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Facile synthesis of porous SnO2 film grown on Ni foam applied for high-performance supercapacitors
Accepted Manuscript Facile synthesis of porous SnO2 film grown on Ni foam applied for high-performance supercapacitors Shiyun Hao, Youyi Sun, Yaqing Liu, Yinghe Zhang, Guosheng Hu PII:
S0925-8388(16)32313-1
DOI:
10.1016/j.jallcom.2016.07.278
Reference:
JALCOM 38447
To appear in:
Journal of Alloys and Compounds
Received Date: 6 May 2016 Revised Date:
19 July 2016
Accepted Date: 26 July 2016
Please cite this article as: S. Hao, Y. Sun, Y. Liu, Y. Zhang, G. Hu, Facile synthesis of porous SnO2 film grown on Ni foam applied for high-performance supercapacitors, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.07.278. 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.
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Facile synthesis of porous SnO2 film grown on Ni foam applied for high-performance supercapacitors Shiyun Hao1, Youyi Sun2*, Yaqing Liu1*, Yinghe Zhang3, Guosheng Hu1 1. School of Materials science and Engineering, North University of China, Taiyuan 030051, P.R.China. 2. Shanxi Province Key Laboratory of Functional Nanocomposites, North University of China, Taiyuan 030051, P.R.China.
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3. Nanotechnology Department, Helmholtz Association, 21502, Hamburg, Germany.
Abstract: Here, a novel three dimensional Ni foam-supported porous SnO2 film (SnO2/Ni composite foam) was successfully synthesized by the simple hydrothermal method. The porous SnO2 film had closely coated on
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the backbones of Ni foam without assistant of surfactant at mild conditions. At the same time, the pore size of SnO2/Ni composite foam was easily controlled by the reaction temperature. Furthermore, it was found that the
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capacitive properties of SnO2/Ni composite foam were related to the pore size of SnO2 film. The SnO2 film with smaller pore size showed higher capacitive properties, in which the porous SnO2/Ni composite foam with pore size of ca.60.0nm showed high rate capability of 541.0 F g-1 at 10.0 A g-1 and good cycle stability with capacitance retention of 98.1% after 1000 cycles. The high capacitive properties was due to the SnO2 active materials grown on the backbone of Ni foam and porous structure of SnO2 materials, resulting in the reduce of resistance and enhancement of active surface area. Our work not only demonstrates the controlled synthesis of
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high-quality porous SnO2 /Ni composite foam at mild conditions on a large scale, but also provides a universal
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route for the rational design of supercapacitors with high performance.
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Key words: SnO2 ; Ni foam; hydrothermal method; low temperature; supercapacitor.
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Responding author: Fax: 86-351-3559669 E-mail address:[email protected] (YY Sun)
ACCEPTED MANUSCRIPT 1. Introduction Supercapacitors as ideal energy storage devices have attracted intense interest, due to their outstanding electrochemical performance such as high power density, fast charging-discharging rate and excellent cycle stability [1]. It was well-known that the performance of supercapacitors strongly depended on the properties and structure of electrodes. So, a large number of materials have been used for supercapacitor electrodes, such as carbon-based materials, metal oxides and conducting polymers [2]. Among various candidates, SnO2 has
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been considered to be one of the most attractive materials in terms of its high theoretical specific capacitance, low cost, natural abundance and environmental friendliness [3]. Unfortunately, in most cases, the SnO2 powder was coated on the current collector with the assistance of binder (such as PVDF, PTFE) [4].The high contact resistance between SnO2 powder and current collector and the nonconductive binder would greatly decrease
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the rate capability and poor cycle stability of supercapacitors [4]. Furthermore, SnO2-based electrodes usually suffered from low electrical conductivity and large volume expansion/shrinkage during the charge-discharge
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process, which greatly precluded their practical application in supercapacitors [5-8]. To overcome these problems, the SnO2 electrode material and current collector Ni foam should be integrated together in depth rather than that electrode material was simply coated on the current collector by binder, and a porous SnO2 film should replace the traditional SnO2 powder to reduce contact resistance and volume expansion/shrinkage, enhance the active surface area and electrical conductivity of electrode material. Recently, the SnO2 film deposition on three-dimensional (3D) Ni foam for Li-ion batteries application were synthesized by
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electrostatic spray deposition (ESD) technique and hydrothermal method [9-10]. However, the high temperature and long reaction time was required in these works, which restricted its practical application.
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Therefore, it was still a challenge to the integration of SnO2 film into metal conductive network at mild conditions as binder-free electrode materials. In addition to this, there was few works reporting the porous
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SnO2 film deposition on 3D Ni foam for application in supercapacitors. In the present study, SnO2/Ni composite foam was synthesized via hydrothermal method at low
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temperature for the first time. In this case, the SnO2 film was directly grown on the backbones of the 3D Ni foam without assistant of any surfactant, which reduced the contact resistance and enhanced the transport rate of ion/electron. Furthermore, the SnO2 film possessed porous structure, which enhanced electrochemically active surface area and cycle stability. In the electrochemical measurement, the SnO2/Ni composite foam were directly tested as the electrode, which did not need any additional metal current collector, displayed remarkable high rate capability performance, excellent cycle stability and low cost. 2. Experimental 2.1 Preparation of SnO2/Ni composite foam The SnO2/Ni composite foam was synthesized by hydrothermal method at mild conditions as shown in Scheme 1. Ni foam (110 PPI, 40 mg/cm2, 0.5 mm thick, Shenzhen Po Xon Machinery Technology Co LTD)
ACCEPTED MANUSCRIPT was cleaned with a 2.0M HCl solution in an ultrasonic bath for 0.5h in order to remove the possibly existing nickel oxide layer on the outer surface, and then washed with deionized water and absolute ethanol for several times, respectively. Afterward, Ni foam was dried at 70.0℃for 6.0h. In a typical procedure, 0.8765g SnCl4·5H2O4 and 0.055g Zn(CH3COO)2·2H2O were dissolved in 40.0ml NaOH water (0.96g). Subsequently, 40ml ethanol was drop-wised，and the solution, after being magnetically stirred for 10 min in the air at room
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temperature, was transferred into Teflon-lined autoclave containing a clean Ni foam with a diameter of 3.0 cm and the autoclave was maintained at 100.0℃, 120.0℃ or 140.0℃ for 6.0 h and then cooled down to room temperature. The final SnO2/Ni composite foam was washed with distilled water and ethanol for several times to remove any salts, and then dried in a vacuum oven at 50.0℃ for 6.0 h. The mass of SnO2 on composite
Scheme 1. 2.2 Characterization
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foam was measured by change of Ni foam deposited before and after SnO2.
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X-ray diffraction (XRD) was recorded on a Rigaku Dmax-r C X-ray diffractometer using Cu Kα radiation (λ=0.154 nm) operated at 40 kV and 100 mA.
The morphology and structure of composite foam was observed by scanning electron microscopy (SEM) (Su-4700, HITACHI Japan) . 2.3. Electrochemical characterization
All electrochemical measurements were carried out in a conventional three-electrode system using an
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electrochemical workstation (CHI660D, Chenhua Instruments, China) with 6M KOH solution as the
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electrolyte. The porous SnO2/Ni composite foam was directly tested as the electrode, while saturated calomel electrode (SCE) as the reference and a platinum plate as the counter electrode. The electrochemical
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performances of the prepared electrodes were characterized by cyclic voltammetry, electrochemical impedance spectroscopy (EIS) measurements and galvanostatic charge-discharge tests. The specific capacitance was obtained from the discharge process according to the following equation, Cs (F/g) = IΔt/ΔE·m, where I is the
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current loaded (A), Δt is the discharge time (s), ΔE is the potential change during the discharge process, and m is the mass of active material (g). 3. Results and Discussion
Fig.1 shows the XRD patterns of the samples prepared at various temperatures by the hydrothermal method. It clearly showed three sharp peaks at 44.4o, 51.8o and 76.3o, corresponding to the diffractions of (111), (200) and (220) planes of Ni, respectively (JCPDS card No. 04-0850) [11]. In addition to this, for the all samples, excluding the diffraction peaks of Ni foam, others diffraction peaks at round 26.4°, 34.0°, 58.2°, 61.0° and 65.0° were assigned to (110), (101), (002), (310) and (112) planes of SnO2 (JCPDS, No. 41-1445) [12]. These results indicated the formation of SnO2/Ni composite foam. These peaks of products prepared at
ACCEPTED MANUSCRIPT 100.0℃ were broad and weak, demonstrating that the crystallinity of the SnO2 obtained from hydrothermal method at low temperature was relatively poor [12]. When the reaction temperature was increased to 120.0℃ and 140.0℃, there were obvious increase in the intensity of XRD patterns. These results indicated that the SnO2/Ni composite foam could be prepared at relatively low reaction temperature (<150.0℃). Although SnO2 grown on Ni foam has been synthesized by the hydrothermal method at low temperature, it was still difficult
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for scientists to properly explain the formation mechanism. Some formation mechanism related to diameter and morphologies of SnO2 depended on the Sn+/OH- ratio in solution has been proposed [13-14]. Here, this may be explained by both crystal growth and nucleation theory in which the synthesis was divided into two
the chemical reaction given below:
Sn 4 6OH SnOH 6
2
Zn 2 4OH ZnOH 4
2
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steps: nucleation stage and crystal growth. In the nucleation stage, ZnSn(OH)6 called nucleus was created by
SnOH 6 ZnOH 4 ZnSnOH 6 2
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2
ZnSnOH 6 SnO2 Zn 2 OH H 2O Zn ion in this process did not act as a reactant and might be understood as a catalyst, which decreased the decomposed temperature of nuclei. The nucleation might go on until the first stage of hydrothermal process (T<150.0℃) in which nuclei start growing in size by collision with other nuclei. Hence, the SnO2
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grown on Ni foam could be synthesized by the hydrothermal method at lower temperature comparing to
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previous works[5-8]. In our opinion, the Zn ion played a decisive role not only in the formation of the SnO2 at low temperature but also in the morphology and size of SnO2 sample in hydrothermal route.
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Fig.1.
Fig.2 shows the microstructure of SnO2/Ni foam prepared at various temperatures. As shown in Fig.2A-C, there were no materials on interval of Ni foam, indicating the SnO2 grown on backbones of Ni foam.
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By a closer examination of the SnO2/Ni composite foam prepared at various temperatures as shown in the Fig.2D-F, it was found that the SnO2 film was composed of numerous SnO2 particles. Furthermore, the SnO2 film was a highly open and porous structure, which was benefit to improve electrochemical surface area of active materials and electron transportation among the active materials. At the same time, it could also be seen from the Fig.2D-F that the pore size tended to increase with increasing in reaction temperature. The pore size of SnO2/Ni composite foam prepared at 100.0℃, 120.0℃ and 140.0℃ was about 60.0nm, 200.0nm and 450.0nm, respectively. This might be ascribed to the growth of the larger particles with a higher reaction temperature, and then the larger particles tended to assemble larger pore [15]. These results indicated that the porous SnO2/Ni composite foam could be prepared by hydrothermal method without assistant of any
ACCEPTED MANUSCRIPT surfactant at low temperature of 100.0℃. The mild preparation conditions were very importance for application of SnO2/Ni composite foam in the electrodes materials of supercapacitors. The schematic diagram of the porous SnO2/Ni composite foam structure was shown in Fig.2G. The porous SnO2 film directly grown on backbones of the Ni foam showed highly open and porous structure, which provided effective accessible channels for electrolyte on transportation and shorten the distance for electrolyte diffusion. The 3D network
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revealed the existence of more channels and active surface area within the composite foam, which allowed electrolyte easily penetrating to the inner of composite and thus accelerating the interfacial reaction. These characteristics make the porous SnO2/Ni composite foam to act as good electrode of supercapacitors. Fig.2.
Fig. 3A shows the CV curves of the SnO2/Ni composite foam electrode at same scan rate of 100.0mV s-1
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within the potential range of 0~0.8V. For comparison, the measured currents were normalized to the total mass of electroactive material. The CV curves of the SnO2/Ni composite foam electrode exhibits nearly a rectangle
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shape implying the good capacitive behavior [16]. At the same time, a couple of redox peaks at 0.15~0.20V and 0.35~0.42V were clearly observed for all the electrodes, indicating a typical faradic capacitance performance [16]. In addition, the CV area of the SnO2/Ni composite foam electrode prepared at 100.0℃ was much larger than that of the SnO2/Ni composite foam electrode prepared at 120.0℃ and 140.0℃. Due to the specific capacitance of an electrode was directly proportional to the area of its CV, the results suggested that the SnO2/Ni composite foam electrode prepared at 100.0℃ has a larger specific capacitance than the SnO2/Ni
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composite foam electrode prepared at 120.0℃ and 140.0℃. The high-power characteristic of the SnO2/Ni
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composite foam electrodes could be identified from their voltammetric response at various scan rates ranging from 10.0mV s-1 to 200.0mV s-1 as shown in Fig. 3B. All curves at different scan rates exhibited a similar
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shape. At the sweep rate of 10mV s-1, a pair of redox peaks was located at ca. 0.22V and 0.32V, respectively. With the scan rate increasing from 10.0mV s-1 to 200.0mV s-1, the anodic peaks shifted towards positive potential and the cathodic peaks shifted towards negative potential. At the sweep rate of 200.0mV s-1, a pair of
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redox peaks was located at ca. 0.12V and 0.43V, respectively. Even at a high scan rate of 200.0mV s-1, the CV curve still clearly showed a pair of redox peaks, indicating that the porous SnO2/Ni composite foam was beneficial to fast charge-discharge due to the high interface area, easy ion diffusion, low resistance and super-fast electronic transport rate. Remarkably, the peak potential shifts only ca. 30.0mV for a 5-time increase in the scan rate, suggesting that the 3D hierarchical nanostructured electrode possessed low polarization, resulting from the efficient electron and ion transportation in the porous integrated electrodes of porous SnO2 film grown on the high conductive and porous nickel films [17]. Fig.3.
ACCEPTED MANUSCRIPT Fig.4A shows the charge-discharge profiles of three SnO2/Ni composite foam electrodes at 1.0 A/g. All SnO2/Ni composite foam electrodes exhibited a symmetrical and closely linear slope during charging and discharging processes, suggesting a good capacitive behavior [18]. The specific capacitances of SnO2/Ni composite foam electrodes prepared at 100.0℃, 120.0℃ and 140.0℃ were 310.4 F g-1, 295.2 F g-1 and 180.7 F g-1, respectively. Especially, the specific capacitance of SnO2/Ni composite foam electrode prepared at
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100.0℃ was the highest value compared to that of the electrode based on SnO2 reported in previous works as shown in Table 1. Here, the specific capacitance of SnO2/Ni composite foam electrode decreased with increase in the reaction temperature. The result was attributed to different pore size of the SnO2/Ni composite foam electrode prepared at various reaction temperatures. The pore size of SnO2/Ni composite foam increased with increase in reaction temperature, which was confirmed by the SEM images. It was well-known that the
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porous materials with smaller pore size show larger active surface area, corresponding to higher specific capacitance [18]. Fig.4B shows the charge-discharge properties of the SnO2/Ni composite foam electrode
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prepared at 100.0℃ at different current densities. The specific capacitance could reach as high as 310.4 F g-1, 428.0 F g-1, 372.4 F g-1, 468.3 F g-1 and 541.0F g-1 at 1.0 A g-1, 2.0 A g-1, 4.0 A g-1, 8.0 A g-1 and 10.0 A g-1, respectively. It clearly showed that when the charge/discharge current density increases from 1.0 A g-1 to 10 A g-1 (10 times), the capacitance retention of SnO2/Ni composite foam electrode could reach to 174.3%, indicating a relatively good rate capability [20]. Fig.4C shows the cycling performance of the SnO2/Ni composite foam electrode prepared at 100.0℃ at 1.0 A g-1. The specific capacitance slightly decreased with
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the cycling number and 98.1% of initial capacitance could be remained after 1000 cycles, indicating excellent
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cycling performance. The high capacitance, good rate capability and cycling stability of the SnO2/Ni composite foam might be closely related to the intrinsic characteristics of nanostructured porous SnO2 film
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grown on the backbone of Ni foam as shown in Fig.2G. Firstly, the active material SnO2 grown on backbone of the Ni foam (current collector) without polymer binder was directly used as a electrode of supercapacitor, which could build a highway for charge storage, a pathway for electrolyte diffusion and continuous conductive
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paths for the transport of electrons due to be good conductivity and network structure of Ni foam, and low contact resistance between active materials SnO2 and current collector Ni foam [17]. Secondly, the SnO2 film grown on backbone of Ni foam has a 3D network structure, which could offer a large active surface area and more active sites to be accessed by electrolyte, resulting in a high utilization of active materials [15]. Fig.4. Table 1. In order to study the kinetics and mechanism of charge transfer at the electrode materials/electrolyte surface, the electrochemical impedance spectroscopy measurement was conducted as shown in Fig.5. The impedance plots for all porous SnO2/Ni composite foam electrodes were straight line at the low and high
ACCEPTED MANUSCRIPT frequency region, indicating excellent capacitive behavior [21]. At high frequency (close to 100 kHz), the equivalent series resistance (ESR) obtained from the first intersection with the real axis is about 0.74 Ω (100.0℃), 0.81 Ω (120.0℃) and 0.92 Ω (140.0℃). The result indicates a low internal resistance for all porous SnO2/Ni composite foam electrodes. Moreover, a semicircle in the high-to-medium frequency region is almost not observed, indicating the low charge-transfer resistance (Rct) and a short path for the ion diffusion for
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porous SnO2/Ni composite foam electrodes [22]. Here, the low ESR and Rct values are attributed to the binder-free nature of the electrode and a very tight connection between the SnO2 nanosheets and Ni foam. These results further revealed that this strategy of integrating porous SnO2 film into the current collector as a whole electrode could improve the performance of SnO2 applied in supercapacitors. Although the ESR and Rct of the porous SnO2/Ni composite foam prepared at 100.0℃ was almost same, the specific capacitance was
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larger than that of porous SnO2/Ni composite foam prepared at 120.0℃ and 140.0℃. This was due to that the effect from improvement of surface area for charge storage is predominant as above discussion.
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Fig.5.
4. Conclusions
In summary, porous SnO2/Ni composite foam was prepared via a hydrothermal method at low temperature for the first time, in which the pore size of porous SnO2/Ni composite foam was easily controlled by the reaction temperature. Furthermore, the effect of pore size of porous SnO2/Ni composite foam on specific capacitance was investigated in detail. It was found that the pore size of porous SnO2/Ni composite
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foam had a great effect on the specific capacitance. The porous SnO2/Ni composite foam with smallest pore
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size exhibited the highest capacitance of 310.4 F g-1 at 1.0 A g-1, good rate capability of 541.0 F g-1 at 10.0 A g-1 and the capacitance retention of 98.1% of its initial value after 1000 cycles. Therefore, this simple hydrothermal synthetic approach may provide a convenient route for the large-scale preparation of 3D porous
storage in future.
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Acknowledgments
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SnO2 film grown directly on the current collector with low ESR, Rct and large surface area for high energy
The authors are grateful for the support by National Natural Science Foundation of China under grants (11202006 and 11202007), and the Shanxi provincial natural science foundation of China (2014021018-6). References
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carbon cloth as a binder-free electrode for supercapacitors, J. Mater. Chem. A. 3(2015)15057-15067. [22]Y.H.Jin, M.Q. Jia, Design and synthesis of nanostructured graphene-SnO2-polyanilineternary composite and their excellent supercapacitor performance, Colloids and Surfaces A: Physicochem. Eng. Aspects. 464
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ACCEPTED MANUSCRIPT Table 1. Structure and properties of SnO2 based on electrodes materials. Scheme 1. The schematic diagram of the synthesis process of SnO2/Ni composite foam. Fig.1. XRD of SnO2/Ni composite foam prepared at various temperature of (A)100.0℃, (B)120.0℃ and (C)140.0℃.
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Fig.2. SEM images of the SnO2/Ni composite foams prepared at different temperature of (A)100.0℃, (B)120.0℃ and (C)140.0℃. (D), (E) and (F) are the enlarged view of the black square dashed box marked in (A), (B) and (C), respectively. (G) is the schematic diagram of the porous SnO2/Ni composite foam structure. Fig.3. (A) CV curves of porous SnO2/Ni composite foams prepared at different temperature of (a)100.0℃,
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(b)120.0℃ and (c)140.0℃, (B) CV curves of porous SnO2/Ni composite foam at various scan rates. Fig.4. (A) galvanostatic charging-discharging curves of porous SnO2/Ni composite foams prepared at different temperature of (a)100.0℃, (b)120.0℃ and (c)140.0℃, (B) galvanostatic charging-discharging curves of
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porous SnO2/Ni composite foams at different current densities of (a) 1.0 A g-1, (b) 2.0 A g-1, (c) 4.0 A g-1, (d) 8.0 A g-1 and (e) 10.0 A g-1 and (C) cycling stability tests of porous SnO2/Ni composite foams at a current density of 1.0 A g-1.
Fig.5. Nyquist plots of the porous SnO2/Ni composite foam prepared at different temperature of (A)100.0℃,
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(B)120.0℃ and (C)140.0℃.
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Table 1.
high-rate retention/%
Cycle retention/%
(current density)
(increasing times )
(cycle times)
310.4 (1.0 A g-1)
174.3 (10.0 times)
548.0(1.0A g-1)
50.0 (3.0 times)
25.8(0.1A g )
--------
SnO2/Graphene
126.0 (1.0A g-1)
36.8 (10.0 times)
SnO2/Graphene
112.0 (0.4A g-1)
74.1(8.0 times)
396.0(4.5A g-1)
66.4 (10.0 times)
Porous SnO2/NF Hollow SnO2
-1
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Hollow SnO2@C
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Capacitance/F g-1
Sample
Flower SnO2/Graphene
98.1
Rct(Ω)
Re.
No
present
-------
------
4
--------
------
7
1.1
5
1.9
3
5.9
6
(1000.0 times)
90.0 (1000.0 times) 73.2 (2000.0 times) 95.6 (1000.0 times)
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Zn2+ OH-
+ Assemble
Growth Sn(OH)62+
Decompose
Zn(OH)42- ZnSn(OH)6 nuclei ZnSn(OH)6
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Scheme 1.
600 ★
A 300
B 150
△
★ Ni foam
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20
△△ △
40
60
80
o
2 theta/
Fig.1.
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△
(C)
.
(D)
(E)
★
△
D
C 0
△ SnO2
★
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Intensity(a.u)
450
(A)
SnO2
Backbone of Ni foam
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Sn4+ OH-
(F)
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B
C
100μm
100μm
D
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A
100μm
F
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E
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pore
pore
pore
600nm
600nm
G OH-
K+
Penetrating
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Electrolyte Transportation
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Ni foam
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s
SnO2
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Backbone e
Fig.2.
600nm
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Highlights
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>The porous 3D SnO2/Ni composite foam was successfully synthesized through a facile method.> the capacitive properties of composite foam as function of pore size of SnO2 was investigated.>porous SnO2@Ni composite foam exhibited high capacitive properties.>
S0925-8388(16)32313-1
DOI:
10.1016/j.jallcom.2016.07.278
Reference:
JALCOM 38447
To appear in:
Journal of Alloys and Compounds
Received Date: 6 May 2016 Revised Date:
19 July 2016
Accepted Date: 26 July 2016
Please cite this article as: S. Hao, Y. Sun, Y. Liu, Y. Zhang, G. Hu, Facile synthesis of porous SnO2 film grown on Ni foam applied for high-performance supercapacitors, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.07.278. 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.
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Facile synthesis of porous SnO2 film grown on Ni foam applied for high-performance supercapacitors Shiyun Hao1, Youyi Sun2*, Yaqing Liu1*, Yinghe Zhang3, Guosheng Hu1 1. School of Materials science and Engineering, North University of China, Taiyuan 030051, P.R.China. 2. Shanxi Province Key Laboratory of Functional Nanocomposites, North University of China, Taiyuan 030051, P.R.China.
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3. Nanotechnology Department, Helmholtz Association, 21502, Hamburg, Germany.
Abstract: Here, a novel three dimensional Ni foam-supported porous SnO2 film (SnO2/Ni composite foam) was successfully synthesized by the simple hydrothermal method. The porous SnO2 film had closely coated on
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the backbones of Ni foam without assistant of surfactant at mild conditions. At the same time, the pore size of SnO2/Ni composite foam was easily controlled by the reaction temperature. Furthermore, it was found that the
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capacitive properties of SnO2/Ni composite foam were related to the pore size of SnO2 film. The SnO2 film with smaller pore size showed higher capacitive properties, in which the porous SnO2/Ni composite foam with pore size of ca.60.0nm showed high rate capability of 541.0 F g-1 at 10.0 A g-1 and good cycle stability with capacitance retention of 98.1% after 1000 cycles. The high capacitive properties was due to the SnO2 active materials grown on the backbone of Ni foam and porous structure of SnO2 materials, resulting in the reduce of resistance and enhancement of active surface area. Our work not only demonstrates the controlled synthesis of
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high-quality porous SnO2 /Ni composite foam at mild conditions on a large scale, but also provides a universal
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route for the rational design of supercapacitors with high performance.
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Key words: SnO2 ; Ni foam; hydrothermal method; low temperature; supercapacitor.
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Responding author: Fax: 86-351-3559669 E-mail address:[email protected] (YY Sun)
ACCEPTED MANUSCRIPT 1. Introduction Supercapacitors as ideal energy storage devices have attracted intense interest, due to their outstanding electrochemical performance such as high power density, fast charging-discharging rate and excellent cycle stability [1]. It was well-known that the performance of supercapacitors strongly depended on the properties and structure of electrodes. So, a large number of materials have been used for supercapacitor electrodes, such as carbon-based materials, metal oxides and conducting polymers [2]. Among various candidates, SnO2 has
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been considered to be one of the most attractive materials in terms of its high theoretical specific capacitance, low cost, natural abundance and environmental friendliness [3]. Unfortunately, in most cases, the SnO2 powder was coated on the current collector with the assistance of binder (such as PVDF, PTFE) [4].The high contact resistance between SnO2 powder and current collector and the nonconductive binder would greatly decrease
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the rate capability and poor cycle stability of supercapacitors [4]. Furthermore, SnO2-based electrodes usually suffered from low electrical conductivity and large volume expansion/shrinkage during the charge-discharge
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process, which greatly precluded their practical application in supercapacitors [5-8]. To overcome these problems, the SnO2 electrode material and current collector Ni foam should be integrated together in depth rather than that electrode material was simply coated on the current collector by binder, and a porous SnO2 film should replace the traditional SnO2 powder to reduce contact resistance and volume expansion/shrinkage, enhance the active surface area and electrical conductivity of electrode material. Recently, the SnO2 film deposition on three-dimensional (3D) Ni foam for Li-ion batteries application were synthesized by
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electrostatic spray deposition (ESD) technique and hydrothermal method [9-10]. However, the high temperature and long reaction time was required in these works, which restricted its practical application.
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Therefore, it was still a challenge to the integration of SnO2 film into metal conductive network at mild conditions as binder-free electrode materials. In addition to this, there was few works reporting the porous
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SnO2 film deposition on 3D Ni foam for application in supercapacitors. In the present study, SnO2/Ni composite foam was synthesized via hydrothermal method at low
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temperature for the first time. In this case, the SnO2 film was directly grown on the backbones of the 3D Ni foam without assistant of any surfactant, which reduced the contact resistance and enhanced the transport rate of ion/electron. Furthermore, the SnO2 film possessed porous structure, which enhanced electrochemically active surface area and cycle stability. In the electrochemical measurement, the SnO2/Ni composite foam were directly tested as the electrode, which did not need any additional metal current collector, displayed remarkable high rate capability performance, excellent cycle stability and low cost. 2. Experimental 2.1 Preparation of SnO2/Ni composite foam The SnO2/Ni composite foam was synthesized by hydrothermal method at mild conditions as shown in Scheme 1. Ni foam (110 PPI, 40 mg/cm2, 0.5 mm thick, Shenzhen Po Xon Machinery Technology Co LTD)
ACCEPTED MANUSCRIPT was cleaned with a 2.0M HCl solution in an ultrasonic bath for 0.5h in order to remove the possibly existing nickel oxide layer on the outer surface, and then washed with deionized water and absolute ethanol for several times, respectively. Afterward, Ni foam was dried at 70.0℃for 6.0h. In a typical procedure, 0.8765g SnCl4·5H2O4 and 0.055g Zn(CH3COO)2·2H2O were dissolved in 40.0ml NaOH water (0.96g). Subsequently, 40ml ethanol was drop-wised，and the solution, after being magnetically stirred for 10 min in the air at room
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temperature, was transferred into Teflon-lined autoclave containing a clean Ni foam with a diameter of 3.0 cm and the autoclave was maintained at 100.0℃, 120.0℃ or 140.0℃ for 6.0 h and then cooled down to room temperature. The final SnO2/Ni composite foam was washed with distilled water and ethanol for several times to remove any salts, and then dried in a vacuum oven at 50.0℃ for 6.0 h. The mass of SnO2 on composite
Scheme 1. 2.2 Characterization
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foam was measured by change of Ni foam deposited before and after SnO2.
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X-ray diffraction (XRD) was recorded on a Rigaku Dmax-r C X-ray diffractometer using Cu Kα radiation (λ=0.154 nm) operated at 40 kV and 100 mA.
The morphology and structure of composite foam was observed by scanning electron microscopy (SEM) (Su-4700, HITACHI Japan) . 2.3. Electrochemical characterization
All electrochemical measurements were carried out in a conventional three-electrode system using an
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electrochemical workstation (CHI660D, Chenhua Instruments, China) with 6M KOH solution as the
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electrolyte. The porous SnO2/Ni composite foam was directly tested as the electrode, while saturated calomel electrode (SCE) as the reference and a platinum plate as the counter electrode. The electrochemical
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performances of the prepared electrodes were characterized by cyclic voltammetry, electrochemical impedance spectroscopy (EIS) measurements and galvanostatic charge-discharge tests. The specific capacitance was obtained from the discharge process according to the following equation, Cs (F/g) = IΔt/ΔE·m, where I is the
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current loaded (A), Δt is the discharge time (s), ΔE is the potential change during the discharge process, and m is the mass of active material (g). 3. Results and Discussion
Fig.1 shows the XRD patterns of the samples prepared at various temperatures by the hydrothermal method. It clearly showed three sharp peaks at 44.4o, 51.8o and 76.3o, corresponding to the diffractions of (111), (200) and (220) planes of Ni, respectively (JCPDS card No. 04-0850) [11]. In addition to this, for the all samples, excluding the diffraction peaks of Ni foam, others diffraction peaks at round 26.4°, 34.0°, 58.2°, 61.0° and 65.0° were assigned to (110), (101), (002), (310) and (112) planes of SnO2 (JCPDS, No. 41-1445) [12]. These results indicated the formation of SnO2/Ni composite foam. These peaks of products prepared at
ACCEPTED MANUSCRIPT 100.0℃ were broad and weak, demonstrating that the crystallinity of the SnO2 obtained from hydrothermal method at low temperature was relatively poor [12]. When the reaction temperature was increased to 120.0℃ and 140.0℃, there were obvious increase in the intensity of XRD patterns. These results indicated that the SnO2/Ni composite foam could be prepared at relatively low reaction temperature (<150.0℃). Although SnO2 grown on Ni foam has been synthesized by the hydrothermal method at low temperature, it was still difficult
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for scientists to properly explain the formation mechanism. Some formation mechanism related to diameter and morphologies of SnO2 depended on the Sn+/OH- ratio in solution has been proposed [13-14]. Here, this may be explained by both crystal growth and nucleation theory in which the synthesis was divided into two
the chemical reaction given below:
Sn 4 6OH SnOH 6
2
Zn 2 4OH ZnOH 4
2
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steps: nucleation stage and crystal growth. In the nucleation stage, ZnSn(OH)6 called nucleus was created by
SnOH 6 ZnOH 4 ZnSnOH 6 2
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ZnSnOH 6 SnO2 Zn 2 OH H 2O Zn ion in this process did not act as a reactant and might be understood as a catalyst, which decreased the decomposed temperature of nuclei. The nucleation might go on until the first stage of hydrothermal process (T<150.0℃) in which nuclei start growing in size by collision with other nuclei. Hence, the SnO2
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grown on Ni foam could be synthesized by the hydrothermal method at lower temperature comparing to
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previous works[5-8]. In our opinion, the Zn ion played a decisive role not only in the formation of the SnO2 at low temperature but also in the morphology and size of SnO2 sample in hydrothermal route.
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Fig.1.
Fig.2 shows the microstructure of SnO2/Ni foam prepared at various temperatures. As shown in Fig.2A-C, there were no materials on interval of Ni foam, indicating the SnO2 grown on backbones of Ni foam.
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By a closer examination of the SnO2/Ni composite foam prepared at various temperatures as shown in the Fig.2D-F, it was found that the SnO2 film was composed of numerous SnO2 particles. Furthermore, the SnO2 film was a highly open and porous structure, which was benefit to improve electrochemical surface area of active materials and electron transportation among the active materials. At the same time, it could also be seen from the Fig.2D-F that the pore size tended to increase with increasing in reaction temperature. The pore size of SnO2/Ni composite foam prepared at 100.0℃, 120.0℃ and 140.0℃ was about 60.0nm, 200.0nm and 450.0nm, respectively. This might be ascribed to the growth of the larger particles with a higher reaction temperature, and then the larger particles tended to assemble larger pore [15]. These results indicated that the porous SnO2/Ni composite foam could be prepared by hydrothermal method without assistant of any
ACCEPTED MANUSCRIPT surfactant at low temperature of 100.0℃. The mild preparation conditions were very importance for application of SnO2/Ni composite foam in the electrodes materials of supercapacitors. The schematic diagram of the porous SnO2/Ni composite foam structure was shown in Fig.2G. The porous SnO2 film directly grown on backbones of the Ni foam showed highly open and porous structure, which provided effective accessible channels for electrolyte on transportation and shorten the distance for electrolyte diffusion. The 3D network
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revealed the existence of more channels and active surface area within the composite foam, which allowed electrolyte easily penetrating to the inner of composite and thus accelerating the interfacial reaction. These characteristics make the porous SnO2/Ni composite foam to act as good electrode of supercapacitors. Fig.2.
Fig. 3A shows the CV curves of the SnO2/Ni composite foam electrode at same scan rate of 100.0mV s-1
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within the potential range of 0~0.8V. For comparison, the measured currents were normalized to the total mass of electroactive material. The CV curves of the SnO2/Ni composite foam electrode exhibits nearly a rectangle
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shape implying the good capacitive behavior [16]. At the same time, a couple of redox peaks at 0.15~0.20V and 0.35~0.42V were clearly observed for all the electrodes, indicating a typical faradic capacitance performance [16]. In addition, the CV area of the SnO2/Ni composite foam electrode prepared at 100.0℃ was much larger than that of the SnO2/Ni composite foam electrode prepared at 120.0℃ and 140.0℃. Due to the specific capacitance of an electrode was directly proportional to the area of its CV, the results suggested that the SnO2/Ni composite foam electrode prepared at 100.0℃ has a larger specific capacitance than the SnO2/Ni
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composite foam electrode prepared at 120.0℃ and 140.0℃. The high-power characteristic of the SnO2/Ni
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composite foam electrodes could be identified from their voltammetric response at various scan rates ranging from 10.0mV s-1 to 200.0mV s-1 as shown in Fig. 3B. All curves at different scan rates exhibited a similar
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shape. At the sweep rate of 10mV s-1, a pair of redox peaks was located at ca. 0.22V and 0.32V, respectively. With the scan rate increasing from 10.0mV s-1 to 200.0mV s-1, the anodic peaks shifted towards positive potential and the cathodic peaks shifted towards negative potential. At the sweep rate of 200.0mV s-1, a pair of
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redox peaks was located at ca. 0.12V and 0.43V, respectively. Even at a high scan rate of 200.0mV s-1, the CV curve still clearly showed a pair of redox peaks, indicating that the porous SnO2/Ni composite foam was beneficial to fast charge-discharge due to the high interface area, easy ion diffusion, low resistance and super-fast electronic transport rate. Remarkably, the peak potential shifts only ca. 30.0mV for a 5-time increase in the scan rate, suggesting that the 3D hierarchical nanostructured electrode possessed low polarization, resulting from the efficient electron and ion transportation in the porous integrated electrodes of porous SnO2 film grown on the high conductive and porous nickel films [17]. Fig.3.
ACCEPTED MANUSCRIPT Fig.4A shows the charge-discharge profiles of three SnO2/Ni composite foam electrodes at 1.0 A/g. All SnO2/Ni composite foam electrodes exhibited a symmetrical and closely linear slope during charging and discharging processes, suggesting a good capacitive behavior [18]. The specific capacitances of SnO2/Ni composite foam electrodes prepared at 100.0℃, 120.0℃ and 140.0℃ were 310.4 F g-1, 295.2 F g-1 and 180.7 F g-1, respectively. Especially, the specific capacitance of SnO2/Ni composite foam electrode prepared at
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100.0℃ was the highest value compared to that of the electrode based on SnO2 reported in previous works as shown in Table 1. Here, the specific capacitance of SnO2/Ni composite foam electrode decreased with increase in the reaction temperature. The result was attributed to different pore size of the SnO2/Ni composite foam electrode prepared at various reaction temperatures. The pore size of SnO2/Ni composite foam increased with increase in reaction temperature, which was confirmed by the SEM images. It was well-known that the
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porous materials with smaller pore size show larger active surface area, corresponding to higher specific capacitance [18]. Fig.4B shows the charge-discharge properties of the SnO2/Ni composite foam electrode
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prepared at 100.0℃ at different current densities. The specific capacitance could reach as high as 310.4 F g-1, 428.0 F g-1, 372.4 F g-1, 468.3 F g-1 and 541.0F g-1 at 1.0 A g-1, 2.0 A g-1, 4.0 A g-1, 8.0 A g-1 and 10.0 A g-1, respectively. It clearly showed that when the charge/discharge current density increases from 1.0 A g-1 to 10 A g-1 (10 times), the capacitance retention of SnO2/Ni composite foam electrode could reach to 174.3%, indicating a relatively good rate capability [20]. Fig.4C shows the cycling performance of the SnO2/Ni composite foam electrode prepared at 100.0℃ at 1.0 A g-1. The specific capacitance slightly decreased with
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the cycling number and 98.1% of initial capacitance could be remained after 1000 cycles, indicating excellent
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cycling performance. The high capacitance, good rate capability and cycling stability of the SnO2/Ni composite foam might be closely related to the intrinsic characteristics of nanostructured porous SnO2 film
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grown on the backbone of Ni foam as shown in Fig.2G. Firstly, the active material SnO2 grown on backbone of the Ni foam (current collector) without polymer binder was directly used as a electrode of supercapacitor, which could build a highway for charge storage, a pathway for electrolyte diffusion and continuous conductive
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paths for the transport of electrons due to be good conductivity and network structure of Ni foam, and low contact resistance between active materials SnO2 and current collector Ni foam [17]. Secondly, the SnO2 film grown on backbone of Ni foam has a 3D network structure, which could offer a large active surface area and more active sites to be accessed by electrolyte, resulting in a high utilization of active materials [15]. Fig.4. Table 1. In order to study the kinetics and mechanism of charge transfer at the electrode materials/electrolyte surface, the electrochemical impedance spectroscopy measurement was conducted as shown in Fig.5. The impedance plots for all porous SnO2/Ni composite foam electrodes were straight line at the low and high
ACCEPTED MANUSCRIPT frequency region, indicating excellent capacitive behavior [21]. At high frequency (close to 100 kHz), the equivalent series resistance (ESR) obtained from the first intersection with the real axis is about 0.74 Ω (100.0℃), 0.81 Ω (120.0℃) and 0.92 Ω (140.0℃). The result indicates a low internal resistance for all porous SnO2/Ni composite foam electrodes. Moreover, a semicircle in the high-to-medium frequency region is almost not observed, indicating the low charge-transfer resistance (Rct) and a short path for the ion diffusion for
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porous SnO2/Ni composite foam electrodes [22]. Here, the low ESR and Rct values are attributed to the binder-free nature of the electrode and a very tight connection between the SnO2 nanosheets and Ni foam. These results further revealed that this strategy of integrating porous SnO2 film into the current collector as a whole electrode could improve the performance of SnO2 applied in supercapacitors. Although the ESR and Rct of the porous SnO2/Ni composite foam prepared at 100.0℃ was almost same, the specific capacitance was
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larger than that of porous SnO2/Ni composite foam prepared at 120.0℃ and 140.0℃. This was due to that the effect from improvement of surface area for charge storage is predominant as above discussion.
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Fig.5.
4. Conclusions
In summary, porous SnO2/Ni composite foam was prepared via a hydrothermal method at low temperature for the first time, in which the pore size of porous SnO2/Ni composite foam was easily controlled by the reaction temperature. Furthermore, the effect of pore size of porous SnO2/Ni composite foam on specific capacitance was investigated in detail. It was found that the pore size of porous SnO2/Ni composite
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foam had a great effect on the specific capacitance. The porous SnO2/Ni composite foam with smallest pore
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size exhibited the highest capacitance of 310.4 F g-1 at 1.0 A g-1, good rate capability of 541.0 F g-1 at 10.0 A g-1 and the capacitance retention of 98.1% of its initial value after 1000 cycles. Therefore, this simple hydrothermal synthetic approach may provide a convenient route for the large-scale preparation of 3D porous
storage in future.
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Acknowledgments
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SnO2 film grown directly on the current collector with low ESR, Rct and large surface area for high energy
The authors are grateful for the support by National Natural Science Foundation of China under grants (11202006 and 11202007), and the Shanxi provincial natural science foundation of China (2014021018-6). References
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carbon cloth as a binder-free electrode for supercapacitors, J. Mater. Chem. A. 3(2015)15057-15067. [22]Y.H.Jin, M.Q. Jia, Design and synthesis of nanostructured graphene-SnO2-polyanilineternary composite and their excellent supercapacitor performance, Colloids and Surfaces A: Physicochem. Eng. Aspects. 464
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ACCEPTED MANUSCRIPT Table 1. Structure and properties of SnO2 based on electrodes materials. Scheme 1. The schematic diagram of the synthesis process of SnO2/Ni composite foam. Fig.1. XRD of SnO2/Ni composite foam prepared at various temperature of (A)100.0℃, (B)120.0℃ and (C)140.0℃.
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Fig.2. SEM images of the SnO2/Ni composite foams prepared at different temperature of (A)100.0℃, (B)120.0℃ and (C)140.0℃. (D), (E) and (F) are the enlarged view of the black square dashed box marked in (A), (B) and (C), respectively. (G) is the schematic diagram of the porous SnO2/Ni composite foam structure. Fig.3. (A) CV curves of porous SnO2/Ni composite foams prepared at different temperature of (a)100.0℃,
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(b)120.0℃ and (c)140.0℃, (B) CV curves of porous SnO2/Ni composite foam at various scan rates. Fig.4. (A) galvanostatic charging-discharging curves of porous SnO2/Ni composite foams prepared at different temperature of (a)100.0℃, (b)120.0℃ and (c)140.0℃, (B) galvanostatic charging-discharging curves of
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porous SnO2/Ni composite foams at different current densities of (a) 1.0 A g-1, (b) 2.0 A g-1, (c) 4.0 A g-1, (d) 8.0 A g-1 and (e) 10.0 A g-1 and (C) cycling stability tests of porous SnO2/Ni composite foams at a current density of 1.0 A g-1.
Fig.5. Nyquist plots of the porous SnO2/Ni composite foam prepared at different temperature of (A)100.0℃,
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(B)120.0℃ and (C)140.0℃.
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Table 1.
high-rate retention/%
Cycle retention/%
(current density)
(increasing times )
(cycle times)
310.4 (1.0 A g-1)
174.3 (10.0 times)
548.0(1.0A g-1)
50.0 (3.0 times)
25.8(0.1A g )
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SnO2/Graphene
126.0 (1.0A g-1)
36.8 (10.0 times)
SnO2/Graphene
112.0 (0.4A g-1)
74.1(8.0 times)
396.0(4.5A g-1)
66.4 (10.0 times)
Porous SnO2/NF Hollow SnO2
-1
AC C
Hollow SnO2@C
EP
Capacitance/F g-1
Sample
Flower SnO2/Graphene
98.1
Rct(Ω)
Re.
No
present
-------
------
4
--------
------
7
1.1
5
1.9
3
5.9
6
(1000.0 times)
90.0 (1000.0 times) 73.2 (2000.0 times) 95.6 (1000.0 times)
ACCEPTED MANUSCRIPT
Zn2+ OH-
+ Assemble
Growth Sn(OH)62+
Decompose
Zn(OH)42- ZnSn(OH)6 nuclei ZnSn(OH)6
SC
Scheme 1.
600 ★
A 300
B 150
△
★ Ni foam
TE
20
△△ △
40
60
80
o
2 theta/
Fig.1.
AC C
EP
△
(C)
.
(D)
(E)
★
△
D
C 0
△ SnO2
★
M AN U
Intensity(a.u)
450
(A)
SnO2
Backbone of Ni foam
RI PT
Sn4+ OH-
(F)
ACCEPTED MANUSCRIPT
B
C
100μm
100μm
D
RI PT
A
100μm
F
SC
E
M AN U
pore
pore
pore
600nm
600nm
G OH-
K+
Penetrating
D
Electrolyte Transportation
AC C
Ni foam
EP
s
SnO2
TE
Backbone e
Fig.2.
600nm
ACCEPTED MANUSCRIPT
0.15
0.15 -1
10mV s -1 50mV s -1 100mV s -1 150mV s -1 200mV s
0.10
Current/A
a
0.05 c
0.00
0.05
0.00
b
-0.05
-0.05 0.0
0.2
0.4
0.6
-0.10
0.8
0.0
a
0.07V
0.6
Potential/V
Potential/V
0.8
A
0.03V
0.06V
TE
D
0.3
0
100
200
300
400
240
AC C
Potential/V
0.8
160
80
0.6
0.4
0.2
0
1000
2000
3000
4000
5000
Time (s)
0 200
400
600
800
b
0.8
B
a
0.6
0.4
0.2 100
200
Time/s
EP
C
e d c
0
Time/s
320
0.6
SC
M AN U
0.9 b
0.4
Potential/V
Fig.3.
c
B
0.2
Potential/V
Specific capacitance (F/g)
Current/A
0.10
RI PT
A
1000
Cycle number
Fig.4.
300
400
ACCEPTED MANUSCRIPT 15
B C
-Z''/Ohm
10
0
0
5
10
Z'/Ohm
AC C
EP
TE
D
M AN U
Fig.5.
15
SC
5
RI PT
A
ACCEPTED MANUSCRIPT
Highlights
AC C
EP
TE
D
M AN U
SC
RI PT
>The porous 3D SnO2/Ni composite foam was successfully synthesized through a facile method.> the capacitive properties of composite foam as function of pore size of SnO2 was investigated.>porous SnO2@Ni composite foam exhibited high capacitive properties.>