Balasubramaniam Gnana Sundara Raja,
Jerry J. Wub,
Abdullah M. Asiric and
Sambandam Anandan*a
aNanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Trichy 620 015, India. E-mail: sanand@nitt.edu; Fax: +91-431-230-0133; Tel: +91-431-230-3639
bDepartment of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan
cThe Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21413, Saudi Arabia
First published on 30th March 2016
A cubic-like nanostructured SnO2–Co3O4 hybrid was sonochemically synthesized via formation of CoSn(OH)6 nanocubes as an intermediate at room temperature in the presence of stannous and cobalt precursors, without the assistance of any surfactant or template. The obtained hybrid maintained the original frame structure of CoSn(OH)6 nanocubes. The above samples were characterized by thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The electrochemical performance of the hybrid SnO2–Co3O4 nanocubes was investigated as a supercapacitor electrode material through cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS) studies. The cubic-like nanostructured hybrid SnO2–Co3O4 exhibits enhanced supercapacitor performance. The hybrid SnO2–Co3O4 nanocubes delivered a maximum specific capacitance of 484 F g−1, which was more than four-fold higher than bare SnO2 (101 F g−1) and intermediate CoSn(OH)6 nanocubes at the same current density of 0.5 mA cm−2 in the potential range between 0 and +1 V. The enhancement of the overall electrochemical behaviour of the electrode material can be attributed to the synergetic effect between nanostructured SnO2 and Co3O4. About 77% of its initial capacitance was retained after 1000 cycles, which demonstrates its high electrochemical stability and capacitance retention. The obtained results confirmed that this material is a promising candidate for supercapacitor applications.
Recently, composites of metal oxides (Co3O4–NiO,27 MnO2–NiO,28 etc.) and metal oxide/hydroxide composite (Ni(OH)2–MnO2,29 Co3O4–Ni(OH)2,30 etc.) has been used to develop supercapacitors. Based on available literature reported that multiple oxide materials exhibit superior capacitive performance compared to single transition metal oxides. It has been demonstrated that various metal oxides such as Fe3O4,31 RuO2,22 V2O5 (ref. 32) and MnO2 (ref. 33) oxides added to bare SnO2, shows enhanced electrical conductivity and specific capacitance compared that of bare SnO2. Cobalt oxide (Co3O4) has been studied as a promising potential candidate for supercapacitor electrode materials because of its environmental friendliness, low cost and pseudocapacitance allows larger charge storage.34 However, the electrochemical capacitive properties of cobalt oxide coupled with SnO2 have received relatively little attention.35,36 To prepare homogeneous dispersions of multiple oxides depending on the synthetic route,36 various synthetic methods such as hydrothermal method,35 solvothermal,37 co-precipitation,38 etc. are used for the preparation of tin oxide composite. Among these synthetic approaches, sonochemical method was possible method towards synthesizing multiple oxides through the principle of acoustic cavitation; the formation, growth, and implosive collapse of bubbles in a liquid. Bubble collapse stimulated by cavitation produces intense local heating and high pressures.11
In this work, cubic like nanostructured SnO2–Co3O4 coupled hybrid was sonochemically synthesized via formation of CoSn(OH)6 nanocubes as intermediate and investigate their strength towards supercapacitor applications. Remarkably, these cubic like nanostructured SnO2–Co3O4 coupled hybrid exhibit high specific capacitance and attractive rate capability compared to CoSn(OH)6 nanocubes intermediate, bare SnO2 and could be a promising candidate for electrode material in supercapacitor applications.
The supercapacitance studies were carried out in a standard three-electrode system containing the black colour SnO2–Co3O4 coupled hybrid coated stainless steel plate as a working electrode, Pt foil as a counter electrode and Ag/AgCl as a reference electrode. The electrolyte used was aqueous solution of 1 M Na2SO4. The performance of supercapacitor studies were evaluated by cyclic voltammetry (CV) and galvanostatic charge–discharge techniques within the potential range between 0 and 1 V at different scan rates (5, 10, 20, 40, 80 and 160 mV s−1) and different current densities (0.5–15 mA cm−2) respectively. Electrochemical impedance spectroscopy measurements were performed under open circuit voltage in an alternating current frequency range of 0.1–100000 Hz with an excitation signal of 10 mV. As a comparison, the electrochemical performance of CoSn(OH)6 and bare SnO2 was also investigated under the same conditions.
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Fig. 1 (a) XRD pattern (b) FT-IR spectrum of (c) TGA curve (d) FESEM image of intermediate CoSn(OH)6. |
However upon calcination at 500 °C, CoSn(OH)6 yields the black colour SnO2–Co3O4 coupled hybrid which is identified by the typical powder XRD pattern (Fig. 2). Upon viewing the XRD pattern, found that the diffraction peaks of CoSn(OH)6 get disappeared and new diffraction peaks at 2θ values of 26.5° (110), 33.7° (101), 42.4° (210), 51.7° (211), 54.7° (220), 61.9° (221), 64.7° (301), 71.6° (202), 77.6° (321), 83.9° (410) are perfectly indexed and matches well with the corresponding tetragonal SnO2 nanoparticles (JCPDS no. 77-0452). Beside this, the diffraction peaks appeared at 2θ values of 18.9° (111), 31.1° (220), 36.6° (311), 38.0° (222), 44.4° (400), 59.3° (511) are perfectly indexed and matches well with the corresponding cubic Co3O4 nanoparticles (JCPDS no. 78-1969). The absence of any other peak indicates that the SnO2–Co3O4 coupled hybrid phase formation is made of both tetragonal SnO2 and cubic Co3O4 phase only. Absence of CoSnO3 diffraction peaks also illustrates that the formation of SnO2–Co3O4 coupled hybrid only.
Fig. 3 shows the FT-IR spectra of the SnO2–Co3O4 coupled hybrid, the broad absorption band at 573 cm−1 due to Co–O stretching vibration mode. The sharp Sn–OH intermediate peak at 1182 cm−1 was disappeared and a new peak noticed at 653 cm−1 can be assigned as Sn–O stretching vibration,41 which clearly demonstrates that the presence of multiple oxide composite. The strong and weak absorption bands at 3425 cm−1 and 1632 cm−1 may be assigned due to O–H stretching and bending vibrations and these peaks represents the adsorption of moisture present on the surface of the sample.
The morphologies of the SnO2–Co3O4 coupled hybrid have been investigated by using FE-SEM and HR-TEM analysis. FE-SEM image (Fig. 4a and b) of SnO2–Co3O4 coupled hybrid also looks like nanocube morphology similar to the structure of CoSn(OH)6 but the sizes of nanocubes found decreased from 20–100 nm due to heat treatment. For comparison FESEM images of (a) bare SnO2 and (b) pristine Co3O4 as shown in Fig. S1.†
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Fig. 4 FESEM images of SnO2–Co3O4 coupled hybrid (a and b) at different magnifications, respectively. |
HR-TEM images (Fig. 5a and b) of SnO2–Co3O4 coupled hybrid also illustrate the cubical shapes as confirmed by the FE-SEM observations. Fig. 5(c) describes a typical HR-TEM image of the coupled hybrid in which the lattice fringes are clearly seen and matches well with d-spacing 0.336 nm and 0.243 nm, corresponding to the (110) and (311) planes of tetragonal SnO2 (JCPDS no. 77-0452) and cubic Co3O4 (JCPDS no. 78-1969) form respectively. The selected area electron diffraction (SAED) pattern (Fig. 5(d)) shows the crystalline nature of SnO2–Co3O4 coupled hybrid and in addition the observed four diffraction rings can be assigned as (110), (331), (220), and (511) planes of tetragonal SnO2 and cubic Co3O4 nanoparticles. The energy-dispersive X-ray spectroscopy (EDX) shows the presence of Sn, Co, O composition of elements only in the coupled hybrid (Fig. 5(e)).
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Fig. 5 HR-TEM images of SnO2–Co3O4 coupled hybrid (a–c), corresponding SAED pattern (d) and EDX (e) spectrum. |
The electrochemical properties of SnO2–Co3O4 coupled hybrid as electrode material for supercapacitor were evaluated by cyclic voltammetry (CV), galvanostatic charge–discharge and EIS. As a comparison, the electrochemical performance of bare SnO2 intermediate CoSn(OH)6 and pristine Co3O4 was also investigated under the same conditions. Fig. 6 (a) shows the CV curves of bare SnO2, intermediate CoSn(OH)6, pristine Co3O4 and SnO2–Co3O4 coupled hybrid electrode measured at a scan rate of 40 mV s−1 between 0 and +1 V vs. Ag/AgCl in 1 M Na2SO4 aqueous electrolyte. All the curves are rectangular in shape and in addition no redox transitions in the potential region illustrates the ideal capacitive behavior of the prepared electrodes. It is known that the specific capacitance is proportional to the integrated area of CVs.42 Among the CV curves, the integrated area of SnO2–Co3O4 coupled hybrid is the biggest, which means that this coupled hybrid has the largest capacitance in comparison with bare SnO2, intermediate CoSn(OH)6 and pristine Co3O4 sample. The capacitance of the SnO2–Co3O4 coupled hybrid was enhanced due to the synergistic effect between the two different metal oxides which is responsible for the improved electrical conductivity, provide free diffusion pathways for the fast ion transport and facile ion accessibility to storage sites.19 Fig. 6(b) shows the CV curves of SnO2–Co3O4 coupled hybrid electrode at different scan rates of 5–160 mV s−1. All the CV curves are rectangular in shape and exhibit mirror image characteristics which clearly indicate that the electrochemical reactions are reversible and as well as an ideal electrochemical capacitive behavior.43 As increasing the scan rate, the capacitive current density increased, indicating a good rate capability and lower internal resistances of the electrode materials. And the CV curves of coupled hybrid electrode retain a similar shape even up to the scan rate (160 mV s−1), suggesting the good reversibility of the electrode materials in 1 M Na2SO4. For comparison the different scan rates (5–160 mV s−1) CV curves of intermediate CoSn(OH)6 and bare SnO2 electrode are shown in Fig. S2a and b† are also rectangular in shape and exhibit mirror image characteristics which indicate that the electrochemical reactions are reversible as well as an ideal electrochemical capacitive behavior. However the current density of intermediate CoSn(OH)6 and bare SnO2 electrode material is lower than that of SnO2–Co3O4 coupled hybrid. The higher current density of the coupled hybrid is due to the synergistic effect between the metal oxides in the electrode materials.
The galvanostatic charge–discharge behaviors of bare SnO2, intermediate CoSn(OH)6, pristine Co3O4 and SnO2–Co3O4 coupled hybrid is investigated by chronopotentiometry from the potential range between 0 and +1 V vs. Ag/AgCl at the current density of 0.5 mA cm−2 in 1 M Na2SO4 electrolyte and it is shown in Fig. 7(a). A linear variation of potential vs. time is exhibited, which is another criterion for capacitive behavior of the electrode materials. This pseudocapacitance performance occurs from the electrochemical adsorption–desorption or redox reaction at an interface between the electrode and the electrolyte.44 Evidently, the SnO2–Co3O4 coupled hybrid possess considerably longer discharge time indicates that the higher capacitance of the SnO2–Co3O4 coupled hybrid upon compared to that of bare SnO2, intermediate CoSn(OH)6 and pristine Co3O4. The specific capacitance can be calculated from the following formula
SC = It/mΔE | (1) |
The obtained specific capacitance of our synthesized SnO2–Co3O4 coupled hybrid was compared to other pristine Co3O4 and Sn–Co based mixed oxide. For example, porous Co3O4 materials prepared by solid-state thermolysis approach, exhibits a specific capacitance of 150 F g−1 at a current density of 1 A g−1 in 2 M KOH electrolyte.45 Pang et al.46 reported that dendrite-like Co3O4 nanostructure can deliver a specific capacitance of 207 F g−1 at a current density of 0.5 A g−1 in 3 M KOH electrolyte. Furthermore, mesoporous Co3O4 nanocubes were prepared by solid-state crystal re-construction route and displays a specific capacitance of 220 F g−1 at a current density of 0.6 A g−1 in 6 M KOH electrolyte.47 Xie et al.48 prepared layered Co3O4 material as a electrode material can deliver a specific capacitance 263 F g−1 at a current density of 1 A g−1 in 6 M KOH electrolyte. He et al.49 prepared Sn–Co mixed oxide by co-precipitation method and reported supercapacitance of about 285 F g−1. Ferreira et al.50 also reported the supercapacitance of about 328 F g−1 for Ni–Co–Sn mixed oxide thin films prepared by Pechini method. The above results clearly indicate that our prepared SnO2–Co3O4 coupled hybrid can deliver superior specific capacitance compared with these reported other cobalt oxide materials. The capacitance value reported in this work is much higher than these earlier reports, which is mainly assigned to the effective utilization of SnO2 and Co3O4 materials and combined supercapacitive properties of both materials. Fig. 7(b) shows the galvanostatic charge–discharge curves of SnO2–Co3O4 coupled hybrid electrode at a current density of 0.5 mA cm−2. For comparison the galvanostatic charge–discharge curves of intermediate CoSn(OH)6 and bare SnO2 electrode at a current density of 0.5 mA cm−2 are shown in Fig. S3(a and b).†
Cycling performance is another criterion in determining the applicability of supercapacitors for many practical applications. In this study, a long-term cycle stability of the SnO2–Co3O4 coupled hybrid as an electrode material was evaluated by repeating the galvanostatic charge–discharge test at a current density of 0.5 mA cm−2 for 1000 cycles (Fig. 7(c)). During the first 100 cycles, the specific capacitance fades fast, decreasing by ∼13% of its initial capacitance. During the next 600 cycles, the specific capacitance fades slightly and becomes stable, decreasing by ∼5% of its initial capacitance. A considerable specific capacitance of 375 F g−1, about 77% of initial capacitance is still retained after 1000 cycles indicates that good cycling stability and capacity retention of the coupled hybrid. The cycling performance and capacity retention in this work is higher compared to earlier report available in the literature.45 For comparison a long-term cycle stability of the intermediate CoSn(OH)6 and bare SnO2 electrode was evaluated by repeating the galvanostatic charge–discharge test at a current density of 0.5 mA cm−2 for 1000 cycles (Fig. S4a and b†).
Rate capability is one of the key factors for evaluating the power applications of supercapacitors. Fig. 7(d) shows the galvanostatic charge–discharge curves of the supercapacitors electrode material made with SnO2–Co3O4 coupled hybrid at different current densities (0.5–15 mA cm−2) within the potential range between 0 and +1 V vs. Ag/AgCl. The specific capacitance values of the composite electrodes obtained from the discharge curves are 484, 472, 429, 400, 390, 386, 377, 371 and 343 F g−1 at the current density of 0.5, 1, 3, 5, 7, 9, 11, 13 and 15 mA cm−2, respectively (Fig. 7(e)). In general, the specific capacitance decreases with the increase in discharge current density, it may be caused by the increase of potential drop due to the resistance of the materials. In this cycling test, the charge–discharge time increases at lower current densities, ions from the electrolyte can diffuse into the inner-structure of electrode material, having access to almost all active sites of the electrode. Similarly, the charge–discharge time decreases at higher current densities, an effective utilization of the material is limited number of active sites to the outer surface of electrode. It reveals that the specific capacitance is inversely proportional to the current density.51 When the current density increases from 0.5 to 15 mA cm−2 (484 F g−1 to 343 F g−1), there is still around 71% of initial capacitance was retained, indicating the relatively excellent high-rate capability. These results confirmed that the coupled hybrid is ideally suitable for fast energy storage in supercapacitor applications.
The electrochemical parameters, such as energy and power density are an efficient way to evaluate the capacitive performance of supercapacitors. The energy density (E, W h kg−1) and the power density (P, W kg−1) for supercapacitors can be calculated using the following equations:19
E = (ItV)/(7.2M), W h kg−1 | (2) |
P = 3.6E/t, W kg−1 | (3) |
The electrochemical impedance spectroscopy (EIS) can be applied for evaluating the kinetic and mechanistic information of electrode materials. The EIS are tested in the frequency range from 0.1 to 100000 Hz at open circuit potential with amplitude of 10 mV, where Z′ and Z′′ are the real and imaginary parts of the impedance, respectively. Fig. 8 (a) shows the Nyquist plots of the EIS for bare SnO2, intermediate CoSn(OH)6, pristine Co3O4 and SnO2–Co3O4 coupled hybrid. According to the analysis, the Nyquist plot of electrodes displays a semicircle in the high frequency region and a linear part in the low-frequency region. The diameter of semicircle presents in the high frequency region suggests that there is a charge transfer resistance (Rct) in the electrochemical system, which is related with the diffusion of charge. Obviously, the smaller semicircle in SnO2–Co3O4 coupled hybrid means charge transfer resistance (Rct) decreased can be attributed due to the synergistic effect between the nanostructured metal oxides (SnO2 and Co3O4), which may in turn responsible for improved electrical conductivity when compared to bare SnO2, intermediate CoSn(OH)6 and pristine Co3O4. This result indicates that the composite providing an ideal pathway for easy and fast penetration of the electrolyte ions into the inner layer of the coupled hybrid electrode. The vertical line at lower frequencies indicates an ideal capacitive behavior, representative of the ion diffusion of electrolyte within the pores of the electrode. Fig. 8(b) shows Nyquist plots of the EIS for SnO2–Co3O4 coupled hybrid before and after 1000 cycles, the measured pseudo charge transfer resistance (Rct) was increased, this further contributes to the decrease in specific capacitance after long term cyclic stability test.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25299g |
This journal is © The Royal Society of Chemistry 2016 |