Amorphous RuO₂ coated on carbon spheres as excellent electrode materials for supercapacitors

Abstract

Significant enhancement in supercapacitor performances was achieved by the first fabrication of amorphous RuO₂ hybrid by utilising carbon spheres as templates. The as-prepared carbon spheres (Cs)/RuO₂ and reduced carbon spheres (rCs)/RuO₂ composites exhibit high specific capacitances of 387 F g⁻¹ (44.2 wt% RuO₂ loading) and 614 F g⁻¹ (72.6 wt% RuO₂ loading) at a current density of 1 A g⁻¹ at 2 mg cm⁻² loading mass, showing remarkable rate capability with capacitances of 336 F g⁻¹ and 491 F g⁻¹ at a current density of 20 A g⁻¹ and excellent cycling stability with capacity retention of 97.7% and 90.8% for 5000 cycles. The enhanced electrochemical performances could be attributed to the unique structure of the resulted composites as well as the high utilization of well-dispersed RuO₂ nanoparticles on the carbon surface. These results demonstrate the great potential of carbon sphere-based composites in the development of high-performance electrode materials for supercapacitors.

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Introduction

The diminishing availability of fossil fuels and worsening global warming issues have become major concerns nowadays, and thus the search for renewable and sustainable resources would be considered indispensable to the current world. To resolve the shortage of resources and relieve the burden on the environment, extensive efforts were made by researchers to develop various energy conversion and storage devices, especially batteries and electrochemical capacitors (ECs).^1–5 As the ECs possess high power capability, excellent reversibility and long cycle life when compared with batteries, they are considered promising for various applications especially in transportation such as electric vehicles and hybrid electric vehicles.^6–9 Generally, according to the charge storage mechanisms, ECs can be classified into two types. One is the electrochemical double-layer capacitor (EDLC), mainly focusing on carbon-based active materials with high surface areas, which stores energy through electrostatic forces existed in the formed double layers. While the other is pseudocapacitor, often utilizing transition metal oxides or electrically conducting polymers as the electrode materials, which works by the principle of fast and reversible faradic redox reactions.^10–13

Transition metal oxides have been widely used in ECs as active electrode materials due to their good conductivity and excellent power density. Among them, ruthenium oxide (RuO₂), especially amorphous RuO₂ has been regarded as the most potential electrode material for use in ECs owing to its good conductivity, large specific capacitance and highly reversible redox reactions. However, the observed electrochemical performance is far from its theoretical property.^14–18 The most important reason is that the high surface energy of RuO₂ nanoparticles could lead to severe aggregation, bringing some difficulties in the ion and electron transportation which would increase the electrical resistance. In addition, RuO₂ as a noble transition metal oxide would largely raise the cost of the whole capacitor. Thereby, the research for RuO₂-based composite has focused on reducing the used amount of RuO₂ and achieving their outstanding pseudo-capacitive behaviour due to the compositing effects at the same time.^19–22 Herein, RuO₂/carbon composites have been employed widely because carbon and carbon-based materials could significantly improve the utilization efficiency of RuO₂.^19,23–26

To enhance the specific capacitance, various carbon and carbon-based materials have been widely applied to fabricate this composite electrode.^12,27–30 Carbon spheres (Cs) are considered as ideal ones because of their superior physical and chemical properties^31–34. Firstly, the Cs are obviously advantageous as components in the composites due to the ease of processability and relatively inert electrochemistry. Secondly, the surface of Cs has a large number of oxygen functional groups which could supply many electrocatalytic active sites, making it easy for a variety of redox reactions. Thirdly, the cost of Cs is relatively low and Cs are environment-friendly as a special kind of carbon-based materials. To the best of our knowledge, there have been no reports about the application of carbon sphere/RuO₂ composites in supercapacitor.

In this work, a simple sol–gel method is used to synthesize Cs/RuO₂ and reduced Cs/RuO₂ (rCs/RuO₂) hybrid as electrode materials and they are tested in a three-electrode system. The results show that the Cs/RuO₂ and rCs/RuO₂ have excellent electrochemical performances with remarkable specific capacitance, ultrahigh rate property and rather high cycling stability, which demonstrate that a suitable carbon-compositing strategy is an effective way to improve the electrochemical performances of ruthenium oxide as electrode used in supercapacitors.

Experiment section

Materials and chemicals

Pt foil (0.1 mm thick, 99.99%, 25 × 25 mm) was obtained from Alfa-Aesar and RuCl₃·xH₂O was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Glucose and sodium hydroxide were supplied by Sinopharm Chemical Reagent Beijing Co., Ltd. All reagents were of analytical grade and subjected without any further treatment.

Preparation of carbon spheres

In a typical experiment,³¹ 8 g glucose was dissolved in 40 mL distilled water to form a clear solution. Then the solution was sealed in a 60 mL Teflon-lined autoclave and maintained at 180 °C for 4 h. The dark products were centrifuged, washed with ethanol and distilled water for several times, then dried at 80 °C for 4 h under vacuum.

Preparation of Cs/RuO₂ and rCs/RuO₂ hybrid

The Cs/RuO₂ hybrid was prepared by a sol–gel method. Newly prepared carbon spheres were dispersed in distilled water and ethanol with the help of ultrasonication for 2 hours to form a dark suspension. Then a required amount of RuCl₃·xH₂O was added to the reaction media to obtain a desired concentration (0.1 M), and the mixed solvent was stirred vigorously for 30 min. Then 0.3 M NaOH was slowly added to the mixture to adjust the pH to 7.0. Then the solution was kept stirring overnight. Dark products could be obtained after the filteration and wash with distilled water for several times. The resulted particles were dried at 60 °C for 10 h and calcined in air at 150 °C for 2 h. To get rCs/RuO₂, the precursor of carbon spheres calcined in argon atmosphere at 400 °C for 2 h, and the subsequent procedures were the same as described above³⁵. For comparison, RuO₂ powders were prepared without Cs or rCs by the same method.

Characterization of the samples

X-ray diffraction (XRD) patterns of the samples were carried out on a Rigaku D/max 2550 VB⁺ 18 kW X-ray diffractometer with alpha radiation at a scanning rate of 0.1°2θ s⁻¹. Transmission electron microscopy (TEM, JEM-2100F) was applied to characterize the sample morphologies. Thermogravimetric analysis (TGA) data were collected on a thermal analysis instrument (NETZSCH STA449F3) at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C in air.

Electrochemical testing

The electrochemical measurements including cyclic voltammetry (CV) and galvanostatic current charge–discharge (CD) were carried out by a Modulab (Solartron Analytical) electrochemical workstation in a three-electrode cell. The obtained samples, Cs/RuO₂ (or rCs/RuO₂) were mixed with acetylene black, and polyvinylidene difluoride (PVDF) with a weight ratio of 80 : 10 : 10. Then the mixture was ground uniformly to form the coating slurry by dropping into N-methyl-2-pyrrolidone (NMP). This slurry was extracted by using a micropipette and dropped onto the platinum sheet to serve as working electrode after being dried in a vacuum oven at 50 °C overnight. The loading mass of active materials was around 2.0 mg cm⁻². In addition, platinum foil was used as counter electrode, saturated calomel electrode as reference electrode, and an aqueous solution of H₂SO₄ (1 M) as the electrolyte, respectively.

Results and discussion

Structure and morphological analysis

Fig. 1 (a–d) displays the morphological structure of Cs, rCs, Cs/RuO₂ and rCs/RuO₂, respectively. The TEM image of Cs (Fig. 1a) clearly suggests that the carbon spheres obtained by hydrothermal method from glucose at 180 °C for 4 h is uniform with a diameter about 100–200 nm, while the surface of rCs observed from TEM image (Fig. 1b) seems more smooth indicating that rCs are fitter as templates. Fig. 1c shows the TEM micrograph of the obtained Cs/RuO₂ hybrid for a nominal film deposition with a diameter of 30–40 nm, from which the nanoparticle clusters on Cs could be clearly observed. It is clearly suggested that RuO₂ has formed a uniform shell coated on the surface of the carbon sphere template. For comparison, the TEM image of rCs/RuO₂ (Fig. 1d) manifests that the layer of small particles attached on the surface are more uniform and dispersible with a shell thickness about 20 nm.

Fig. 1 TEM images of Cs (a), rCs (b), Cs/RuO₂ (c), and rCs/RuO₂ (d).

X-ray diffraction and thermogravimetric analysis

Fig. 2a and b displays the powder XRD patterns of the as-prepared Cs, rCs, Cs/RuO₂ and rCs/RuO₂, respectively. The low and broad diffraction peaks observed from the as-prepared Cs and rCs illustrate the amorphous structure. For the Cs/RuO₂ and rCs/RuO₂, the diffraction peak corresponding to the as-prepared Cs, rCs almost disappears and no crystalline RuO₂ diffraction peaks are observed, revealing that the RuO₂ is well coated on the surface of carbon and the RuO₂ is typically amorphous due to the low annealing temperature of 150 °C. In order to determine the content of RuO₂ in the as-prepared samples, a thermogravimetric analysis (TGA) was carried out in a flow of air gas with a heating rate of 10 °C min⁻¹ to 800 °C for RuO₂, Cs/RuO₂ and rCs/RuO₂ (Fig. 2c). The results show that the weight loss of as-prepared RuO₂ commences to happen all the time while the loss before 450 °C could be mainly due to the removal of crystalline water and some residues absorbed on the surface of the products. Furthermore, the subsequent slower weight loss should be attributed to an amorphous phase rather than crystalline of the as-prepared RuO₂.^19,36 The total weight loss of RuO₂ is about 8% in the range of 30–800 °C. For rCs/RuO₂ and Cs/RuO₂, the first weight loss step in the range of 30–300 °C is ascribed to the loss of absorbed water, crystalline water, and oxygen-containing groups, and the second weight loss step in the range of 300–450 °C is mostly attributed to the burning of carbon sketch of rCs or Cs. Under the tested conditions, the total weight loss of rCs/RuO₂ is about 35.4%, and thus the content of hydrous RuO₂ in rCs/RuO₂ hybrid is about 72.6%. Moreover, the total weight loss of Cs/RuO₂ is about 63.8%, presenting 44.2% content of hydrous RuO₂ in Cs/RuO₂ hybrid.

Fig. 2 Powder XRD patterns of Cs and rCs (a), Cs/RuO₂ and rCs/RuO₂ (b), TGA curves of RuO₂, Cs/RuO₂ and rCs/RuO₂ hybrid (c).

Electrochemical properties analysis

Comparison of electrochemical properties of Cs and rCs. Before investigating the pseudocapacitive behaviours of the Cs/RuO₂ and rCs/RuO₂ hybrid, it is necessary to know the capacitance of the underlying Cs and rCs electrodes themselves. The galvanostatic charge–discharge measurements of Cs and rCs were carried out at a current density of 1 A g⁻¹ within a potential range from 0 to 1 V and the corresponding results are shown in Fig. 3. The specific capacitance (C_m) could be calculated by the following equation,^37,38

C_m = (I × △t)/(△V × m) (1)

where I (A) is the discharging current, △t (s) is the discharging time, △V (V) is the potential drop during discharge, and m (g) is the mass of the active materials. It is obviously noted that the rCs exhibit a considerable improvement in the specific capacitance compared to Cs. This reasonable result should be attributed to the fact that the rCs was prepared by thermal reduction in Ar atmosphere with significantly declined oxygen-containing groups. Moreover, it is concluded that a lower oxygen content of carbon-based material would be responsible for their better electronic conductivity and higher specific capacitance, which is in good agreement with our previous work.^27,38

Fig. 3 Galvanostatic charge–discharge curves of Cs and rCs.

Effect of Cs and rCs on the electrochemical properties of RuO₂. In order to study the effects of different templates of Cs and rCs on the capacitive behaviours of the Cs/RuO₂ and rCs/RuO₂ hybrid, cyclic voltammetry (CV) and galvanostatic charge–discharge techniques were applied to test their electrochemical properties. Fig. 4c exhibits the CV curve of Cs/RuO₂ hybrid measured in 1 M H₂SO₄ with a potential range of 0–1 V at scan rates of 10, 20, 50 and 100 mV s⁻¹, respectively. Observed from the tested CV curves, the rectangle shapes suggest good capacitive behaviour for Cs/RuO₂ hybrid. Furthermore, this rectangular CV curves do not change distinctly with the increase of scan rates demonstrating a satisfied electrical conductivity of the electrode material. In the case of rCs/RuO₂ hybrid, a much higher current of electrochemical response is observed from Fig. 4e. The CV curves of Cs/RuO₂ and rCs/RuO₂ hybrid exhibit more rectangular shape compared with that of pure RuO₂ (Fig. 4a), revealing that the Cs/RuO₂ and rCs/RuO₂ hybrid have better capacitive behaviour than pure RuO₂. The CV curves of the Cs/RuO₂ (Fig. 4c) and rCs/RuO₂ (Fig. 4e) composites imply that the capacitance of the material is mainly produced by the redox reactions as following:²⁷

RuO_x(OH)_y + δH⁺ + δe⁻ → RuO_x−δ(OH)_y+δ (2)

Fig. 4 Cyclic voltammograms of pure RuO₂ (a), Cs/RuO₂ (c) and rCs/RuO₂ (e) at different scan rates. Galvanostatic charge–discharge curves of pure RuO₂ (b), Cs/RuO₂ (d) and rCs/RuO₂ (f) at different current densities.

Fig. 4b displays the galvanostatic charge–discharge curves of pure RuO₂. The almost triangular shape indicates its ideal capacitive behaviour because of the high degree of symmetry in charge and discharge. The specific capacitance of pure RuO₂ electrode is 669, 630, 609, 560 and 492 F g⁻¹ at current densities of 1, 2, 5, 10 and 20 A g⁻¹, respectively. Fig. 4d exhibits the galvanostatic charge–discharge curves of Cs/RuO₂ hybrid. The specific capacitances of the Cs/RuO₂ electrode at current densities of 1, 2, 5, 10 and 20 A g⁻¹ are 387, 376, 365, 350 and 336 F g⁻¹, respectively. For rCs/RuO₂ hybrid, the galvanostatic charge–discharge curves have been shown in Fig. 4f. The specific capacitances of the rCs/RuO₂ electrode at current densities of 1, 2, 3, 5, 10 and 20 A g⁻¹ are 614, 596, 553, 522 and 491 F g⁻¹, respectively. The comparison of the specific capacitances of Cs/RuO₂, rCs/RuO₂ and pure RuO₂ are demonstrated in Fig. 5a. It can be clearly noted that the specific capacitance of RuO₂ in rCs/RuO₂ hybrid is also improved as high as 839 F g⁻¹ at 1 A g⁻¹ when the weight of only RuO₂ in this hybrid is used to calculate.

Fig. 5 Comparison of the specific capacitance change of pure RuO₂, Cs/RuO₂ and rCs/RuO₂ (a) electrodes as a function of scan rate. Comparison of the cycling stability of pure RuO₂, Cs/RuO₂ and rCs/RuO₂(b) at a current density of 5 A g⁻¹.

It can be clearly know that the specific capacitance of RuO₂ in Cs/RuO₂ hybrid is greatly improved up to 864 F g⁻¹ at 1 A g⁻¹ when the percentage of RuO₂ in hybrid is considered. Meanwhile, the results have shown that the capacity retention rate of the Cs/RuO₂ and rCs/RuO₂ hybrid is 86.8% and 80.0%, which is higher than pure RuO₂ (73.5%). This improvement of electrochemical properties is ascribed to the significantly enhanced utilization of RuO₂ achieved by the well-dispersed RuO₂ nanoparticles on Cs or rCs surface. It is known to all that an excellent cyclability of the utilized electrode material is a very important quality required for its application in supercapacitors. Thus, the Cs/RuO₂ and rCs/RuO₂ hybrids were tested to evaluate the cycle stability by galvanostatic charge–discharge measurements for 5000 cycles at a high current density of 5 A g⁻¹ within a voltage range between 0 and 1 V in 1 M H₂SO₄ solution. The cycling performances of Cs/RuO₂ and rCs/RuO₂ hybrid are presented in Fig. 5b, while the cycling property of pure RuO₂ is also displayed for comparison. The exhibited results could indicate the rather excellent cycling performances of Cs/RuO₂ and rCs/RuO₂ hybrid, from which the capacitances of Cs/RuO₂ and rCs/RuO₂ hybrid have retained 97.7% and 90.8% after 5000 cycles, respectively. Note that, the coulombic efficiency of Cs/RuO₂ and rCs/RuO₂ hybrid remains at almost 100% during the cycling process. However, the capacitance retention of pure RuO₂ after 5000 cycles is just 73.9%, as shown in Fig. 5b. Thereby, it could be concluded that the presence of Cs and rCs in the RuO₂-based hybrid would be able to greatly improve the cycling stability.

Conclusions

In summary, we have developed a simple sol–gel method to firstly synthesize Cs/RuO₂ and rCs/RuO₂ hybrid with uniform size distribution. The utilization of RuO₂ is found to be significantly enhanced by coating its nanoparticles onto the surface of Cs or rCs. The investigation for the electrochemical properties has clearly shown that the unique structure of Cs/RuO₂ and rCs/RuO₂ composites could achieve high specific capacitance, ultrahigh rate capability and excellent cycling stability. The specific capacitances of Cs/RuO₂ and rCs/RuO₂ hybrid could reach 336 F g⁻¹ and 491 F g⁻¹ at an ultrahigh current density of 20 A g⁻¹ in H₂SO₄ solution, respectively. They also have exhibit excellent capacity retention of 97.7% and 90.8% after 5000 charge–discharge cycles. These results demonstrate the potential use of Cs/RuO₂ and rCs/RuO₂ hybrid electrode materials for high-performance energy storage systems.

The work is financially supported by the National Natural Science Foundation of China (51134007, 21003161 and 21350110326), the Distinguished Young Scientists of Hunan Province (13JJ1004), the Program for the New Century Excellent Talents in University (NCET-11-0513), the Hunan Provincial Innovation Foundation for Postgraduate (CX2013B048), and the Open-End Fund for the Valuable and Precision Instruments of Central South University (CSUZC2013004).

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