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SnO2–RuO2 composite films by chemical deposition for supercapacitor application
Materials Chemistry and Physics xxx (2013) 1e7
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SnO2eRuO2 composite films by chemical deposition for supercapacitor application S.N. Pusawale a, P.R. Deshmukh a, J.L. Gunjakar b, C.D. Lokhande a, * a b
Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India Ewha Woman’s University, 11-1, Seoul 120-750, South Korea
h i g h l i g h t s < Chemical bath deposition of SnO2eRuO2 composite films. < Supercapacitor application. < Maximum specific capacitance of 150 F g1. < Maximum energy density of 4.9 Wh kg1 at a power density of 0.42 kW kg1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2011 Received in revised form 12 June 2012 Accepted 22 December 2012
A chemical bath deposition method is used for preparing SnO2eRuO2 composite films. The films with varying RuO2 composition are prepared by varying the vol% of RuCl3 in the bath and their structural, morphological and supercapacitive properties are studied. The structural study revealed that the composite films are amorphous. FT-Raman spectrum showed the fundamental vibrations of SnO2 and RuO2, confirming the formation of SnO2eRuO2 composite. The elongated particle structures are observed from morphological studies. The supercapacitive studies showed highest specific capacitance of 150 F g1 for SnO2eRuO2 composite containing 15 vol% of RuCl3 in the deposition bath. The chargeedischarge curves showed the pseudocapacitive performance with maximum energy density of 4.9 Wh kg1 at a power density of 0.42 kW kg1 for optimized SnO2eRuO2 composite. An equivalent series resistance (ESR) of 12 U is observed from the electrochemical impedance studies. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Thin films Composite materials Chemical synthesis Electrochemical properties
1. Introduction In recent years there is a growing demand of supercapacitors in various applications such as, hybrid electric vehicles, telecommunications, particularly associated with cellular phones for a reduction of the size of the batteries [1], memory protection of computer electronics [2,3] etc. The charge storage mechanism in supercapacitors divide it into two types; electrochemical double layer capacitors (EDLC) and pseudocapacitors. An electrochemical double-layer capacitor (EDLC) stores electrical charge in an electrical double layer at the electrodeeelectrolyte interface. High surface area electrode materials, such as activated carbons maximize this interface resulting in larger capacitance [4]. In addition, the higher capacitance can be obtained by using redox active materials such as metal oxides and conducting polymers. The charge
* Corresponding author. Tel.: þ91 231 2609225; fax: þ91 231 26092333. E-mail address: [email protected] (C.D. Lokhande).
storage in these materials is due to the highly reversible surface redox reactions commonly referred as pseudocapacitance [5]. The research on metal oxides electrodes for supercapacitors has attracted much attention in past few years. Lokhande and Joo discussed in detail the supercapacitive performance of various metal oxide electrodes [6]. Among the various metal oxides, RuO2 has been proved one of the most promising electrode material for supercapacitor application, which possess high-energy storage capabilities with large specific capacitance and good reversibility [5]. However, the lack of abundance and high cost of this oxide inhibit its commercial applications. To overcome this, another approach is to prepare composites in which RuO2 is combined with other metal oxide materials such as IrO2 [7], NiO [8,9], Co3O4 [10], SnO2 [11e13] and TiO2 [14,15]. The idea of composite is due to the intention of combining the high specific capacitance of RuO2 with a good dispersion of this oxide in order to get electrode material of high capacitance at low moderate price. In the search of second electrode material for combining with RuO2, SnO2 is found to be one of the suitable
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
Please cite this article in press as: S.N. Pusawale, et al., SnO2eRuO2 composite films by chemical deposition for supercapacitor application, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
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S.N. Pusawale et al. / Materials Chemistry and Physics xxx (2013) 1e7
materials due its low cost and environmental friendly nature. Moreover, SnO2 has same rutile structure as RuO2 with the lattice parameters of both the oxides quite similar to each other [16]. SnO2eRuO2 composite coated titanium electrodes were widely used as dimensionally stable anodes (DSA) for chlorine evolution [17e19]. Introducing SnO2 into RuO2 was shown to increase the electrochemical active sites and stability of RuO2 in developing dimensionally stable anodes [20e22]. In previous studies Hu et al. [11] used modified solegel process for deposition of Ru1dSndO2 with a maximum specific capacitance of 690 F g1 for Sn content d ¼ 0.2. Kim et al. used a DC reactive sputtering method for the preparation of composite RuO2eSnO2 electrode and a maximum specific capacitance of 88.8 F g1 was observed [23]. Wang and Hu adopted a mild hydrothermal process to synthesize hydrous ruthenium oxide tin oxide composites ((RueSn)O2$nH2O) where a maximum specific capacitance of 566 F g1 was observed [24]. A composite SnO2eRuO2 supercapacitor electrode synthesized by cyclic voltammetry plating of RuO2 onto a porous and highly conductive SnO2 showed a specific capacitance of 15 F g1 [12]. Both Chemical bath deposition and electrochemical deposition methods are low temperature chemical methods they are simple and inexpensive. However, the growth mechanism in both the methods is rather different. In electrochemical deposition, the thickness and morphology of the films can be easily controlled by electrochemical parameters such as electrode potential and current density. While in case of chemical bath deposition method it refers to the deposition of films on a solid substrate from a reaction occurring in a solution. Since the growth mechanism in both the methods is different it would result in films with different structural and morphological features that will affect its electrochemical properties. In comparing with other deposition methods, chemical bath deposition (CBD) method is simple, inexpensive and reproducible one in which direct deposition of metal halides, chalogenides or oxides takes place on any type of substrate surface [25]. The final product in this method is a thin film coated on the substrate surface. This feature makes it promising method to fabricate electrodes for supercapacitor, as there are no extra preparation steps, which are commonly required for fabricating the supercapacitor electrode in the methods like solegel and hydrothermal with a final product in powder form. The CBD method is based on the slow controlled precipitation of the desired compound from its ions in the reaction bath on the substrate surface [25]. In the present paper, for the first time, we report on the synthesis of SnO2eRuO2 composite films by CBD method for supercapacitor application. 2. Experimental 2.1. Deposition of SnO2eRuO2 composite film The deposition of SnO2eRuO2 composite film by CBD method was carried out by heating the mixed solution of 0.05 M stannic (Sn4þ) chloride and 0.01 M ruthenium (Ru3þ) chloride. The pH of the solution was adjusted in the range of 1e2 using urea (NH2CONH2) solution with constant stirring, the stainless steel substrates were immersed vertically in the above bath and the bath was heated at 353 K. After 10 min, the precipitation was started in the bath and during the precipitation, heterogeneous reaction occurred and deposition of SnO2eRuO2 films took place on the substrate. The substrates coated with SnO2eRuO2 thin films were removed after 2 h, and washed with double distilled water and dried. The films prepared were uniform, blackish in color and well adherent to the substrate. In order to vary the amount of RuO2 in the deposition, the volume percentage of RuCl3 in the deposition
bath was varied as 5%, 10%, 15% and 20% and the corresponding composite films were named as C1, C2, C3 and C4 respectively, and used in further studies. 2.2. Characterization techniques The thickness of the film was measured by using computerized AMBIOS make XP-1 surface profiler with 1 A vertical resolution. To study the structural property of composite thin films, X-ray diffraction patterns were obtained using a Philips (PW-3710) diffractometer with a Cr Ka (l ¼ 2.2870 A) target. The FT-Raman spectrograms were obtained using Bruker Raman spectrophotometer. The microstructure and EDS mapping were observed using field emission scanning electron microscopy (FE-SEM) (JSM 7100F). The electrochemical study was performed in a three-electrode configuration cell consisting of SnO2eRuO2 composite film as a working electrode, platinum as a counter electrode and saturated calomel electrode (SCE) as a reference electrode. The chargee discharge analysis was performed by WonATech Automatic Battery Cycler WBCS system, interfaced to a computer. The electrochemical impedance measurement was conducted with a versastat 3G frequency response analyzer (FRA) under Zplot program (Scribner Associates Inc.). 3. Results and discussion 3.1. Reaction mechanism of SnO2eRuO2 composite film formation The chemical reaction of SnO2eRuO2 composite film formation can be enlightened as follows. The primary role of urea is to provide a slow controlled supply of OH ions through slow thermal decomposition [26].
COðNH2 Þ2 þ H2 O/CO2 þ 2NH3
(1)
In aqueous solution, ammonia slowly releases the OH ions as,
NH3 þ H2 O5NH4þ þ OH
(2)
4þ
The Sn ions in the solution reacts with OH ions to form stannic hydroxide (Sn(OH)4) precipitate which then hydrolyzed to form SnO2 through the reaction,
Sn4þ þ 4OH /SnðOHÞ4 /SnO2 þ 2H2 O 3þ
(3)
Similarly the Ru ions reacts with OH ions in the solution to form ruthenium hydroxide (Ru(OH)3) precipitate which then react with oxygen to form RuO2 as,
Ru3þ þ 3OH /RuðOHÞ3
(4)
1 D 2RuðOHÞ3 þ O2 / 2RuO2 þ 3H2 O 2
(5)
This describes the SnO2eRuO2 film formation mechanism by CBD. The films with varying vol% of RuCl3 in the bath were deposited. Fig. 1 shows the variation of thickness of SnO2eRuO2 composite film for different vol% of RuCl3. From the graph, it is observed that the thickness increases with increase in the volume of RuCl3 in the bath. The maximum thickness of 0.51 mm is observed for sample C4 (20 vol% of RuCl3). 3.2. Structural analysis The X-ray diffraction patterns of SnO2eRuO2 composite films for varying vol% of RuCl3 are presented in Fig. 2. The X-ray diffraction patterns show only peaks corresponding to stainless steel
Please cite this article in press as: S.N. Pusawale, et al., SnO2eRuO2 composite films by chemical deposition for supercapacitor application, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
5
10
15
20
Volume of RuCl3 in bath (%)
200
3.3. FT-Raman studies The typical Raman spectra of chemical bath deposited SnO2, RuO2 and SnO2eRuO2 composite electrode are shown in Fig. 3. In Fig. 3(a), the Raman spectrum of SnO2 showed two peaks centered
Δ -stainless steel
Δ
Intensity (counts)
Δ
Δ
Δ
d)
Δ
c)
Δ
b)
Δ Δ
0
20
40
60
a)
80
100
2 θ (degree) Fig. 2. The X-ray diffraction patterns of SnO2eRuO2 composite films for sample a) C1 (5 vol%), b) C2 (10 vol%), c) C3 (15 vol%) and d) C4 (20 vol%).
794 cm ,N SnO 685 cm ,B
1g
Fig. 1. Variation of thickness of SnO2eRuO2 composite film with variation of volume (5, 10, 15 and 20 vol%) of RuCl3.
substrate. Thus, the XRD studies showed the possibility of formation of amorphous SnO2eRuO2 composite film. The amorphous phase for SnO2eRuO2 composite was also observed by Hu et al. [11], Yanqun, and Dian [27]. The amorphous phase is suitable for electrochemical applications, since the protons can easily permeate through the bulk of the amorphous electrode and whole amount of electrode is utilized [28].
622 cm ,A
-1
b)
400
763 cm , B
0.325
c)
589 cm ,SnO
0.390
436 cm ,N group
0.455
320 cm ,N group
0.520
668 cm ,B ,RuO
0.585
564 cm ,SnO
295 cm ,N SnO
0.650
Relative intensity (A.U.)
Thickness (μm)
0.715
3
377 cm ,N SnO
S.N. Pusawale et al. / Materials Chemistry and Physics xxx (2013) 1e7
600
800
a)
1000
-1
Wavenumber (cm ) Fig. 3. FT-Raman spectra of a) SnO2, b) RuO2 and c) SnO2eRuO2 composite film.
at 763 cm1 corresponding to the crystallites of SnO2 with a tetragonal rutile structure for B2g vibration [29]. The Raman peak at 589 cm1 corresponds to amorphous SnO2. The other Raman peaks observed at 320 and 436 cm1 correspond to the N group vibration modes of SnO2 nanocrystallites [29]. In Fig. 3(b) the Raman peak at 622 cm1 correspond to the A1g whereas the peak at 685 cm1 is correspond to the B2g vibration mode of RuO2 respectively [30,31]. Fig. 3(c) shows the Raman spectrum for SnO2eRuO2 composite (sample C3). The peaks at 668 cm1 correspond to crystalline RuO2 (i.e. B2g) in the rutile form, whereas as the Raman peak at 564 cm1 is due to the amorphous SnO2. In comparison with Raman spectrum of pure SnO2 and RuO2 the shift in peak positions of SnO2 and RuO2 is observed in Fig. 3(c) is may be due to the quantum effects of decrease in particle size [32]. However, the certain new peaks observed in Fig. 3(c) are may be due to the formation of SnO2eRuO2 composite. 3.4. Surface morphological studies The SEM images of pure SnO2 and SnO2eRuO2 composite samples are shown in Fig. 4. The SEM image for SnO2 is shown in Fig. 4(a), which revealed the smooth and compact morphology. It is observed that the smooth and compact morphology of pure SnO2 changed to rough and interconnected network like with overgrowth structure for sample C1 (Fig. 4(b)). With further increment of RuCl3 in the deposition, the network structure becomes relatively compact with small pits and cracks on the surface for sample C2 (Fig. 4(c)). The small pits observed becomes larger and irregularly shaped with further addition of RuCl3 for sample C3 (Fig. 4(d)), the size of the pits however decreased with more RuCl3 in the deposition and surface becomes rough and elongated structure like for sample C4 (Fig. 4(e)). The SEM studies clearly shows the transition of smooth and compact morphology of pure SnO2 to the rough surface with irregular pits and elongated structure after incorporation of RuO2 in the deposition. This may be due to different growth kinetics associated with the variation in the composition of bath for deposition. Fig. 5 shows the corresponding EDS mapping images of sample C3 that shows the uniform distributions of Sn, Ru and O in the composite. The observed Sn, Ru and O content in the composite films are given in Table 1.
Please cite this article in press as: S.N. Pusawale, et al., SnO2eRuO2 composite films by chemical deposition for supercapacitor application, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
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Fig. 4. The SEM images of (a) pure SnO2 and SnO2eRuO2 composite films for samples: (b) C1 (5 vol%), (c) C2 (10 vol%), (d) C3 (15 vol%) and (e) C4 (20 vol%) at 3000 magnification.
3.5. Supercapacitive studies The cyclic voltammetry (CV) curves for SnO2 and SnO2eRuO2 composite electrodes in the potential region between 0 and þ0.8 V vs. SCE at 20 mV s1 were employed to evaluate the electrochemical characteristics of the supercapacitors based on SnO2eRuO2 composites. The CV curve for SnO2 is shown in Fig. 6(a), which revealed the double layer capacitance mechanism as no redox transitions are observed in this potential region. The observed current is less compared with composite electrodes and a double layer capacitance of w4 F g1 is observed for SnO2. The CV curves for composite electrodes are shown in Fig. 6(b)e(d) which shows a different type of CV nature compared with pure SnO2 with the presence of redox transitions clearly confirming the pseudocapacitive mechanism. As the capacitance originating from SnO2 is very small, the pseudocapacitance coming from SnO2eRuO2 composite electrodes is mainly attributed to the RuO2. From the curve, it is observed that the current under the curve increases with increase in volume of RuCl3 in the deposited bath. This may be due to the increase in volume of RuCl3 in the bath it gives more number of Ru3þ ions, which increases the content of RuO2 in the deposition. The increase in the RuO2 content increases the current of CV curves. Therefore, specific capacitance of the composite electrode increases from 42 (sample C1) to 150 F g1 (sample C3) as the vol% of RuCl3 in
the bath increases from 5 to 15 respectively. For further increment in vol% of RuCl3 in deposition (20 vol%), there observed a decrease in the voltammetric current that means 15 vol% is the maximum limit of volume of RuCl3 in the deposition bath which gives the maximum current response. The less CV response above this composition is due to the decrease in the RuO2 content in composite film decreasing specific capacitance to 121 F g1 this can be attributed to the decrease in electroactive sites for 20 vol% of RuCl3 in the deposition bath [24]. The observed value of specific capacitance in this work is less than earlier reported values [10,11,13,21,33]. The work described in Ref. [11] is about Sb doped SnO2 xerogel impregnated with RuO2 prepared by incipent wetness method. In this case, Sb was used as dopant to increase the conductivity of SnO2, as it is well known that pure SnO2 is bad conductor, with doping its conductivity increases that results in lowering the electrode resistance and improve the current performance. The observed capacitance of the net composite electrode is of 15 F g1 (The capacitance of 710 F g1 reported in the work is for RuO2 only not for composite electrode). In our present work we don’t use any such dopant for SnO2 also the observed capacitance is greater. The specific capacitance of our studies is less than other references mentioned. The methods used in these references are solegel and hydrothermal method. On the other hand, in sol gel and hydrothermal method the final product is
Please cite this article in press as: S.N. Pusawale, et al., SnO2eRuO2 composite films by chemical deposition for supercapacitor application, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
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Table 1 The observed Sn, Ru and O content in at.% for samples C1eC4. Sample
Sn (at.%)
Ru (at.%)
O (at.%)
C1 C2 C3 C4
23.19 18.41 12.32 13.43
6.59 12.44 20.67 16.44
70.22 69.15 67.01 70.13
(5 vol%) (10 vol%) (15 vol%) (20 vol%)
morphological features which are also responsible for the good capacitive behavior. The substrate used in the sol gel study is graphite so there may be chances of contributing the double layer capacitance of graphite to the total capacitance. However, in our study we have used inexpensive stainless steel substrates which don’t have capacitive behavior. 3.6. Chargeedischarge studies The chargeedischarge behaviors of the samples (C1, C2, C3 and C4) is examined by chronopotentiometry from 0 to þ0.8 V vs. SCE at the current density of 0.5 mA cm2 in 0.5 M H2SO4 electrolyte is shown in Fig. 7. A nonlinear variation of potential vs. time is displayed, which may be a pseudocapacitance performance arisen from the electrochemical adsorptionedesorption or redox reaction at an interface between the electrode and the electrolyte [34]. All the samples exhibit coulombic efficiency greater than 90%. The specific power and specific energy was calculated for all the samples using the formula,
Specific power ðSPÞ ¼ IV=2m
(6)
And Specific Energy ðSEÞ ¼ IVtd =2m
(7)
where, ‘I’ is the discharging current in Amperes, ‘V’ is the applied potential range for charging and discharging, ‘m’ is the mass of electrode in kg and ‘td’ is the discharging time in h. Fig. 8 shows Ragone plot relating power density to achievable energy density. From the plot, it is observed that the values of energy density and power density of composite samples are according to Ragone plot typically calculated for supercapacitors [35]. From the plot, it is observed that the sample C3 has maximum energy density of 4.9 Wh kg1 at still high power density of 0.42 kW kg1.
c) d)
2
Current density (mA cm )
2
Fig. 5. EDS mapping of SnO2eRuO2 composite film for sample C3 (15 vol%).
in powder form which then undergoes through various preparation steps to make final electrode. However, in our study we have used a simple chemical bath deposition method, which results in the direct deposition of composite films on the substrate surface. Also as the methods are different it results into different structural and
1
b) a)
0
-1
-2 0.0
0.2
0.4
0.6
0.8
Voltage (V vs. SCE) Fig. 6. The cyclic voltammograms (CV) of SnO2eRuO2 composite films for samples: a) C1 (5 vol%), b) C2 (10 vol%), c) C3 (15 vol%) and d) C4 (20 vol%) in the potential window of 0 to þ0.8 V vs. SCE at the 20 mV s1scan rate in 0.5 M H2SO4 as an electrolyte.
Please cite this article in press as: S.N. Pusawale, et al., SnO2eRuO2 composite films by chemical deposition for supercapacitor application, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
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S.N. Pusawale et al. / Materials Chemistry and Physics xxx (2013) 1e7
120
0.6 0.4
80 Z'' (ohm)
Z''(ohm)
Voltage (V vs. SCE)
0.8
40
0.2 0.0
a) b) 0
20
40
60
c)
d) 80
Z' (ohm)
0
100
0
Time (s)
3.7. Electrochemical impedance studies In order to investigate the electrochemical characteristics of the electrode and electrolyte in quantitative manner impedance measurement was performed. The electrochemical impedance measurement (at open circuit voltage, in the frequency range 105e 102 Hz) of SnO2eRuO2 composite sample C3 (15 vol% RuCl3) was carried out in 0.5 M H2SO4. The Nyquist depictions (Z0 vs. Z00 ) of the raw impedance data are shown in Fig. 9. For the convenience of the interpretation, this plot can be divided in to high and low frequency regions. In the high frequency range, the intercept at real part (Z0 ) is a combinational resistance of ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance between the active material and the current collector [36]. The presence of semicircle in the high frequency region suggests that there is a charge transfer resistance while the line in the low frequency region angled at w45 to the real axis is the characteristic of ion diffusion into the porous structure of the electrode [37]. Inset (Fig. 9) shows the expanded high frequency region of the same plot.
120
Fig. 9. Nyquist plot of SnO2eRuO2 composite (sample C3, 15 vol%) in 0.5 M H2SO4.
The 12 U, high frequency impedance intercept reflects the equivalent series resistance (ESR) of the electrolyte. The ESR observed in the present study is higher and which is probably the reason to get the specific capacitance value of only 150 F g1. The further experiments on lowering the ESR and maximizing its specific capacitance value are going on. 4. Conclusions The amorphous SnO2eRuO2 composite films are prepared by varying volume of RuCl3 in the deposition bath using CBD method. The FT-Raman spectrum study showed the characteristic vibrations modes of SnO2 and RuO2 in the composite sample, confirming the formation of SnO2eRuO2 composite. As revealed from CV studies, a specific capacitance of 150 F g1 is observed for the SnO2eRuO2 composite film with 15 vol% of RuCl3 in the deposition bath limited by an ESR of 12 U. This means that the CBD method is useful for depositing SnO2eRuO2 with minimum uses of RuCl3 precursor and optimum specific capacitance value. The specific energy of 4.9 Wh kg1 with specific power of 0.42 kW kg1 observed for SnO2eRuO2 composite makes it promising electrode in supercapacitor application. Acknowledgment
6 (c) -1
80 Z' (ohm)
Fig. 7. The charge discharge curves of SnO2eRuO2 composite films for samples: a) C1 (5 vol %), b) C2 (10 vol%), c) C3 (15 vol%) and d) C4 (20 vol%) in the potential window of 0 to þ0.8 V vs. SCE at the current density of 0.5 mA cm2 in 0.5 M H2SO4 as an electrolyte.
Specific Energy (Wh kg )
40
One of the authors Prof. C. D. Lokhande is grateful to All India Council for Technical Education (AICTE), New Delhi for financial support through the scheme F. No.8023/BOR/RID/RPS-165/2009-10.
5 4
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3 (a) 2 1 0.50
(b)
(d)
0.48
0.46
0.44
0.42
0.40
0.38
0.36
-1
Specific Power (kW kg ) Fig. 8. The Ragone plot for SnO2eRuO2 composite films for samples: a) C1 (5 vol%), b) C2 (10 vol%), c) C3 (15 vol%) and d) C4 (20 vol%).
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[27] S. Yanqun, T. Dian, J. Wuhan Univ. Technol. Mater. Sci. Ed. 22 (2007) 626e629. [28] T.P. Gujar, V.R. Shinde, C.D. Lokhande, W.Y. Kim, K.D. Jung, O.S. Joo, Electrochem. Commun. 9 (2007) 504e510. [29] K.N. Yu, Y. Xiong, Y. Liu, C. Xiong, Phys. Rev. B 55 (1997) 2666e2671. [30] S. Bhaskar, P.S. Dobal, S.B. Majumder, R. S Katiyar, J. Appl. Phys. 89 (2001) 2987e2991. [31] K.H. Chang, C.C. Hu, C.Y. Chou, Chem. Mater. 19 (2007) 2112e2119. [32] J.V. Ryan, A.D. Berry, M.L. Anderson, J.W. Long, R.M. Stroud, V.M. Cepak, V.M. Browning, D.R. Rolison, C.I. Merzbacher, Nature 406 (2000) 169e172. [33] C.C. Hu, K.H. Chang, C.C. Wang, Electrochim. Acta 52 (2007) 4411e4418. [34] F. Tao, Y.Q. Zhao, G.Q. Zhang, H.L. Li, Electrochem. Commun. 9 (2007) 1282e 1287. [35] A. Burke, J. Power Sources 91 (2003) 37e50. [36] M.S. Wu, H.H. Hsieh, Electrochim. Acta 53 (2008) 3427e3435. [37] C.C. Hu, H.Y. Guo, K.H. Chang, C.C. Huang, Electrochem. Commun. 11 (2009) 1631e1634.
Please cite this article in press as: S.N. Pusawale, et al., SnO2eRuO2 composite films by chemical deposition for supercapacitor application, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
SnO2eRuO2 composite films by chemical deposition for supercapacitor application S.N. Pusawale a, P.R. Deshmukh a, J.L. Gunjakar b, C.D. Lokhande a, * a b
Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India Ewha Woman’s University, 11-1, Seoul 120-750, South Korea
h i g h l i g h t s < Chemical bath deposition of SnO2eRuO2 composite films. < Supercapacitor application. < Maximum specific capacitance of 150 F g1. < Maximum energy density of 4.9 Wh kg1 at a power density of 0.42 kW kg1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2011 Received in revised form 12 June 2012 Accepted 22 December 2012
A chemical bath deposition method is used for preparing SnO2eRuO2 composite films. The films with varying RuO2 composition are prepared by varying the vol% of RuCl3 in the bath and their structural, morphological and supercapacitive properties are studied. The structural study revealed that the composite films are amorphous. FT-Raman spectrum showed the fundamental vibrations of SnO2 and RuO2, confirming the formation of SnO2eRuO2 composite. The elongated particle structures are observed from morphological studies. The supercapacitive studies showed highest specific capacitance of 150 F g1 for SnO2eRuO2 composite containing 15 vol% of RuCl3 in the deposition bath. The chargeedischarge curves showed the pseudocapacitive performance with maximum energy density of 4.9 Wh kg1 at a power density of 0.42 kW kg1 for optimized SnO2eRuO2 composite. An equivalent series resistance (ESR) of 12 U is observed from the electrochemical impedance studies. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Thin films Composite materials Chemical synthesis Electrochemical properties
1. Introduction In recent years there is a growing demand of supercapacitors in various applications such as, hybrid electric vehicles, telecommunications, particularly associated with cellular phones for a reduction of the size of the batteries [1], memory protection of computer electronics [2,3] etc. The charge storage mechanism in supercapacitors divide it into two types; electrochemical double layer capacitors (EDLC) and pseudocapacitors. An electrochemical double-layer capacitor (EDLC) stores electrical charge in an electrical double layer at the electrodeeelectrolyte interface. High surface area electrode materials, such as activated carbons maximize this interface resulting in larger capacitance [4]. In addition, the higher capacitance can be obtained by using redox active materials such as metal oxides and conducting polymers. The charge
* Corresponding author. Tel.: þ91 231 2609225; fax: þ91 231 26092333. E-mail address: [email protected] (C.D. Lokhande).
storage in these materials is due to the highly reversible surface redox reactions commonly referred as pseudocapacitance [5]. The research on metal oxides electrodes for supercapacitors has attracted much attention in past few years. Lokhande and Joo discussed in detail the supercapacitive performance of various metal oxide electrodes [6]. Among the various metal oxides, RuO2 has been proved one of the most promising electrode material for supercapacitor application, which possess high-energy storage capabilities with large specific capacitance and good reversibility [5]. However, the lack of abundance and high cost of this oxide inhibit its commercial applications. To overcome this, another approach is to prepare composites in which RuO2 is combined with other metal oxide materials such as IrO2 [7], NiO [8,9], Co3O4 [10], SnO2 [11e13] and TiO2 [14,15]. The idea of composite is due to the intention of combining the high specific capacitance of RuO2 with a good dispersion of this oxide in order to get electrode material of high capacitance at low moderate price. In the search of second electrode material for combining with RuO2, SnO2 is found to be one of the suitable
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
Please cite this article in press as: S.N. Pusawale, et al., SnO2eRuO2 composite films by chemical deposition for supercapacitor application, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
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S.N. Pusawale et al. / Materials Chemistry and Physics xxx (2013) 1e7
materials due its low cost and environmental friendly nature. Moreover, SnO2 has same rutile structure as RuO2 with the lattice parameters of both the oxides quite similar to each other [16]. SnO2eRuO2 composite coated titanium electrodes were widely used as dimensionally stable anodes (DSA) for chlorine evolution [17e19]. Introducing SnO2 into RuO2 was shown to increase the electrochemical active sites and stability of RuO2 in developing dimensionally stable anodes [20e22]. In previous studies Hu et al. [11] used modified solegel process for deposition of Ru1dSndO2 with a maximum specific capacitance of 690 F g1 for Sn content d ¼ 0.2. Kim et al. used a DC reactive sputtering method for the preparation of composite RuO2eSnO2 electrode and a maximum specific capacitance of 88.8 F g1 was observed [23]. Wang and Hu adopted a mild hydrothermal process to synthesize hydrous ruthenium oxide tin oxide composites ((RueSn)O2$nH2O) where a maximum specific capacitance of 566 F g1 was observed [24]. A composite SnO2eRuO2 supercapacitor electrode synthesized by cyclic voltammetry plating of RuO2 onto a porous and highly conductive SnO2 showed a specific capacitance of 15 F g1 [12]. Both Chemical bath deposition and electrochemical deposition methods are low temperature chemical methods they are simple and inexpensive. However, the growth mechanism in both the methods is rather different. In electrochemical deposition, the thickness and morphology of the films can be easily controlled by electrochemical parameters such as electrode potential and current density. While in case of chemical bath deposition method it refers to the deposition of films on a solid substrate from a reaction occurring in a solution. Since the growth mechanism in both the methods is different it would result in films with different structural and morphological features that will affect its electrochemical properties. In comparing with other deposition methods, chemical bath deposition (CBD) method is simple, inexpensive and reproducible one in which direct deposition of metal halides, chalogenides or oxides takes place on any type of substrate surface [25]. The final product in this method is a thin film coated on the substrate surface. This feature makes it promising method to fabricate electrodes for supercapacitor, as there are no extra preparation steps, which are commonly required for fabricating the supercapacitor electrode in the methods like solegel and hydrothermal with a final product in powder form. The CBD method is based on the slow controlled precipitation of the desired compound from its ions in the reaction bath on the substrate surface [25]. In the present paper, for the first time, we report on the synthesis of SnO2eRuO2 composite films by CBD method for supercapacitor application. 2. Experimental 2.1. Deposition of SnO2eRuO2 composite film The deposition of SnO2eRuO2 composite film by CBD method was carried out by heating the mixed solution of 0.05 M stannic (Sn4þ) chloride and 0.01 M ruthenium (Ru3þ) chloride. The pH of the solution was adjusted in the range of 1e2 using urea (NH2CONH2) solution with constant stirring, the stainless steel substrates were immersed vertically in the above bath and the bath was heated at 353 K. After 10 min, the precipitation was started in the bath and during the precipitation, heterogeneous reaction occurred and deposition of SnO2eRuO2 films took place on the substrate. The substrates coated with SnO2eRuO2 thin films were removed after 2 h, and washed with double distilled water and dried. The films prepared were uniform, blackish in color and well adherent to the substrate. In order to vary the amount of RuO2 in the deposition, the volume percentage of RuCl3 in the deposition
bath was varied as 5%, 10%, 15% and 20% and the corresponding composite films were named as C1, C2, C3 and C4 respectively, and used in further studies. 2.2. Characterization techniques The thickness of the film was measured by using computerized AMBIOS make XP-1 surface profiler with 1 A vertical resolution. To study the structural property of composite thin films, X-ray diffraction patterns were obtained using a Philips (PW-3710) diffractometer with a Cr Ka (l ¼ 2.2870 A) target. The FT-Raman spectrograms were obtained using Bruker Raman spectrophotometer. The microstructure and EDS mapping were observed using field emission scanning electron microscopy (FE-SEM) (JSM 7100F). The electrochemical study was performed in a three-electrode configuration cell consisting of SnO2eRuO2 composite film as a working electrode, platinum as a counter electrode and saturated calomel electrode (SCE) as a reference electrode. The chargee discharge analysis was performed by WonATech Automatic Battery Cycler WBCS system, interfaced to a computer. The electrochemical impedance measurement was conducted with a versastat 3G frequency response analyzer (FRA) under Zplot program (Scribner Associates Inc.). 3. Results and discussion 3.1. Reaction mechanism of SnO2eRuO2 composite film formation The chemical reaction of SnO2eRuO2 composite film formation can be enlightened as follows. The primary role of urea is to provide a slow controlled supply of OH ions through slow thermal decomposition [26].
COðNH2 Þ2 þ H2 O/CO2 þ 2NH3
(1)
In aqueous solution, ammonia slowly releases the OH ions as,
NH3 þ H2 O5NH4þ þ OH
(2)
4þ
The Sn ions in the solution reacts with OH ions to form stannic hydroxide (Sn(OH)4) precipitate which then hydrolyzed to form SnO2 through the reaction,
Sn4þ þ 4OH /SnðOHÞ4 /SnO2 þ 2H2 O 3þ
(3)
Similarly the Ru ions reacts with OH ions in the solution to form ruthenium hydroxide (Ru(OH)3) precipitate which then react with oxygen to form RuO2 as,
Ru3þ þ 3OH /RuðOHÞ3
(4)
1 D 2RuðOHÞ3 þ O2 / 2RuO2 þ 3H2 O 2
(5)
This describes the SnO2eRuO2 film formation mechanism by CBD. The films with varying vol% of RuCl3 in the bath were deposited. Fig. 1 shows the variation of thickness of SnO2eRuO2 composite film for different vol% of RuCl3. From the graph, it is observed that the thickness increases with increase in the volume of RuCl3 in the bath. The maximum thickness of 0.51 mm is observed for sample C4 (20 vol% of RuCl3). 3.2. Structural analysis The X-ray diffraction patterns of SnO2eRuO2 composite films for varying vol% of RuCl3 are presented in Fig. 2. The X-ray diffraction patterns show only peaks corresponding to stainless steel
Please cite this article in press as: S.N. Pusawale, et al., SnO2eRuO2 composite films by chemical deposition for supercapacitor application, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2012.12.059
5
10
15
20
Volume of RuCl3 in bath (%)
200
3.3. FT-Raman studies The typical Raman spectra of chemical bath deposited SnO2, RuO2 and SnO2eRuO2 composite electrode are shown in Fig. 3. In Fig. 3(a), the Raman spectrum of SnO2 showed two peaks centered
Δ -stainless steel
Δ
Intensity (counts)
Δ
Δ
Δ
d)
Δ
c)
Δ
b)
Δ Δ
0
20
40
60
a)
80
100
2 θ (degree) Fig. 2. The X-ray diffraction patterns of SnO2eRuO2 composite films for sample a) C1 (5 vol%), b) C2 (10 vol%), c) C3 (15 vol%) and d) C4 (20 vol%).
794 cm ,N SnO 685 cm ,B
1g
Fig. 1. Variation of thickness of SnO2eRuO2 composite film with variation of volume (5, 10, 15 and 20 vol%) of RuCl3.
substrate. Thus, the XRD studies showed the possibility of formation of amorphous SnO2eRuO2 composite film. The amorphous phase for SnO2eRuO2 composite was also observed by Hu et al. [11], Yanqun, and Dian [27]. The amorphous phase is suitable for electrochemical applications, since the protons can easily permeate through the bulk of the amorphous electrode and whole amount of electrode is utilized [28].
622 cm ,A
-1
b)
400
763 cm , B
0.325
c)
589 cm ,SnO
0.390
436 cm ,N group
0.455
320 cm ,N group
0.520
668 cm ,B ,RuO
0.585
564 cm ,SnO
295 cm ,N SnO
0.650
Relative intensity (A.U.)
Thickness (μm)
0.715
3
377 cm ,N SnO
S.N. Pusawale et al. / Materials Chemistry and Physics xxx (2013) 1e7
600
800
a)
1000
-1
Wavenumber (cm ) Fig. 3. FT-Raman spectra of a) SnO2, b) RuO2 and c) SnO2eRuO2 composite film.
at 763 cm1 corresponding to the crystallites of SnO2 with a tetragonal rutile structure for B2g vibration [29]. The Raman peak at 589 cm1 corresponds to amorphous SnO2. The other Raman peaks observed at 320 and 436 cm1 correspond to the N group vibration modes of SnO2 nanocrystallites [29]. In Fig. 3(b) the Raman peak at 622 cm1 correspond to the A1g whereas the peak at 685 cm1 is correspond to the B2g vibration mode of RuO2 respectively [30,31]. Fig. 3(c) shows the Raman spectrum for SnO2eRuO2 composite (sample C3). The peaks at 668 cm1 correspond to crystalline RuO2 (i.e. B2g) in the rutile form, whereas as the Raman peak at 564 cm1 is due to the amorphous SnO2. In comparison with Raman spectrum of pure SnO2 and RuO2 the shift in peak positions of SnO2 and RuO2 is observed in Fig. 3(c) is may be due to the quantum effects of decrease in particle size [32]. However, the certain new peaks observed in Fig. 3(c) are may be due to the formation of SnO2eRuO2 composite. 3.4. Surface morphological studies The SEM images of pure SnO2 and SnO2eRuO2 composite samples are shown in Fig. 4. The SEM image for SnO2 is shown in Fig. 4(a), which revealed the smooth and compact morphology. It is observed that the smooth and compact morphology of pure SnO2 changed to rough and interconnected network like with overgrowth structure for sample C1 (Fig. 4(b)). With further increment of RuCl3 in the deposition, the network structure becomes relatively compact with small pits and cracks on the surface for sample C2 (Fig. 4(c)). The small pits observed becomes larger and irregularly shaped with further addition of RuCl3 for sample C3 (Fig. 4(d)), the size of the pits however decreased with more RuCl3 in the deposition and surface becomes rough and elongated structure like for sample C4 (Fig. 4(e)). The SEM studies clearly shows the transition of smooth and compact morphology of pure SnO2 to the rough surface with irregular pits and elongated structure after incorporation of RuO2 in the deposition. This may be due to different growth kinetics associated with the variation in the composition of bath for deposition. Fig. 5 shows the corresponding EDS mapping images of sample C3 that shows the uniform distributions of Sn, Ru and O in the composite. The observed Sn, Ru and O content in the composite films are given in Table 1.
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Fig. 4. The SEM images of (a) pure SnO2 and SnO2eRuO2 composite films for samples: (b) C1 (5 vol%), (c) C2 (10 vol%), (d) C3 (15 vol%) and (e) C4 (20 vol%) at 3000 magnification.
3.5. Supercapacitive studies The cyclic voltammetry (CV) curves for SnO2 and SnO2eRuO2 composite electrodes in the potential region between 0 and þ0.8 V vs. SCE at 20 mV s1 were employed to evaluate the electrochemical characteristics of the supercapacitors based on SnO2eRuO2 composites. The CV curve for SnO2 is shown in Fig. 6(a), which revealed the double layer capacitance mechanism as no redox transitions are observed in this potential region. The observed current is less compared with composite electrodes and a double layer capacitance of w4 F g1 is observed for SnO2. The CV curves for composite electrodes are shown in Fig. 6(b)e(d) which shows a different type of CV nature compared with pure SnO2 with the presence of redox transitions clearly confirming the pseudocapacitive mechanism. As the capacitance originating from SnO2 is very small, the pseudocapacitance coming from SnO2eRuO2 composite electrodes is mainly attributed to the RuO2. From the curve, it is observed that the current under the curve increases with increase in volume of RuCl3 in the deposited bath. This may be due to the increase in volume of RuCl3 in the bath it gives more number of Ru3þ ions, which increases the content of RuO2 in the deposition. The increase in the RuO2 content increases the current of CV curves. Therefore, specific capacitance of the composite electrode increases from 42 (sample C1) to 150 F g1 (sample C3) as the vol% of RuCl3 in
the bath increases from 5 to 15 respectively. For further increment in vol% of RuCl3 in deposition (20 vol%), there observed a decrease in the voltammetric current that means 15 vol% is the maximum limit of volume of RuCl3 in the deposition bath which gives the maximum current response. The less CV response above this composition is due to the decrease in the RuO2 content in composite film decreasing specific capacitance to 121 F g1 this can be attributed to the decrease in electroactive sites for 20 vol% of RuCl3 in the deposition bath [24]. The observed value of specific capacitance in this work is less than earlier reported values [10,11,13,21,33]. The work described in Ref. [11] is about Sb doped SnO2 xerogel impregnated with RuO2 prepared by incipent wetness method. In this case, Sb was used as dopant to increase the conductivity of SnO2, as it is well known that pure SnO2 is bad conductor, with doping its conductivity increases that results in lowering the electrode resistance and improve the current performance. The observed capacitance of the net composite electrode is of 15 F g1 (The capacitance of 710 F g1 reported in the work is for RuO2 only not for composite electrode). In our present work we don’t use any such dopant for SnO2 also the observed capacitance is greater. The specific capacitance of our studies is less than other references mentioned. The methods used in these references are solegel and hydrothermal method. On the other hand, in sol gel and hydrothermal method the final product is
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S.N. Pusawale et al. / Materials Chemistry and Physics xxx (2013) 1e7
5
Table 1 The observed Sn, Ru and O content in at.% for samples C1eC4. Sample
Sn (at.%)
Ru (at.%)
O (at.%)
C1 C2 C3 C4
23.19 18.41 12.32 13.43
6.59 12.44 20.67 16.44
70.22 69.15 67.01 70.13
(5 vol%) (10 vol%) (15 vol%) (20 vol%)
morphological features which are also responsible for the good capacitive behavior. The substrate used in the sol gel study is graphite so there may be chances of contributing the double layer capacitance of graphite to the total capacitance. However, in our study we have used inexpensive stainless steel substrates which don’t have capacitive behavior. 3.6. Chargeedischarge studies The chargeedischarge behaviors of the samples (C1, C2, C3 and C4) is examined by chronopotentiometry from 0 to þ0.8 V vs. SCE at the current density of 0.5 mA cm2 in 0.5 M H2SO4 electrolyte is shown in Fig. 7. A nonlinear variation of potential vs. time is displayed, which may be a pseudocapacitance performance arisen from the electrochemical adsorptionedesorption or redox reaction at an interface between the electrode and the electrolyte [34]. All the samples exhibit coulombic efficiency greater than 90%. The specific power and specific energy was calculated for all the samples using the formula,
Specific power ðSPÞ ¼ IV=2m
(6)
And Specific Energy ðSEÞ ¼ IVtd =2m
(7)
where, ‘I’ is the discharging current in Amperes, ‘V’ is the applied potential range for charging and discharging, ‘m’ is the mass of electrode in kg and ‘td’ is the discharging time in h. Fig. 8 shows Ragone plot relating power density to achievable energy density. From the plot, it is observed that the values of energy density and power density of composite samples are according to Ragone plot typically calculated for supercapacitors [35]. From the plot, it is observed that the sample C3 has maximum energy density of 4.9 Wh kg1 at still high power density of 0.42 kW kg1.
c) d)
2
Current density (mA cm )
2
Fig. 5. EDS mapping of SnO2eRuO2 composite film for sample C3 (15 vol%).
in powder form which then undergoes through various preparation steps to make final electrode. However, in our study we have used a simple chemical bath deposition method, which results in the direct deposition of composite films on the substrate surface. Also as the methods are different it results into different structural and
1
b) a)
0
-1
-2 0.0
0.2
0.4
0.6
0.8
Voltage (V vs. SCE) Fig. 6. The cyclic voltammograms (CV) of SnO2eRuO2 composite films for samples: a) C1 (5 vol%), b) C2 (10 vol%), c) C3 (15 vol%) and d) C4 (20 vol%) in the potential window of 0 to þ0.8 V vs. SCE at the 20 mV s1scan rate in 0.5 M H2SO4 as an electrolyte.
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S.N. Pusawale et al. / Materials Chemistry and Physics xxx (2013) 1e7
120
0.6 0.4
80 Z'' (ohm)
Z''(ohm)
Voltage (V vs. SCE)
0.8
40
0.2 0.0
a) b) 0
20
40
60
c)
d) 80
Z' (ohm)
0
100
0
Time (s)
3.7. Electrochemical impedance studies In order to investigate the electrochemical characteristics of the electrode and electrolyte in quantitative manner impedance measurement was performed. The electrochemical impedance measurement (at open circuit voltage, in the frequency range 105e 102 Hz) of SnO2eRuO2 composite sample C3 (15 vol% RuCl3) was carried out in 0.5 M H2SO4. The Nyquist depictions (Z0 vs. Z00 ) of the raw impedance data are shown in Fig. 9. For the convenience of the interpretation, this plot can be divided in to high and low frequency regions. In the high frequency range, the intercept at real part (Z0 ) is a combinational resistance of ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance between the active material and the current collector [36]. The presence of semicircle in the high frequency region suggests that there is a charge transfer resistance while the line in the low frequency region angled at w45 to the real axis is the characteristic of ion diffusion into the porous structure of the electrode [37]. Inset (Fig. 9) shows the expanded high frequency region of the same plot.
120
Fig. 9. Nyquist plot of SnO2eRuO2 composite (sample C3, 15 vol%) in 0.5 M H2SO4.
The 12 U, high frequency impedance intercept reflects the equivalent series resistance (ESR) of the electrolyte. The ESR observed in the present study is higher and which is probably the reason to get the specific capacitance value of only 150 F g1. The further experiments on lowering the ESR and maximizing its specific capacitance value are going on. 4. Conclusions The amorphous SnO2eRuO2 composite films are prepared by varying volume of RuCl3 in the deposition bath using CBD method. The FT-Raman spectrum study showed the characteristic vibrations modes of SnO2 and RuO2 in the composite sample, confirming the formation of SnO2eRuO2 composite. As revealed from CV studies, a specific capacitance of 150 F g1 is observed for the SnO2eRuO2 composite film with 15 vol% of RuCl3 in the deposition bath limited by an ESR of 12 U. This means that the CBD method is useful for depositing SnO2eRuO2 with minimum uses of RuCl3 precursor and optimum specific capacitance value. The specific energy of 4.9 Wh kg1 with specific power of 0.42 kW kg1 observed for SnO2eRuO2 composite makes it promising electrode in supercapacitor application. Acknowledgment
6 (c) -1
80 Z' (ohm)
Fig. 7. The charge discharge curves of SnO2eRuO2 composite films for samples: a) C1 (5 vol %), b) C2 (10 vol%), c) C3 (15 vol%) and d) C4 (20 vol%) in the potential window of 0 to þ0.8 V vs. SCE at the current density of 0.5 mA cm2 in 0.5 M H2SO4 as an electrolyte.
Specific Energy (Wh kg )
40
One of the authors Prof. C. D. Lokhande is grateful to All India Council for Technical Education (AICTE), New Delhi for financial support through the scheme F. No.8023/BOR/RID/RPS-165/2009-10.
5 4
References
3 (a) 2 1 0.50
(b)
(d)
0.48
0.46
0.44
0.42
0.40
0.38
0.36
-1
Specific Power (kW kg ) Fig. 8. The Ragone plot for SnO2eRuO2 composite films for samples: a) C1 (5 vol%), b) C2 (10 vol%), c) C3 (15 vol%) and d) C4 (20 vol%).
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Please cite this article in press as: S.N. Pusawale, et al., SnO2eRuO2 composite films by chemical deposition for supercapacitor application, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2012.12.059