Fabrication and characterization of ultra light SiC whiskers decorated by RuO₂ nanoparticles as hybrid supercapacitors

Abstract

Ultra light SiC whiskers decorated by RuO₂ nanoparticles fabricated combining carbon thermal reduction and a hydrothermal route were investigated as novel hybrid supercapacitors with improved performance. The morphology and microstructure of the composite indicate that RuO₂ nanoparticles are dispersed on the surface of SiC whiskers. The electrochemical behavior of the hybrid electrode systematically measured in 1.0 M H₂SO₄ demonstrates that SiC whiskers can considerably improve the supercapacitance properties of RuO₂. The specific capacitance of RuO₂ nanoparticles in the composite can be obtained as high as 1454.8 F g⁻¹ with a mass ratio of RuO₂/modified SiC whiskers of 2 : 3. Moreover, the hybrid electrode displays a longer cycle life with a better capacitance retention (∼93.6%) than that of pure RuO₂ after 2000 CV cycles.

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1. Introduction

In the face of ever-increasing energy consumption and global climate change, clean, as well as renewable energy storage in applications ranging from portable electronics and consumer electronics to large industrial equipment and aviation, has triggered tremendous research endeavors.^1,2 Among various power source devices, supercapacitors that store energy through either ion adsorption (electrochemical double-layer capacitors, EDLCs) or fast surface redox reactions (pseudocapacitors), have been an attractive power solution due to their remarkable properties such as distinguished power output, high power density, noticeable reliability, fast charge–discharge rate and long cycle life.^3–5

Up to now, carbon with high specific surface⁶ and semiconductors with nanostructure, e.g., titanium dioxide (TiO₂) nanotubes/nanowires, titanium nitride (TiN) nanowires, silicon nanowires and silicon carbide (SiC) nanowires^7–9 are adopted as EDLCs. Especially cubic SiC (3C-SiC) exerts a widely fascination as EDLCs due to its excellent electronic conductivity of all SiC polytypes, the smallest band gap (∼2.4 eV) and the largest electron mobility (∼800 cm² V⁻¹ s⁻¹ in a low-doped material).¹⁰ As for the materials as pseudocapacitors, metal oxides such as RuO₂,¹¹ MnO₂,¹² NiO,¹³ and Co₃O₄ (ref. 14) are usually adopted because they are capable of storing more charge during highly reversible redox reactions between metal oxides and adoptive electrolyte. Among these oxides, RuO₂ has received extensive attention owing to its wide potential window, reversible redox reaction, high theoretical specific capacitance and high electrical conductivity (3 × 10² S cm⁻¹).¹⁵ However the commercial application of RuO₂ as electrode is limited due to the high cost of Ru. In addition, the phenomenon that RuO₂ nanoparticles generally undergo grievous particle aggregation will result in poor cycling ability and deteriorative rate performance.

Aiming to improve the capacitive performances of supercapacitors, incorporating pseudocapacitance into EDLCs is a common method.⁶ For instance, Kim et al. applied microsphere SiC/nanoneedle manganese oxide composites as supercapacitor electrodes with the specific capacitance of 251.3 F g⁻¹ at a scan rate of 10 mV s⁻¹.¹⁶ Recently many investigators have attempted to combine RuO₂ nanoparticles with low-cost materials including conducting polymers (polyaniline (PANI) and polypyrrole (PPy)),¹⁷ carbon activated carbons,¹⁸ carbon nanotubes (CNTs),¹⁹ carbon nanofibers²⁰ and grapheme²¹ etc. to improve the rate capability of RuO₂. The fabrication of hybrid electrode using various materials is still a hotspot for researchers.

In this work, a novel hybrid electrode as supercapacitors is proposed. The experimental idea is put forward as follows. First, large-scale ultra light SiC whiskers as EDLCs are synthesized using carbon thermal reduction method. Then RuO₂ nanoparticles as pseudocapacitor are inlaid on the outer surface of SiC whiskers via a moderate hydrothermal route. The obtained RuO₂/SiC whiskers composite was applied as hybrid electrode for supercapacitors for the first time, aiming to fabricate a novel hybrid electrode with enhanced electrochemical properties.

2. Experimental

2.1 Fabrication of RuO₂/SiC whiskers composite

SiC whiskers were obtained via carbon thermal reduction using gangue (SiO₂ > 99%) and carbon black as raw materials. The reaction was carried out at 1500 °C for 2 h at argon atmosphere and the detail has been reported in our previous work.²² For simplification, SiC whiskers are denoted as SiC_w. SiC_w was washed using 10% HF at ambient temperature for 1 h to remove silica and the resulting whiskers are symbolized as H-SiC_w. The RuO₂/H-SiC_w composite was prepared as the following procedures: at first, 0.1 g H-SiC_w was uniformly dispersed in 10 mL RuCl₃ solution with continuous magnetical stirring. As for the influence of RuO₂ with different content on the capacitances, the concentration of RuCl₃ in 10 mL RuCl₃ solution ranging from 0.0084 to 0.025 mol L⁻¹ was adopted according to the theoretical mass ratio from 1 : 3 to 1 : 1 (RuO₂/H-SiC_w). Then 0.1 mol L⁻¹ sodium hydroxide (NaOH) liquor was slowly dripped into the suspension to adjust the pH of the solution at ∼7. After reacting for 1 h, the solution was equably stirred in thermostatic oil bath and refluxed at 150 °C for 8 h, which could lead H-SiC_w to fully contact with the solution. The resultant product was filtered and centrifuged several times with deionized water until the pH of the filtered water was ∼7. The obtained composite was dried at 80 °C for 12 h and denoted as RuO₂/H-SiC_w. In addition, pure RuO₂ sample was prepared via the same way for comparison.

2.2 Preparation of supercapacitor electrode

The work electrode was prepared by mixing the above material respectively, i.e. H-SiC_w, RuO₂/H-SiC_w or RuO₂, and polyvinylidene fluoride (PVDF) with the mass ratio of 95 : 5 in N-methyl-2-pyrrolidone (NMP). Then, the stable slurry was equably coated on a Ti sheet with the coating mass of about 3 mg (1 × 1 cm). The electrode was heated at 80 °C for 24 h to evaporate the solvent.

2.3 Characterization and electrochemical measurement

X-ray diffraction (XRD) patterns of the samples were obtained on PW-1830 X-ray diffractometer (Philips) with Cu Kα from 10° to 80° (2θ) at a scanning speed of 1° min⁻¹. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB 250Xi (Thermo Fisher Scientific). The structure and morphology of the prepared samples were characterized using field-emission scanning electron microscopy (FE-SEM, FEI-SIRION, operated at 5 kV) and transmission electron microscopy (TEM) (Tecnai G2 F30 S-TWIN). The specific surface area was determined from the nitrogen adsorption–desorption isotherm measured at 77 K on a Quadrasorb SI-MP analyzer using Brunauer–Emmett–Teller (BET) model.

The electrochemical behavior was investigated on a CHI660E electrochemical working station. A three-electrode system was adopted and the auxiliary and reference electrode were platinum foil and Ag/AgCl (KCl-saturated) respectively. The electrolyte used in all of the measurements was 1.0 M H₂SO₄ aqueous solution. Cyclic voltammetry (CV), galvanostatic charge/discharge characteristics and electrochemical impedance spectroscopy (EIS) were carried out. The potential range for CV tests was from 0 to 1 V and the scan rates were in the range from 5 to 500 mV s⁻¹. Galvanostatic charge/discharge measurements were carried out from 0 to 1 V with different current densities at 0.2, 0.5, 1, 2, 5 and 8 A g⁻¹. EIS measurements were done by applying an AC voltage with 5 mV amplitude in a frequency range from 0.1 Hz to 100 kHz.

3. Results and discussion

3.1 Phase and morphology characterization

Fig. 1(a) shows the obtained product was grey whiskers in large scale. The whiskers were so lightweight that they could stably stand on the dandelion (Fig. 1(b)). XRD patterns of SiC_w, RuO₂/H-SiC_w and RuO₂ are shown in Fig. 2. For SiC_w, the broad peak at about 20° corresponds to the amorphous silica shell. And the other five reflection peaks from 30 to 80° can be indexed to be the characteristic peaks of cubic SiC (β-SiC, card no. 01-073-1706). In view of the RuO₂/H-SiC_w composite, the characteristic peaks of β-SiC remain while the broad peak at about 20° disappears, indicating that silica is removed by HF. A weak broad peak at about 28° appears at high magnification, this corresponds to (1 1 0) of tetragonal RuO₂, demonstrating the existence of amorphous RuO₂ phase.

Fig. 1 (a) Photograph of the as-received SiC whiskers and (b) the as-received SiC whiskers standing on the clover.

Fig. 2 XRD patterns of SiC_w, RuO₂/H-SiC_w (2 : 3) and RuO₂.

To confirm the existent state of RuO₂ in the RuO₂/H-SiC_w composite, the detailed surface information was examined using XPS as shown in Fig. 3. Survey spectrum indicates that the sample surface consists of Ru, O, Si and C (Fig. 3(a)). The Ru 3d core-level spectra (Fig. 3(b)) in RuO₂/H-SiC_w composite consist of two main peaks at about 280.7 and 284.8 eV, corresponding to the 5/2 and 3/2 spin–orbit components of Ru (3d) with spinorbit-splitting of about 4.1 eV. It is assigned to ruthenium oxide in RuO₂.²³ A doublet peak with binding energy (BE) at about 282.3 and 286.5 eV corresponds to RuO₃. More critical information can be acquired by analysis of Ru 3p spectra to determine the accurate ruthenium oxidation state. One doublet at 462.5 and 484.7 eV of Ru 3p core-level spectrum corresponding to the spin–orbit splitting of Ru (3p) is observed in Fig. 3(c). This is consistent with the RuO₂ chemical state.²⁴

Fig. 3 (a) XPS wide scan survey spectra of RuO₂/H-SiC_w (2 : 3); (b) XPS spectra of Ru 3d peaks of RuO₂/H-SiC_w (2 : 3); (c) XPS spectra of Ru 3p peaks of RuO₂/H-SiC_w (2 : 3).

The structure and morphology of the obtained SiC_w, H-SiC_w, RuO₂/H-SiC_w and RuO₂ were further characterized using FE-SEM and TEM techniques. SEM image of as-received SiC_w is shown in Fig. 4(a), in which the as-received SiC_w shows a straight filament with a smooth surface. TEM image clearly shows that SiC_w before etching possesses SiC@SiO₂ core–shell structure and the shell of SiO₂ is characterized as a smooth thin layer (Fig. 4(b)). After etching by HF, the surface of H-SiC_w becomes much coarser (Fig. 4(c)). TEM analysis indicates that SiO₂ shell is removed and the surface after etching is rougher (Fig. 4(d)). As for the morphology of the RuO₂/H-SiC_w composite, the filament structure is still retained. However the surfaces become extremely rough and rugged due to the deposition of RuO₂ nanoparticles on the surfaces. The typical morphology of RuO₂/H-SiC_w is shown in Fig. 4(e) and (f). It can be seen that RuO₂ nanoparticles were attached uniformly on the surface of H-SiC_w. Fig. 5 shows that the morphology of pure RuO₂ nanoparticles prepared and the inset in Fig. 5 is the diffraction pattern of the selected area, which illustrates RuO₂ is amorphous.

Fig. 4 FE-SEM images of the as-prepared SiC_w (a), H-SiC_w (c) and RuO₂/H-SiC_w (2 : 3) (e), respectively. TEM images of SiC_w (b), H-SiC_w (d) and RuO₂/H-SiC_w (2 : 3) (f), respectively.

Fig. 5 TEM image of pure RuO₂ nanoparticles, the inset is corresponding SAED patterns of the marked region.

The specific surface area of H-SiC_w is determined by the nitrogen adsorption–desorption isotherm as shown in Fig. 6. The BET model gives a specific surface area of 517 m² g⁻¹ (Fig. 6(a)). The pore diameter is mesopores ranging from 2 to 4 nm (Fig. 6(b)).

Fig. 6 (a) Nitrogen adsorption–desorption isotherm and (b) pore size distribution of H-SiC_w.

3.2 Electrochemical capacitive performance

The electrochemical performances of resulting electrode for supercapacitors were tested using CV and galvanostatic charge/discharge with potential windows ranging from 0 to 1 V in 1.0 M H₂SO₄ electrolyte. In this work, Ti sheet was equably and completely coated by stable slurry. Therefore Ti sheet is not directly exposed to 1.0 M H₂SO₄ and is stable in 1.0 M H₂SO₄ electrolyte. In addition, there was no unusual phenomenon occurred at the surface of Ti sheet during the whole electrochemical measurement. Fig. 7(a) shows a comparison of the CV curves of H-SiC_w, RuO₂/H-SiC_w with different RuO₂/H-SiC_w mass ratio and RuO₂ electrodes at a scan rate of 10 mV s⁻¹. All the CV plots display a typical capacitive behavior, i.e. quasi-rectangular shape. Such capacitive characteristics of H-SiC_w electrode associate with the excellent electrical conductivity, large electroactive areas and electric double-layer capacity. For RuO₂, a larger CV encircled area with a symmetric CV shape represents a higher pseudocapacitance and reversible redox reactions have taken place (Fig. 7(a)). In view of the RuO₂/H-SiC_w composite, the CV loop area increases with the content of RuO₂ increasing (Fig. 7(a)). It should be pointed out that the CV curves of the RuO₂/H-SiC_w composite with the ratio of 2 : 3 over a wider range of scan rates (Fig. 7(b)) exhibits the excellent power handling performance and the capacitive response could be maintained even at the scan rate as high as 500 mV s⁻¹.

Fig. 7 Electrochemical characterization performed in 1.0 M H₂SO₄: (a) CV curves of H-SiC_w, RuO₂/H-SiC_w and RuO₂ at a scan rate of 10 mV s⁻¹. (b) CV curves of RuO₂/H-SiC_w (2 : 3) electrode measured as different scan rates of 5, 10, 20, 50, 100, 200 and 500 mV s⁻¹. (c) Comparison of specific capacitances of H-SiC_w, RuO₂/H-SiC_w (2 : 3) and RuO₂ electrodes at different current densities. (d) The specific capacitances of the RuO₂/H-SiC_w as a function of RuO₂ loading fraction. The specific capacitances in the RuO₂ nanoparticles in the composites are also shown. (e and f) Galvanostatic charge/discharge curves of RuO₂/H-SiC_w (2 : 3) electrodes at current densities of 0.2, 0.5, 1 and 2, 5, 8 A g⁻¹, respectively.

The specific capacitance of the electrode measured by the galvanostatic charge/discharge method can be estimated according to the following equation:²⁵

where C (F g⁻¹) is the specific capacitance, I (A) is the constant discharging current, Δt (s) is the discharge time, ΔV (V) is the potential window, and m (g) is the mass of the active material in the electrode. The specific capacitances of H-SiC_w, RuO₂ and RuO₂/H-SiC_w electrodes with different RuO₂/H-SiC_w mass ratios i.e. 1 : 3, 1 : 2, 2 : 3 and 1 : 1 against the current density are plotted in Fig. 7(c). It is clearly found that RuO₂/H-SiC_w composite possessed 649.2 F g⁻¹ for RuO₂/H-SiC_w (1 : 1), 591.9 F g⁻¹ for RuO₂/H-SiC_w (2 : 3), 416.5 F g⁻¹ for RuO₂/H-SiC_w (1 : 2) and 262.0 F g⁻¹ for RuO₂/H-SiC_w (1 : 3) at 0.2 A g⁻¹. It should be pointed out that the capacitance of all electrodes gradually decreases with the current density increasing from 0.2 to 8 A g⁻¹. The specific capacitance of pure RuO₂ exhibits a rapid decrease when higher currents is applied (Fig. 7(c)). At higher current densities, it became hard to maintain the redox reactions completely at the inner active sites of the electrodes, leading to a reduction of the electrochemical performance of the electrodes. Furthermore, RuO₂ as pseudocapacitive material, usually provide slower ion diffusions than EDLCs, which causes difficulty for RuO₂ to continue appropriate redox reactions at higher current densities.²⁶ By contrast, the hybrid electrode possesses a better rate performance than pure RuO₂ due to the enhanced mechanical and chemical stability provided by the supporting material, i.e. SiC_w, in sustaining the redox reactions at higher current densities.²⁷

Fig. 7(d) shows the specific capacitance of the samples as a function of RuO₂ loading weight in the composites. The capacitance per RuO₂ in the composite has been calculated according to the following equation:⁶

where C_s,H-SiCw and C_s,RuO2 are the specific capacitances of H-SiC_w and RuO₂ component, respectively, while w_H-SiCw and w_RuO2 are the weight ratios of H-SiC_w and RuO₂ components. A maximum specific capacitance of as high as 1454.8 F g⁻¹ was achieved for RuO₂ in the composite with the mass ratio of 2 : 3, which was ascribed to the optimized nanostructure with well-dispersed RuO₂ nanoparticles as shown in Fig. 4. Thus the results hint that the composite could substantially improve the utilization and electrochemical capacitance properties of RuO₂.

For further evaluating the specific capability of the RuO₂/H-SiC_w (2 : 3) composite, the galvanostatic charge/discharge curves were measured between 0 and 1 V at the current density of 0.2, 0.5, 1, 2, 5 and 8 A g⁻¹ respectively (as shown in Fig. 7(e) and (f)). It is obvious that the charge curves are symmetric to their corresponding discharge counterparts, suggesting the occurrence of the reversible pseudocapacitive reaction. There are low voltage losses even when the current density is up to 8 A g⁻¹. In addition, low internal resistances exist within electrode because of the tight insertion of RuO₂ on the outer surface of H-SiC_w and the rough surface of the composite. The phenomenon suggests that the composite electrode exhibits excellent capacitive performance by incorporating pseudocapacitors into EDLCs.

EIS technique was also employed to investigate the conductive and diffusive behavior of the RuO₂/H-SiC_w (2 : 3) electrode. It was carried out at a frequency range from 0.1 Hz to 100 kHz, where Z′ and Z′′ are the real and imaginary parts of the impedance, respectively (Fig. 8). The Nyquist plot consisted of a semicircle in the high-frequency region and a linear part in the low-frequency region. The equivalent circuit diagram (inset in Fig. 8) includes the solution resistance R_s, the charge transfer resistance R_ct, a pseudocapacitive element C_p from redox process and the constant phase element (CPE) to account for the double layer capacitance. The linear region of the plot exhibits an angle between 45 and 90°, demonstrating an ideal capacitive behavior and low diffusion resistance.²⁸ In the high-frequency region, R_s and R_ct are measured to be 0.55 and 1.61 Ω respectively. Therefore the supercapacitor shows the low charge-transfer resistance and equivalent series resistance. This combines the properties of low diffusion and electron-transfer resistance and thus is responsible for the good electrochemical properties.

Fig. 8 Nyquist plots of RuO₂/H-SiC_w (2 : 3) electrode. (inset) Equivalent circuit model.

The good electrochemical properties of RuO₂/H-SiC_w composite can be explained by the following factors. Firstly, β-SiC possesses the smallest band gap and the largest electron mobility among all SiC polytypes. Secondly, the surfactivity of SiC_w is promoted by HF, which promotes the deposition of RuO₂ and results in high connectivity between RuO₂ and H-SiC_w. During the charge/discharge process, RuO₂ nanoparticles are beneficial to the formation of pores for ion-buffering reservoirs. Finally H-SiC_w possesses the microstructure of long and straight filaments, which offers both electrical conductive channels and significant support for the deposition of RuO₂. Thus this results in the perfect interfacial contact to facilitate the speed of electrons transport and enhance the mechanical and chemical stability. The mechanism of charge storage of RuO₂ in 1.0 M H₂SO₄ (aq) may proceed via oxidization and reduction reactions as follows.

RuO₂ + 2H⁺ + 2e⁻ (oxidized state) ↔ Ru(OH)₂ (reduced state)

where H⁺ and e⁻ represent the proton and electron in the H₂SO₄ electrolyte.²⁸ Therefore the redox reactions would cause pseudocapacitance resulting in the considerable improvement of supercapacitor properties.

As for the cycling performance, the cycling stabilities of RuO₂/H-SiC_w (2 : 3) and RuO₂ electrodes were measured with cyclic voltammetry at the scan rate of 10 mV s⁻¹ as shown in Fig. 9. After 2000 continuous cycles, the capacitance retention for RuO₂/H-SiC_w (2 : 3) and RuO₂ electrode respectively attained 93.6% and 85.5% of the original capacitance under the same conditions. The pseudocapacitors such as RuO₂ possess worse cycling performance due to particle aggregation. As for the hybrid electrode material of RuO₂/H-SiC_w, it is true that H-SiC_w serves as conductive and supporting material. Besides that, it also acts as EDLC in hybrid materials. As for the state of RuO₂ nanoparticles, it can be seen that RuO₂ nanoparticles are attached uniformly on the surface of H-SiC_w (Fig. 4(e) and f), therefore the maximum specific capacitance of RuO₂ in the composite with the RuO₂ to H-SiC_w mass ratio of 2 : 3 benefits from the optimized nanostructure with well-dispersed RuO₂ nanoparticles. What's more, the positive role of H-SiC_w for pinning nanosized RuO₂ particles can effectively prevent the aggregation of the nanoparticles during electrochemical cycles, which is similar to that investigated in lithium storage systems²⁹ and thus leads to better cycling performance. The degradation of hybrid electrode occurred mainly due to RuO₂ nanoparticles breaking and collapsing detaching from H-SiC_w upon repeated cycling, which can be inevitable.^30,31 However, H-SiC_w with outstanding mechanical and chemical stabilities, and high flexibility, can effectively prevent the aggregation of the nanoparticles and still play an important role in slowing down the degradation process.

Fig. 9 Comparison of cycling performances measured at a scan rate of 10 mV s⁻¹ for the bare RuO₂ and RuO₂/H-SiC_w (2 : 3) composite.

4. Conclusions

A kind of novel hybrid electrode for supercapacitors, i.e. RuO₂/H-SiC_w is fabricated combing carbon thermal reduction and hydrothermal route. The results reveal that the as-resultant composite nanostructures consist of RuO₂ nanoparticles inlaid on the surfaces of H-SiC_w. When the mass ratio of RuO₂/H-SiC_w is 2 : 3, RuO₂ nanoparticles in the composite achieve the specific capacitance values as high as 1454.8 F g⁻¹. In addition, the as-prepared electrode exhibits a superb charge storage characteristic with large specific capacitance of 591.9 F g⁻¹ at the current density of 0.2 A g⁻¹. It also possesses a long circle life with a good capacitance retention (∼93.6%) after 2000 CV cycles owing to the combination of modified SiC whiskers as EDLC material and RuO₂ as pseudocapacitive material. Our new approach suggests that the hybrid electrode of RuO₂/H-SiC_w possesses promising electrochemical performance as supercapacitors.

Acknowledgements

The authors express their appreciation to National Nature Science Foundation of China (No. 51172021 and 51572019), the National Science-technology Support Plan Projects (Grant 2013BAF09B01) and the National Science Fund for Excellent Young Scholars of China (No. 51522402) for financial support.

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