Performance characteristics of supercapacitor electrodes made of silicon carbide nanowires grown on carbon fabric

Journal of Power Sources 243 (2013) 648e653

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Performance characteristics of supercapacitor electrodes made of silicon carbide nanowires grown on carbon fabric Lin Gu, Yewu Wang*, Yanjun Fang, Ren Lu, Jian Sha* Department of Physics, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China

h i g h l i g h t s
Silicon carbide nanowires were grown on flexible carbon fabric.
Supercapacitor electrodes made of silicon carbide nanowires grown on carbon fabric were fabricated.
Electrochemical properties of the electrodes were tested at various temperatures.
Improved capacitances per area and excellent cycle stability were observed.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 March 2013 Received in revised form 8 June 2013 Accepted 9 June 2013 Available online 18 June 2013

In this paper, we report the supercapacitor electrodes with excellent cycle stability, which are made of silicon carbide nanowires (SiC NWs) grown on flexible carbon fabric. A high areal capacitance of 23 mF cm2 is achieved at a scan rate of 50 mV s1 at room temperature and capacitances increase with the rise of the working temperature. Owing to the excellent thermal stability of SiC NWs and carbon fabric, no observable decrease of capacitance occurs at room temperature (20  C) after 105 cycles, which satisfies the demands of the commercial applications. Further increasing the measurement temperature to 60  C, 90% of the initial capacitance is still retained after 105 cycles. This study shows that silicon carbide nanowires on carbon fabric are a promising electrode material for high temperature and stable micro-supercapacitors. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Supercapacitor Electrical double-layer capacitors Silicon carbide nanowires Carbon fabric Electrode materials Cycle stability

1. Introduction Supercapacitors (SCs), also called electrochemical capacitors (ECs), are a new type of energy-storage device between traditional capacitors and batteries. Because of the high power densities (>10 kW kg1), long cycle lives (>106 cycles) and a wide range of working temperatures (70 to þ100  C) [1e5], supercapacitors are promising candidates for modern power devices. Generally, supercapacitors are divided into two kinds: electrical double-layer capacitors (EDLCs) and pseudocapacitors. For electrical doublelayer capacitors, only adsorption and desorption of electrolyte ions on electrode materials occur during the charge/discharge process. Carbon materials (activated carbons, aerogels, and nanostructures) have been widely studies for EDLC electrodes [6,7]. Recently, EDLCs made of semiconductor or cermet nanowires, such * Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (J. Sha). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.06.050

as silicon nanowires [8,9], silicon carbide nanowires (SiC NWs) [10,11], titanium nitride (TiN) nanowires [12], Titanium dioxide (TiO2) nanotubes and nanowires [13,14], have attracted much attention because of their high specific surface area (SSA) and electrical conductivity. For pseudocapacitors, metal oxides and conducting polymers [15,16] are used as electrode materials and reversible redox reactions occur during cycling process. Though the specific capacitance of the pseudocapacitors is much larger than that of EDLCs, pseudocapacitor materials, such MnO2 and NiO, suffer from poor conductivity and instability [17]. Most recently, the hybrid systems [16,17], which combine the metal oxides and nanowires, such as MnO2 coated WO3x [18], TiO2 [19], Zn2SnO4 [20] and SnO2 [21] nanowires, have been used as the electrode materials. The charge-transfer properties are improved significantly in these hybrid composite electrodes, leading to large specific capacitances (450e637 F g1). Besides the specific capacitance, cycle stability is another important factor of supercapacitors to satisfy the practical demands.

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Though the recent reports on supercapacitors made of hybrid systems get remarkable specific capacitances, most of these supercapacitors can only cycle thousands of times, which is still far from the commercial demand. Compared with pseudocapacitor materials, no chemical reactions happen in EDLC materials and EDLC electrodes have better cycle stability than pseudocapacitor electrodes. Silicon carbide (SiC) nanowire, owing to its thermal and chemical stability, is a promising electrode material for long-lifetime and high-temperature electrical double-layer capacitors. Herein, we report the EDLC electrodes with excellent cycle stability made of SiC NWs grown on carbon fabric. Carbon fabric serves as a conductive and stable substrate, and no binder is needed in the electrodes. The influence of working temperature on the capacitance and stability of the fabricated supercapacitor electrodes is systematically investigated. 2. Experimental section 2.1. Fabrication of SiC NWs on carbon fabric SiC NWs were grown on carbon fabric via a chemical vapor deposition (CVD) method [22,23]. Typically, a carbon fabric (with the thickness of 0.3 mm and the mass of 0.02 g) was cleaned in ethanol and then immersed in 1 M Ni(NO3)2 alcohol solution for 20 min. After being dried at 80  C in air, the carbon fiber cloth was placed in a quartz boat, in which silicon powders had been loaded in advance. Graphite powder (1 g) and WO3 (4 g) were mixed and used for carbon source. Before heating, the furnace tube was evacuated to about 1 Pa. The tubular furnace was heated to 1050  C in 2 h under high purity nitrogen (99.99%) with the flow rate of 100 sccm and the pressure was maintained at about 60 Pa. When

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the temperature reached 1050  C, the pressure in the tube was adjusted to 1000 Pa and maintained for 2 h. After reaction, the carbon fabric was taken out and immersed in dilute nitric acid and HF solution successively to remove the catalyst particles (NiO) at the tips of nanowires [22] and the SiO2 sheath layer around SiC nanowire [23]. 2.2. Characterization and electrochemical measurements Morphology investigations were performed by scanning electron microscopy (SEM) (CorlzeisD, Utral 55). Microstructure of SiC NWs and their EDS analysis were carried out by transmission electron microscopy (TEM) (Tecnai G2 F20 S-TWIN). The electrochemical performance of SiC NWs on carbon fabric was characterized in a three-electrode system with 2 M KCl solution as electrolyte. The region of the fabric without SiC NWs was used for electrical connection. Cyclic voltammetry (CV), galvanostatic chargeedischarge was tested by CHI 660D electrochemical station with a platinum foil (1.5 cm  1.5 cm) as counter electrode and a saturated Ag/AgCl as reference electrode. 3. Results and discussions 3.1. Morphology and structure characterization Fig. 1(a) is a photograph of the carbon fabric after reaction. The SiC NWs (in blue, in the web version) only grow on the center part of the carbon fabric. The two sides of the fabric without nanowires can be used for electrical connection during electrochemical measurement. Fig. 1(b) and (c) shows the low-magnification SEM images of carbon fabric before and after SiC NWs growth. After reaction, the

Fig. 1. (a) A photograph of the carbon fabric with SiC NWs grown at the middle region; (b) and (c) low-magnification SEM images of carbon fabric before and after SiC NWs growth; (d) higher-magnification SEM image of SiC NWS grown on carbon fabric.

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smooth carbon fibers are coated with SiC NWs and become rough. The enlarged SEM image in Fig. 1(d) clearly reveals the long and straight nanowires. Statistical analyses (see Supporting information, Fig. S1) show that the diameter of the nanowire is about 10e50 nm. In our case, SiC NWs grow via the vaporeliquidesolid (VLS) mechanism [22,23]. At high temperature, Ni(NO3)2 on carbon fabric decomposed into NiO particles, which serve as catalyst during VLS growth. The residual oxygen in the tube reacted with the silicon powder to form SiO vapor and graphite powder reacted with WO3 to form CO. The CO and SiO serve as carbon and silicon sources during the growth of SiC NWs. During the reaction process, CO and SiO enter the quasi-liquid state NiO particles. The continuous supplying of CO and SiO makes the alloy particles supersaturated and Si and C precipitate to form SiC NWs [24]. The reaction process can be expressed as the following equations (the (s) and (v) mean solid and vapor states respectively):

that after the HF solution immersion, the SiO2 sheath is removed [23] and the SiC nanowire with rough surface is exposed. Fig. 2(d) is an HRTEM image of the nanowire, which clearly reveals its single crystallinity. The lattice fringe spacing is measured to be 0.25 nm, the same as the interspacing of the (111) lattice planes of b-SiC (3Ce SiC), meaning that the SiC nanowire grows along the [111] direction. The inset shows the selected-area electron diffraction (SAED) pattern, also proving the single crystal structure of the nanowire. Some twins or stacking faults can be found in the SiC nanowire marked by the black square in Fig. 2(b), which can generally be observed in SiC NWs [22,25]. Only signals of C and Si are evident in the energy dispersion spectroscopy (EDS, Fig. 2(e)). Cu signal comes from the TEM grid. Also, the EDS mapping results show that nanowire is composed of Si and C, and only very little O and Ni can be found on the surface of the nanowire.

WO3 ðsÞ þ 3CðsÞ ¼ WðsÞ þ 3COðvÞ 2SiðsÞ þ O2 ðvÞ ¼ 2SiOðvÞ COðvÞ þ SiOðvÞ ¼ SiCðsÞ þ CO2 ðvÞ

3.2. Electrochemical characterization

The as-grown nanowire is SiCeSiO2 coreeshell nanowire with NiO particle at the tip, which is shown in the TEM images of Fig. 2(a) and (b). After acid treatment, NiO particles have been removed as shown in SEM (see Supporting information, Fig. S2) or TEM images. To investigate the interior structures of SiC NWs, HRTEM analyses were also performed. Fig. 2(c) is a bright-field TEM image of an individual SiC nanowire treated by HF solution. It clearly shows

The electrochemical performance of SiC NWs on carbon fabric is measured in 2 M KCl solution. To avoid side reactions at higher voltages in our three-electrode system, the potential window is set to 0e0.6 V vs Ag/AgCl reference electrode. The electrochemical properties of the electrodes at room temperature (20  C) are illustrated in Fig. 3. The CV curves of carbon fabric with and without SiC NWs are shown in Fig. 3(a). The CV of the SiC NWs on carbon fabric exhibits a rectangular shape with no redox peaks. The skewed CV curves show resistance in the electrodes, because the

Fig. 2. (a) and (b) Low-magnification TEM images of SiC nanowire before HNO3 and HF treating; (c) and (d) low-magnification and lattice-resolution TEM images of SiC nanowire after HNO3 and HF treating, the inset in (d) is the corresponding SAED pattern; (e) EDS spectrum taken on the nanowire and the inset is the corresponding EDS mapping results.

L. Gu et al. / Journal of Power Sources 243 (2013) 648e653

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Fig. 3. (a) CV curves of pure carbon fabric and SiC NWs on CF with a scan rate of 0.1 V s1 at room temperature (20  C); (b) CV curves of SiC NWs on CF at room temperature at different scan rates (0.05e2 V s1); (c) areal capacitances of SiC NWs on CF as a function of scan rates; (d) Chargeedischarge curves of SiC NWs on CF at 20  C with different current densities (5e40 mA cm2).

as-grown SiC NWs are intrinsic (with little defects) and the conductivity is not very good. The electrochemical performance was also evaluated by galvanostatic chargeedischarge tests. The results are shown in Fig. 3(d). Though the charge curve appears to be significantly bowing, which may come from side reaction, the discharge curves are nearly straight lines, as shown in Fig. 3(d), meaning a good double-layer charging performance [26]. The specific capacitance of electrodes was calculated in areal capacitance (mF cm2) (see Supporting information). With a scan rate of 0.1 V s1, the capacitance per projected area of SiC NWs on CF is 20.3 mF cm2, which is much larger than that of the pure carbon fabric (1.2 mF cm2). SiC NWs with small diameter (10e50 nm) lead to high specific surface area and large electrical double-layer capacitance. The CV curves over a wide range of scan rates from 50 mV s1e2 V s1 are shown in Fig. 3(b). The shapes of these curves are similar and capacitive behavior of the electrode is maintained over this range. As is shown in Fig. 3(c), the areal capacitance of SiC NWs on carbon fabric is 23 mF cm2 with the scan rate of 50 mV s1. This value is substantially larger than values reported for the similar NWs, including SiC-coated silicon NWs (1.7 mF cm2, 50 mV s1) [9], SiC NWs grown on SiC thin film (0.4 mF cm2, 100 mV s1) [10,11], TiO2 nanotubes (3.24 mF cm2, 100 mV s1) [13], TiO2 NWs grown on carbon fabric (4.4 mF cm2, 10 mV s1) [14] and ZnO NWs on flexible fibers (0.21 mF cm2, 100 mV s1) [27]. When the scan rate increases to 2 V s1, the capacitance decreases to 9.9 mF cm2, 43% retention of the initial capacitance. The specific power/specific energy calculated from the GV curves in Fig. 3(d) are 12 mW cm2/1.7 mJ cm2 at 40 mA cm2 and 1.1 mW cm2/2.3 mJ cm2 at 5 mA cm2, which are much larger than that of SiC NWs grown on SiC film [10,11]. The excellent supercapacitor performance can be attributed to the following reasons: (1) the carbon fabric serves as a highly conductive and

three-dimensional substrate; (2) SiC NWs grow directly on the carbon fabric and no binder is needed, which facilitate the interfacial charge transfer; (3) the quasi-one-dimensional SiC nanowire with small diameter and long length owns a large surface area; (4) the crystalline SiC NWs are favorable for charge transport [12]. The rate-capability and the power property of SiC NWs on carbon fabric can be further improved by in-situ doping [28e30], which enhances the conductivity of the nanowires. For commercial application, supercapacitors should be able to work in a wide range of temperatures. Here, the electrochemical performance of SiC NWs on carbon fabric is investigated at different temperatures (from 0  C to 60  C) and the results are shown in Fig. 4. The capacitances per projected area calculated from CV (with the scan rate of 0.2 V s1) and galvanostatic discharge curves (with the current density of 5 mA cm2) in Fig. 4(a) and (b) are summarized in Table 1. The CV curves at higher temperatures remain rectangular shape in Fig. 4(a). The capacitances calculated by the two methods are comparable and the variation tendencies are also the same. With the increase of temperature from 0  C to 60  C, the areal capacitance increases from 15.6 (14.7) mF cm2 to 21.0 (19.3) mF cm2 calculated from CV (GV) curves. Fig. 4(c) is the plot of specific capacitance vs inverse temperature with the current density of 5 mA cm2. Ln C changes linearly with 1/T, which is consistent with Arrhenius-type equation [26]: C ¼ C0 exp(E/kBT) or ln C ¼ ln C0  E/kBT, where C is the amount of charges formed at the electrodeeelectrolyte interface (proportional to capacitance), C0 is a pre-exponential constant, E is the activation energy, T is absolute temperature, and kB is Boltzmann constant. The activation energy can be seen as the energy needed for charge-complexes in the electrolyte to overcome the transfer and diffusion resistance. The electrolyte here is composed of Kþ and Cl, and the activation energy is calculated to be 2.5 kJ mol1, which is smaller than that of

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Fig. 4. (a) CV curves of SiC NWs on CF with a scan rate of 0.2 V s at different temperatures (0e60  C); (b) discharge curves of SiC NWs on CF with a current density of 5 mA cm2; (c) the plot of specific capacitance vs inverse temperature with the current density of 5 mA cm2; (d) Nyquist plot of the supercapacitor electrodes at different temperatures; (e) Equivalent circuit of Nyquist spectrum; (f) Cycling performance of SiC NWs on CF at 20 and 60  C with a current density of 40 mA cm2 (one point per 1000 cycles).

the TEABF4/PC electrolyte [36]. The motion of the ions (Kþ and Cl) in the electrolyte is responsible for the formation of EDLs. To have a deep understanding of the temperature effect on the electrochemical performance, AC impedance analyses were performed on the supercapacitor electrode. Fig. 4(d) shows the Nyquist spectra of the supercapacitor electrodes measured from 10 kHz to 1 Hz at different temperatures. From the Nyquist plot, solution resistance (Rs) can be got from the Z0 axis intercept at the high frequency end. Normally, the spectrum shows a semicircle which is represented by a parallel combination of interfacial capacitance and resistance. In our case, the absence of the semicircle means that the contact between the electrode materials (SiC NWs) and the current Table 1 Areal capacitance (mF cm2) of SiC NWs on carbon fabric electrode at different temperatures calculated from CV and GV curves in Fig. 4(a) and (b). Temperature

0 C

20  C

40  C

60  C

CV GV

15.6 14.7

17.7 16.7

19.1 17.6

21.0 19.3

collector (carbon fabric) is good [31,33,34]. In the mid-frequency range, the spectrum shows the Warburg diffusion element (W), an important factor to affect the performance of supercapacitors. At the low-frequency range, a vertical spike, which should be parallel to the Z00 axis, can be observed. This spike is represented as electrical double-layer capacitance (Cdl). The inclination of the spike toward Z0 axis means that there’s a leakage resistance (Rl) in parallel with the double-layer capacitance. The leakage resistance implies a leakage current that causes the cell to self-discharge, which may comes from parasitic reactions in the supercapacitor [31,32]. The Nyquist spectrum can be represented by an equivalent circuit as shown in Fig. 4(e) [31]. All of the fitting parameters of the Nyquist spectra in Fig. 4(d) are shown in Table 2. With the increase of the temperature from 0  C to 60  C, the solution resistance decreases because of the enhanced mobility of the ions in the electrolyte. Warburg diffusion element represents the diffusion of ions into the pores of the electrode. The decreased Warburg diffusion element with the elevated temperature means that ions in the electrolyte can diffuse with less resistance and reach the Helmholtz plane more easily, which results in the higher capacitance [36]. With the

L. Gu et al. / Journal of Power Sources 243 (2013) 648e653 Table 2 Equivalent circuit fitting parameters of Fig. 4(d) and (e). Temperature

0 C

20  C

40  C

60  C

R s ( U) W (U s1/2) Cdl (mF cm2) R l ( U)

1.84 2.94 17.0 184.1

1.72 2.38 17.1 119.4

1.59 2.08 17.5 110.1

1.47 1.89 18.1 84.9

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Century Excellent Talents in University, the Fundamental Research Funds for the Central Universities (No. 2011QNA3019), and the Science and Technology Innovative Research Team of Zhejiang Province (2009R50010). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2013.06.050.

increase of temperature, the rate of parasitic reactions may be higher, leading to the decrease of leakage resistance [31,32,35]. Stability is another important issue for supercapacitors. The cycling performance of the SiC NWs on carbon fabric electrode is measured by galvanostatic charge/discharge at 20 and 60  C with a current density of 40 mA cm2 and the results are shown in Fig. 4(d). It clearly shows that no observable decrease of capacitance occurs at room temperature (20  C) after 105 cycles, which will satisfy the demands of the commercial applications. Further increasing the measurement temperature to 60  C, though the capacitance decreases with the cycling, 90% of the initial capacitance is still retained after 105 cycles. No evident change of the morphology of SiC NWs can be observed in SEM images before and after cycling (see Supporting information, Fig. S3). The excellent thermal stability of the SiC NWs as well as carbon fibers is responsible for the great cycling performance of the SiC NWs on carbon fabric electrode. 4. Conclusions In summary, supercapacitor electrodes with excellent cycle stability made of SiC NWs on carbon fabric are developed. The maximum capacitance per area of 23 mF cm2 is achieved at room temperature, which is due to the high specific surface area of SiC NWs. The specific capacitance increases with the elevated temperature because of the decreased Warburg diffusion element. No observable decrease of capacitance occurs at room temperature (20  C) after 105 cycles, which satisfies the demands of the commercial applications. Further increasing the measurement temperature to 60  C, 90% of the initial capacitance is still retained after 105 cycles. The excellent cycle stability of the electrodes is attributed to the thermal and chemical stability of SiC and carbon fiber. Therefore, SiC NWs grown on carbon fabric are a potential material to fabricate high temperature and highly stable micro-supercapacitors. More efforts are going to be paid on in-situ doping of SiC NWs to decrease the resistance of the electrodes, which will further improve the rate-capability and power density. Solid-state electrolyte instead of aqueous electrolyte will be used to make full cell and to improve the electrochemical performance at a wider range of temperatures. Also, pseudocapacitor materials, such as MnO2, can be deposited on the surface of SiC NWs to make hybrid supercapacitor electrodes. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 60976012 and 51272232), Program for New

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