole doped hollow Zn2SnO4 to overcome the expansion problem and improve cycling performance. L. Bao et al. [16] fabricated flexible Zn2SnO4/MnO2 Core/Shell nanocable-carbon microfiber hybrid composites to improve the electrochemical performance supercapacitor electrodes. In addition, the comparative worse conductivity of bare Zn2SnO4 also causes the unsatisfactory electrochemical performances. Tremendous efforts have been devoted to combine carbon based materials and Zn2SnO4 as composites to improve the electrochemical performances. The combined materials could take the advantages of both components: high conductivity and buffer huge volume changes of carbon based materials and high specific capacity of Zn2SnO4. The nanocarbon
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K. Jin et al. / Journal of Alloys and Compounds 767 (2018) 419e423
based materials combined Zn2SnO4, such as, Zn2SnO4/graphene [22,23], Zn2SnO4/CNTs [24], Zn2SnO4/MnO2/carbon microfiber [16], Zn2SnO4/C [25], etc. have been successfully synthesized and the specific capacity and cycle stability were proven to be higher than the bare Zn2SnO4. Though the nanocarbon based materials like CNTs and graphene combined Zn2SnO4 composites exhibit high specific capacity and cycle stability, these nanocarbon materials are costly and difficult for preparation and commercial production [26]. For the application, activated carbon based supercapacitor is more useful due to its high surface area, electrical conductivity, chemical stability and low cost. In this study, combining the advantages of Zn2SnO4 and AC, a series of Zn2SnO4 with higher specific capacity and cycle stability are anchored into the micropores of AC to form Zn2SnO4/AC composites by a co-precipitation method. The textural structure of asprepared samples are characterized and discussed by x-ray diffraction (XRD), thermal gravity analysis (TGA) and surface area and porosity analyzer (ASAP). The electrochemical performances such as charge-discharge behaviors (CP), cycle stability and cycle life are investigated in a three-electrode aqueous configuration. These Zn2SnO4/AC composites electrodes for supercapacitor exhibit high special capacitances and excellent cycling stabilities at high current densities.
2. Experimental 2.1. Materials Commercial activated carbon (100 meshes) is provided by Changnan Activated Carbon Co., LTD. (Zhejiang, China). All other chemicals, including Tin(IV) chloride [SnCl4], Zinc chloride [ZnCl2], Seignette salt [C4O6H4KNa] and Sodium hydroxide (NaOH) purchased from Hangzhou Chemical Co., LTD. (Zhejiang, China) are analytic grade. All solutions are prepared with deionized water.
2.2. Preparation of Zn2SnO4/AC composites A series of Zn2SnO4/AC composites are prepared by coprecipitation method. For details: 0.005 mol SnCl4, 0.01 mol ZnCl2 and 0.002 mol C4O6H4KNa are dissolved in 100 ml deionized water to prepare 4 parts of the same solution. Then 1 mol, 0.750 mol, 0.500 mol and 0.250 mol of activated carbon are added into the solution respectively. The as-prepared samples are labeled as Z200, Z-150, Z-100 and Z-50. After vigorous stirring for 12 h, 100 ml of 2.0 mol/L NaOH solution is added dropwise and continue stirred for 2 h. The mixed liquids are allowed to react in a 100 C water bath for 24 h. After centrifugal separation, the mixture are washed repeatedly with distilled water and anhydrous ethanol, then the materials are placed in a muffle furnace and heated at 400 C for 2 h with a ramping rate of 1 C/min in air atmosphere. The resulting samples are the Zn2SnO4/AC composites.
2.4. Electrodes preparation and measurements The prepared Zn2SnO4/AC composites materials are prepared as the supercapacitor electrode materials according to the following steps: Concretely, the Zn2SnO4/AC composites materials, conductive carbon black, 60% (W/W) PTFE emulsion are mixed according to a mass ratio of 80:10:10. Evenly, a certain amount of absolute ethanol are added under a magnetic stirrer to stir into a certain viscosity slurry. The above slurry are coated on the foamed nickel and pressed into a supercapacitor electrode with a pressure of 10 MPa. The constant current charge-discharge and cycle life test of the obtained supercapacitor electrode materials are measured using three-electrode mode with the electrode materials as a working electrode, the platinum electrode (1 cm2) as the counter electrode, the saturated calomel electrode as reference electrode and 6 M KOH as electrolyte by an electrochemical workstation (CHI 660E, Shanghai Chenhua, China). 3. Results and discussion Fig. 1 shows the XRD patterns of Zn2SnO4/AC composites and pure AC. The diffraction peaks of XRD curves of all samples are normalized according to the strongest peaks. The AC curve shows two broad peaks at about 2q ¼ 25 and 43 can be assigned to the characteristic peaks of amorphous AC. The diffraction peaks of Zn2SnO4 are marked with “◊”. It can be seen from the XRD pattern that the characteristic diffraction peaks of Zn2SnO4 appear at the 2q of 17, 29 , 34 , 35 , 41, 55 and 60 , which representing the (111), (220), (311), (222), (400), (511) and (440) crystal planes of Zn2SnO4 (JCPDF NO:24-1470) and belong to the Fd-3m (227) space group. Moreover, the diffraction peaks of Zn2SnO4 gradually increase and the amorphous peaks of AC gradually decrease with increasing the amount of Zn2SnO4, which indicating that the composites materials contain Zn2SnO4 and AC phases. Fig. 2 shows the TGA curves of Zn2SnO4/AC composites and pure AC. It can be seen from the figure that the steep slope of AC observes at the temperature range from 400 C to 560 C due to decomposition of the amorphous carbon. The pyrolytic temperature of Zn2SnO4/AC composites has almost no change compare to AC, which indicating that the Zn2SnO4/AC composites don't reduce its thermal stability. In addition, from the remaining ratio of Zn2SnO4/ AC composites and pure AC after 600 C, it can be obtained that Z-
2.3. Characterization of Zn2SnO4/AC composites The phase structure of Zn2SnO4/AC composites are characterized by an XRD diffractometer (DX2700 China) with the Cu-Ka radiation (l ¼ 1.54051 Å, step 0.02 ) at 40 kV and 30 mA. Pyrolysis process (TGA) of the prepared composites are tested by a thermal balance instrument (SDT Q600, TA) in air with a heating rate of 5 C/min. BET surface area, pore diameter, and pore volume are carried out by a surface analyzer (ASAP 2020 USA) with N2 as the adsorbate at 196 C.
Fig. 1. XRD patterns of pure AC and Zn2SnO4/AC composites.
K. Jin et al. / Journal of Alloys and Compounds 767 (2018) 419e423
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Table 1 The textural of pure AC and Zn2SnO4/AC composites. Samples
AC
Z-200
Z-150
Z-100
Z-50
SBET (m2$g1) Vtotal (cm3$g1) daverage (nm)
948.75 0.51 3.70
808.33 0.44 3.76
696.26 0.38 3.84
572.91 0.32 4.18
402.07 0.25 4.25
SBET: Brunauer-Emmett-Teller surface area. Vtotal: Adsorption total pore volume. daverage: BJH Desorption average pore diameter.
Fig. 2. TGA curves of pure AC and Zn2SnO4/AC composites.
200, Z-150, Z-100 and Z-50 composites contain 14%, 25%, 36% and 47% of Zn2SnO4 respectively. Fig. 3 depicts the N2 adsorption/desorption isotherms of Zn2SnO4/AC composites and pure AC. It can be clearly seen from the N2 adsorption/desorption isotherms that all Zn2SnO4/AC isotherms exhibit a similar feature of the H2-type hysteresis loop (according to the IUPAC classification) with AC, which illustrates that all of Zn2SnO4/AC retain unique multi-channel structure of AC. Meanwhile, a rising slope of the adsorption isotherm are detected under the low relative pressure, indicating the presence of mesopores. Specific structural parameters of each samples (The BET specific surface area, total pore volume, average pore diameter) calculated according to the corresponding N2 isotherms are listed in Table 1. The specific surface area and total pore volume of AC are 948.75 m2 g1 and 0.51 cm3 g1, which are larger than the Zn2SnO4/ AC composites. The content of Zn2SnO4 has a significant influence on the specific surface area and total pore volume of Zn2SnO4/AC composites. With the increasing content of Zn2SnO4 in composites, the specific surface area of Zn2SnO4/AC decreases continuously. The specific surface area of Z-200, Z-150, Z-100 and Z-50 are 808.33 m2 g1, 696.26 m2 g1, 572.91 m2 g1 and 402.07 m2 g1, respectively. The total pore volume also has a tendency to gradually becomes smaller which reduces from 0.51 cm3 g1 of pure AC and
Fig. 3. N2 adsorptionedesorption isotherms of pure AC and Zn2SnO4/AC composites.
0.44 cm3 g1 of Z-200 to 0.25 cm3 g1 of Z-50. It can be seen from Fig. 4 that the average pore size distribution of Zn2SnO4/AC composites ranges from 3 to 4 nm and part of the micropore distribution decreases, resulting in the average pore diameter of the Zn2SnO4/AC increase slightly. Therefore, it can be concluded that the Zn2SnO4 nanoparticles should enter the mesopores of the AC or block some of the micropores, which resulting in a decrease in specific surface area and total pore volume. In order to investigate the electrochemical performances of Zn2SnO4/AC composites as active supercapacitor electrodes, the specific capacitance values of pure AC and four kinds of Zn2SnO4/AC composites electrode materials are tested. The galvanostatic charge-discharge (GCD) measurements of four kinds of Zn2SnO4/AC composites electrode materials are tested at a current density of 1 A g1. The results are showed in Fig. 5a and Fig. 5b, pure AC shows a symmetrical isosceles triangle with no obvious internal voltage drops, indicating a typical supercapacitor double-layer energy storage. While the potential-time curves of Zn2SnO4/AC composites electrode materials exhibit the nonlinear voltage platform during the charging and discharging process. This mainly associate with a reversible redox reaction between Sn2þ and Sn4þ ions, which is the principle of the typical characteristic of pseudocapacitance. This can be based on the following equations:
Zn2 SnO4 þ OH þ H2 O4SnOOH þ 2ZnOOH þ e
(1)
Meanwhile, the charge and discharge curves are tested at different current densities of 1, 2, 4, 8, and 16 A g1. The specific capacitance values of the electrode are calculated from the GCD curves according to the formula:
CS ¼
I*Dt m*Dv
(2)
Fig. 4. BJH adsorption pore size distribution of pure AC and Zn2SnO4/AC composites.
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Fig. 5. (a) Galvanostatic chargeedischarge curves of AC at 1 A g1, (b) Galvanostatic chargeedischarge curves of Zn2SnO4/AC composites at 1 A g1, (c) The specific capacitance values calculated from various discharge curves, (d) The cycling performance of Z-150 at varying current densities.
where CS (F g1) is the specific capacity, I (A) is the charge-discharge current, △t (s) is the discharge time, m (g) is the electroactive material, △v (V) is the potential window. The results are shown in Fig. 5c. The specific capacitances of pure AC at the current density of 1, 2, 4, 8, 16 A g1 are 69.1, 66, 62.8, 59.2, 54.4 F g1, respectively, which are lower than those of the Z-200 (264.5, 253, 237.6, 212.5, and 178.4 F g1), Z-150 (322.6, 306.4, 280.1, 245.9, and 195.8 F g1), Z-100 (397.2, 377, 340.2, 275.1, and 188.7 F g1) and Z-50 (469.4, 429.7, 356.8, 269, and 180.4 F g1), respectively. By comparison, it is not difficult to find that with the increasing of the current density, the specific capacitance values of the electrode material decrease. Whereas, the decline of different samples exists obvious differences. The specific capacitance values of pure AC, Z-200, Z-150, Z100 and Z-50 are reduced by 21.3%, 32.5%, 39.3%, 52.5% and 61.5% when the current density increase to 16 A g1. This illustrates that with the increasing content of the Zn2SnO4 in the composite electrode, the specific capacitance value increases at a relatively small current density, but greatly decreases at a large current density. This is because of the reduction of the AC content in the composite, which resulting in a decline of the composite conductivity. As seen from Fig. 5c, the specific capacitance value of Z-100 is larger than that of Z-50 at the current density of 8 A g1. Especially at 16 A g1, the specific capacitance value of Z-150 is larger than that of Z-100 and Z-50. Therefore, considering the specific capacitance value at different current densities, we consider that the Z-150 sample (including 25% Zn2SnO4) is optimized for electrochemical performance. In order to further illustrate the excellent electrochemical performance of the Z-150 electrode material, the long-term cycling stability of Z-150 is investigated by continuous
chargingedischarging measurements at different current densities. The results are shown in Fig. 5d. The specific capacitance values of Z-150 electrode material are retained 96.5%, 91.1% and 81.6% at the current density of 1 A g1, 8 A g1 and 16 A g1 after 3000 cycles, respectively. This further confirms the excellent electrochemical performance and good long-term cycling stability of Zn2SnO4/AC composites as supercapacitor electrodes at high current densities. 4. Conclusions In summary, combining the advantages of the good conductive AC and high capacitive Zn2SnO4, a series of Zn2SnO4/AC composites are successfully prepared by a co-precipitation method. The textural structures of Zn2SnO4/AC composites illustrate the Zn2SnO4 nanoparticles are anchored into the micropores of AC. The systematical electrochemical investigation demonstrates that the specific capacitance of the Z-150 sample (including 25% Zn2SnO4) is 322.6 F g1 at a current density of 1 A g1 and retained 96.5% after 3000 cycles. Especially, the specific capacitance values still remains 91.1% at 8 A g1and 81.6% at 16 A g1 after 3000 cycles. This confirms the excellent electrochemical performance and good longterm cycling stability of Zn2SnO4/AC composites as supercapacitor electrodes at high current densities. Acknowledgments The research was funded by the Foundation of Science and Technology Department of Zhejiang Province (No. 2017C37067, 2017C33078, 2016C31113).
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