Hollow SnO2 microspheres and their carbon-coated composites for supercapacitors

Colloids and Surfaces A: Physicochem. Eng. Aspects 444 (2014) 26–32

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Hollow SnO2 microspheres and their carbon-coated composites for supercapacitors Suzhen Ren ∗ , Ying Yang, Meiling Xu, Hongmin Cai, Ce Hao, Xuzhen Wang College of Chemistry, Dalian University of Technology, Dalian 116024, Liaoning Province, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Fine controlled core shell structure of sPS@SnO2 particles.

• Hollow SnO2 microspheres are prepared via template assisted method in hydrothermal environment and high-temperature calcination treatment for removal of sPS. • SnO2 @C composite hollow spheres were fabricated. • SnO2 @C composite leads to an improved electrochemical performance in supercapacitors due to a suitable carbon coating.

a r t i c l e

i n f o

Article history: Received 23 August 2013 Received in revised form 27 November 2013 Accepted 7 December 2013 Available online 25 December 2013 Keywords: Template-assisted method SnO2 hollow sphere Carbon-coated SnO2 hollow sphere (SnO2 @C) Supercapacitor

a b s t r a c t In the presence of sulfonated polystyrene (sPS) template, sPS@SnO2 core shell particles formed via the interaction between the functional group of -SO3 H on the template surface and ions of Sn2+ from the precursor of SnSO4 which were in ethanol-aqueous medium. After high-temperature calcination treatment for removal of sPS, the sPS@SnO2 changed into SnO2 hollow spheres. With the further carbonization of the sPS@SnO2 @glucose composite microspheres, SnO2 @C composite hollow spheres were fabricated. Using SEM, TEM, and N2 adsorption - desorption technology, the structure, specific surface area, and the core-shell structure formation mechanism were determined. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) properties of SnO2 hollow spheres and SnO2 @C composites were investigated, respectively, in the foam nickel electrode under alkaline condition. The specific capacitance of SnO2 @C composite hollow spheres could reach 25.8 F g−1 in 1 mol L−1 KOH aqueous solution and showed excellent charge-discharge behavior. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Electrical double layer capacitors (EDLCs) or supercapacitors have drawn more and more attention in recent years due to their fast charge and discharge rate, high power density (one to two orders of magnitude higher than batteries), long cycle lifetime (two

∗ Corresponding author at: Dalian University of Technology, Chemistry, 2 Linggong Road, Dalian, PR China. Tel.: +86 411 84986073. E-mail address: [email protected] (S. Ren). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.12.028

to three orders of magnitude longer than batteries), and high reliability [1–3]. Basically, supercapacitors have two energy storage mechanisms; one is the electrical double layer capacitance and another pseudocapacitance. The most crucial factor determining the electrochemical property lies in the structure and composition of electrode materials. Research has thus been focused on increasing energy density without sacrificing cycle life or high power density. As we all known, ruthenium oxide is rare and very expensive, leading to limit of practical application in supercapacitor. Therefore, the development of new electrode materials with low

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cost for supercapacitor is still a challenge. Among the oxide nanomaterial, SnO2 has gained prominent interest because of its potential applications in various fields [4]. It is well known that the physical and chemical properties of the materials depend greatly on the size, shape, and composition of the particular material. Therefore, researchers are actively engaged in developing tin oxide nanostructures with different sizes and shapes such as core-shell and hollow microspheres, for a wide range of applications [5–11]. Graphitic carbon is the most commonly used anode materials in lithium ion batteries (LIBs) and supercapacitors. Pristine SnO2 is a typical semiconductor with low electronic conductivity (bandgap is 3.6 eV). In order to improve the electronic conductivity of SnO2 , carbon is often used as a conducting phase to form a composite [12–26]. Carbon coated SnO2 for LIBs leaded to longer cycle lifetime and stability. SnO2 @C nanostructures showed unusual reversible Li storage capacities [18–24]. Hence, combining SnO2 and porous carbon to take advantage of the virtues of the two materials may be a viable alternative. However, SnO2 @C nanocomposite structure for supercapacitor electrode is still a challenge for stability and specific capacitance. Although great efforts have been dedicated to template-free routes for the preparation of hollow particles, coating against colloidal template particles is still the most effective and general method for the preparation of hollow particles with a narrow size distribution and well-defined shape [27–30] . It requires the controlled construction of desirable functional structure. The common difficulties in template-based synthesis are from creating uniform coatings of desired materials (or their precursors) on the surface of templates and maintaining their structural integrity after removing templates. Moreover, controllable preparation of hollow particles with complex architectures, such as multi-shell structure, via a simple coating process still remains difficult. In our recently work, attention has been directed to SnO2 hollow spheres with fine- controlled core-shell structures. The SnO2 hollow spheres were obtained by calcination of sPS@SnO2 composites where sPS is a sacrificial core [30]. In this paper, carbon-coated SnO2 (SnO2 @C) hollow composite spheres can further be easily obtained by calcination the sPS@SnO2 particles which covered glucose as the carbon precursor [23,31,32]. They are well controlled shape and size and with a high specific surface area. The products were examined for their application as supercapacitor electrode materials.

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2.2. Preparation of SnO2 hollow spheres Firstly, PS seed particles were immersed in large quantity of concentrated sulfuric acid and stirred at 40 ◦ C and for the appropriate time, thereby controlling the thickness of the sPS shell [33]. Then, in a typical synthesis, 50 mg of prepared sPS spheres were dispersed in 10.0 ml of ethanol by ultrasonication. After added a small amount polyvinylpyrrolidone (PVP) dissolved in water as surfactant to the sPS ethanol solution, freshly prepared 0.05 mol L−1 SnSO4 (10 ml) solution was added drop by drop and the solution was stirred for 8 h to form sPS@SnO2 particles as shown in Fig. 1. Finally, these dried sPS@SnO2 particles were transferred to furnace and calcinated (heating rate, 5 ◦ C min−1 ) at 550 ◦ C under air for 2 h. Hollow SnO2 particles were then obtained. 2.3. Synthesis of SnO2 @Carbon hollow spheres sPS@SnO2 core-shell spheres was coated with carbon through hydrothermal calcination reaction [23,31,32]. In a typical synthesis, 30 mg of sPS@SnO2 spheres were dispersed in 4 ml of H2 O, 10 ml of ethanol, and 0.4 g of glucose. Then the mixture was transferred to the autoclave. The reaction temperature was 180 ◦ C, and time was set to 15 h. After this reaction, sPS@SnO2 @C was obtained. In the last step, sPS@SnO2 @C was put into tube furnace and treated under N2 for 4 h at 700 ◦ C. The hollow SnO2 @C composite spheres were obtained. 2.4. Characterization Scanning electron microscopy (SEM) images were taken using a QUANTA 450 (FEI, America). Transmission electron microscopy (TEM) was conducted together with energy dispersive X-ray spectrometer as its mode has been utilized to examine the dimensions, structural details, and chemical composition of the samples (Tecnai F30 with a field emission gun operating at 200 kV). N2 adsorption/desorption measurements were used to investigate the porosity structural properties of spheres. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method via a micromeritics ASAP 2010 M instrument. The pore size distribution was determined using the Barrett–Joyner–Halenda (BJH) method [34–36]. 2.5. Electrochemical measurements

2. Experimental section 2.1. Synthesis of PS spheres PS particles with different sizes were prepared by microemulsion polymerization. Typically, 35 ml of DI water and 6.3 ml of styrene were added into a three necked round-bottom flask and the solution was stirred at 1200 rpm for the formation of the emulsion. The emulsion was then bubbled with nitrogen gas to remove oxygen and maintained at an inert atmosphere. The reaction was carried out at 70 ◦ C. When the reaction temperature was attained, 5 ml of 0.02 mol L−1 potassium peroxydisulfate (KPS) solution was added. After a couple of min 5 ml of 0.005 mol L−1 CTAB was added and then 5 ml of 0.0042 mol L−1 SDS solution in DI water was added 5 min later. N2 purge was continued for a period of 15 min after the addition of all the reactants. The polymerization process was then initiated and stirring was continued for a period of 9 h. The end point was determined by the formation of a thick white liquid which was then thoroughly washed with ethanol and DI water and stored in ethanol for further use as seed material.

The electrochemical experiments were carried out using a conventional three-electrode cell. The working electrode was prepared as followed: SnO2 microspheres, carbon blacks, and binder (60% of polytetrafluorethylene aqueous suspension) at a mass ratio of 80:10:10 were added and mixed well in N-methyl-2-pyrrolidone (NMP) until it formed the slurry with proper viscosity, and then the slurry was uniformly coated on a disk-like nickel foam (with the active area of 1 cm2 ) by dipping, dried at 80 ◦ C for 12 h in a vacuum drying oven, then pressed at 4 MPa for 30 s in order to assure a good electronic contact. Thus, the working electrode obtained. The mass of SnO2 microspheres material was 0.0278 g for SnO2 hollow spheres and 0.0194 g for SnO2 @C composite. CV and GCD measurement were carried out in a system, in which platinum foil and Ag/AgCl (saturated KCl) electrodes served as counter and reference electrodes, respectively, and using 1 mol L−1 KOH aqueous solution as the electrolyte. The CV experiment was performed within −1 V to 0 vs. reference electrode and in scan rates range of 10–50 mV s−1 . The charge/discharge curves were measured at different current densities from 0.1 to 1 A g−1 within a potential window (−1.0 to 0 V). The measurements were carried out by means of electrochemical analyzer systems, in CHI 660C (CH Instruments, USA).

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Fig. 1. Scheme of the procedure for the synthesis of hollow SnO2 spheres and SnO2 @C composite spheres.

3. Results and discussion A scheme of synthesis route of hollow SnO2 spheres and SnO2 @C composite spheres is shown in Fig. 1. Monodisperse sPS spheres were coated with SnO2 through hydrothermal reaction and then removed sPS by calcination in air for 2 h at the temperature of 550 ◦ C. After this procedure, SnO2 hollow spheres were obtained. Moreover, a carbon layer with a thickness of about 20 nm was deposited on the surface of the sPS@SnO2 sphere by the hydrothermal reaction. In the last step, the sPS@SnO2 spheres coated with glucose were then put into tube furnace and SnO2 @C spheres were finally obtained.

The experimental procedures and the observations are summarized in Figs. 2 and 3. SEM images show that solid PS microspheres obtained by the emulsion polymerization process are monodispersed and the mean diameter is ca. 600 nm (Fig. 2a). The obtained PS spheres were treated by sulfonation using fume acid. The sPS spheres in Fig. 2b were chosen as template microspheres for later coating reaction. The sPS@SnO2 spheres from hydrothermal reactions are obtained by Sn2+ ions adsorbed on sPS surface react with H2 O molecules. The SnO2 hollow spheres were prepared by removal of the sPS component as described previously [30,33,37]. The hollow SnO2 nanoparticles have a mean diameter of about 550 nm. Some of the hollow spheres were deliberately broken to confirm that the spheres were hollow in Fig. 2c. TEM images of

Fig. 2. SEM images of PS (a), sPS (b), and hollow SnO2 (c) spheres.

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Fig. 3. TEM images of sPS@SnO2 (a,b), sPS@SnO2 @C (c), and SnO2 @C (d,e,f) samples.

area and average pore diameter than that of hollow SnO2 spheres. Furthermore, larger specific surface area and average pore diameter of SnO2 @C composite spheres will benefit their electrochemical performance [17,38,39].

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Volume Adsorption (cm 3/g)

sPS@SnO2 (Fig. 3a and b) clearly indict their core shell structure. After coating with a carbon layer, sPS@SnO2 @C in Fig. 3c and hollow structured SnO2 @C spheres with a thin carbon layer of ca. 20 nm were obtained (Fig. 3d–f). There is a carbon coating layer on particles in Fig. 3f, in comparison to the uncoating particle in Fig. 3b. The shell of the carbon layer has a thickness of about 20 nm. As shown from TEM images in Fig. 3, SnO2 @C spheres are hollow structures. Images depicted in TEM images from Fig. 3 and SEM images from Fig. 2, present the final product of SnO2 @Carbon sphere. Here, one can see that the spherical structure of the samples changed in diameter from 500 to −580 nm. These spherical particles are core-shell structured spheres as revealed by the contrast between the dark edge and pale center in the spheres as well. The N2 adsorption/desorption isotherms at 77 K for hollow SnO2 spheres and their corresponding carbon coated spheres are shown in Fig. 4. The curves indicate that the pore sizes are in the mesoporous range. The specific surface area and average pore diameter of mesoporous SnO2 spheres calculated using the BJH method from the adsorption branch are 34.85 m2 g−1 and 9.39 nm, respectively. Accordingly, the specific surface area and average pore diameter of SnO2 @C composite spheres are 44.92 m2 g−1 and 12.99 nm, respectively. From the above N2 adsorption/desorption results, it can be concluded that our core- shell structured spheres are mesoporous in structure. SnO2 @C composite samples have larger specific surface

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Fig. 5. CVs of SnO2 hollow spheres (A) and SnO2 @C composite hollow spheres (B) at different scan rates: (a) 10 mV s−1 , (b) 20 mV s−1 , (c) 30 mV s−1 , (d) 40 mV s−1 , and (e) 50 mV s−1 . Insert is the current (at the potential of −0.9 V vs. Ag/AgCl)-scan rate plot, respectively.

To test the electrochemical properties of SnO2 @C spheres, they were used as anode materials for supercapacitor. Values for energy and power density were estimated on the basis of the supercapacitor measurements in a 1 mol l−1 KOH electrolyte. Fig. 5 shows CV curves of SnO2 hollow spheres electrode (A) and SnO2 @C composite hollow spheres electrode (B) at different scan rates. From the curves in Fig. 5, the specific capacitance of the two anode was calculated in accordance with the equation C = ʃIdV/vmV. Where C is the specific capacitance based on the mass of electroactive material (F g−1 ), I is the response average current (A), v is the potential scan rate (mV s−1 ), m is the mass of the electrode material in the electrodes (g), and V is the potential (V) [40]. It can be seen that the shapes of the CV curves are more or less rectangular within the measured potential window, and no redox peaks are observed. Similarly, the current under curve was slowly increased by increasing the scan rate, which indicates that the voltammetric currents are directly proportional to the scan rate (see insets of Fig. 5A and B). This also reflects the ideal capacitance behavior of the synthesized SnO2 @C composites. As shown in Fig. 6, the measured specific capacitances were 13.85 and 43.30 F g−1 for hollow SnO2 spheres and SnO2 @C composite spheres, respectively, at the lowest scan rate of 10 mV s−1 . Similarly, a capacitance of 31.06 F g−1 was obtained even at a higher scan rate of 50 mV s−1 for the SnO2 @C composite spheres. This high capacitance value, compared with that of other composites [17], may be due to the increase in specific surface area and average pore diameter of SnO2 @C composite spheres. In addition, carbon material is also a good electrode material for double layer capacitors, so the carbon layer shell also contributes to the specific capacitance.

Fig. 6 also shows the effect of specific capacitance with the scan rate of the two different electrode materials measured in the potential range of 0–1 V. It can be observed that the capacitance values decrease with the increase in scan rate. This is the normal behavior of electrochemical systems. Generally, two different mechanisms have been proposed for the charge storage mechanisms of the oxide materials. One is the intercalation/deintercalation of protons, which leads to the full utilization of the electrode material. This may be the reason for obtaining a higher specific capacitance at a lower scan rate. The second explanation relates to the surface adsorption process for a higher scan rate. This is based on the diffusion effect of the proton within the electrode materials. Hence, it is believed that part of the surface of electrode materials contributed at a high charging-discharging rate, which decreases the specific capacitance. GCD experiments are performed on hollow SnO2 spheres and SnO2 @C spheres. Fig. 7 displays the charge-discharge curves that are measured in 1 mol l−1 KOH electrolyte at different current densities from 0.1 to 1 A g−1 within a potential window (−1.0 to 0 V). It can be found that the capacitor voltage varies linearly with time and the curves are close to isosceles triangle, which indicates that the electrode owns the performance of electrochemical stability and reversibility. We calculated the specific capacitance of two spheres electrodes from the charge-discharge curves shown in Fig. 7, based on the following equation C = it/mV. Where i is the charge-discharge current (A), t is the discharge time, m is the mass of each electrode (g), and V is the potential range [41,42]. The specific capacitance was calculated from the discharge curves in Fig. 7 with values of 3.75 and 25.88 F g−1 obtained at current density of 0.1 A g−1 , respectively. The above work well demonstrated that hollow spheres with large surface areas are beneficial for electrolyte access thus improving the electrochemical properties of the devices. However, there are still some challenges for developing high performance hollow sphere-based supercapacitors: (i) since the specific surface area is the important factor for capacitance delivery, fabrication of hollow spheres with controllable shell size and thickness, versatile composition, and non-aggregated morphology is still challenging to scientists. (ii) the electron transportation in hollow spheres is another restrictive factor for capacitance output, only appropriate degree of crystallinity can improve electron transportation without decreasing ion diffusion ability in active materials. Therefore, controlling the degree of crystallinity of hollow spheres, under the premise of maintaining desired morphology, is also very challenging.

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4. Conclusions By employing the template assisted hydrothermal technique, two samples of hollow SnO2 spheres and SnO2 @C composite have been synthesized. SEM and TEM images of the as-prepared samples confirm the formation of hollow SnO2 spheres and SnO2@C. From the N2 sorption/desorption results, it can be concluded that our hollow structured spheres are mesoporous in structure. SnO2 @C composite sample has larger specific surface area and average pore diameter than that of hollow SnO2 spheres. Electrochemical studies show that the hollow SnO2 @C material is a suitable candidate for supercapacitor application as it has a discharge specific capacitance of 25.8 F g−1 in alkaline solution. Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 21036006, 21137001, 21176043). References [1] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Challenges facing lithium batteries and electrical double-layer capacitors, Angew. Chem. Int. Ed. 51 (2012) 9994–10024. [2] J.R. Miller, P. Simon, Electrochemical capacitors for energy management, Science 321 (2008) 651–652. [3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Mater. 7 (2008) 845–854. [4] J.S. Chen, X.W. David, Lou, SnO2 -based nanomaterials: synthesis and application in lithium-ion batteries, Small 9 (2013) 1877–1893.

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