RuO₂@Co₃O₄ heterogeneous nanofibers: a high-performance electrode material for supercapacitors

Abstract

Novel RuO₂@Co₃O₄ heterogeneous nanofibers (HNFs) were synthesized by a simple electrospinning method, followed by calcination. The possible mechanism of formation of RuO₂@Co₃O₄ HNFs is presented. The synergy between RuO₂ and Co₃O₄ and the unique feature of heterostructure allow the composite to exhibit a high reversible capacity of 1103.6 F g⁻¹ at a current density of 10 A g⁻¹ and a high-rate capability of 500.0 F g⁻¹ at a current density of 100 A g⁻¹. Moreover, excellent cycling performance was observed, where the capacities could be maintained at 93.0%, 91.8%, and 88.0% after 1000, 2000 and 5000 cycles, respectively, at a current density of 10 A g⁻¹. This study therefore confirms that the as-prepared RuO₂@Co₃O₄ HNFs can serve as advanced supercapacitor (SCs) materials. It is highly anticipated that this simple method of electrospinning can be extended to preparing other new types of heterostructural materials that could be used in the field of electrochemical energy storage.

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1. Introduction

The demand for rechargeable battery systems is increasing, due to the growing requirement for electrical grid storage to convert the intermittent renewable sources (such as solar and wind) into electricity.^1,2 Supercapacitors (SCs), a new class of energy storage devices, have attracted much attention for their high power density, fast charging/discharging rate, long cycle life and high reliability.^3–7 However, the energy density of SCs is much lower than that of other batteries (e.g. Li-ion batteries).^8,9 The energy density (E) of SCs depends on the cell potential (V) and capacitance (C); E = 2⁻¹CV².^10–14 As we know, the parameters (V and C) are dominated mainly by electrode properties.¹⁵ To date, many metal oxides, such as RuO₂,¹⁶ MnO₂,^17,18 Co₃O₄ (ref. 19 and 20) and NiO²¹ have been studied as the most promising candidates for electrode materials in the SCs because they can provide higher energy densities. Among the metal oxides, RuO₂ is of significant interest as an electrode material, due to its high energy storage capabilities with large specific capacitance.^22,23 However, RuO₂ is not available in abundance, and this coupled with its prohibitive price, inhibit its commercial applications in SCs.^24–26

To solve this problem, we would need to lower the cost of the RuO₂ electrode. In recent years, a number of promising attempts utilizing the combination of two kinds of metal oxides as electrode materials in SCs have been developed. Pang et al.²⁷ described the fabrication of the Mn₃O₄@Co₃O₄ nanocubic composite for SCs with a specific capacitance of 443 F g⁻¹ at 1.5 A g⁻¹. Youn et al.²⁸ reported RuO₂@Mn₃O₄ composite nanofiber mats for SCs with a specific capacitance of 293 F g⁻¹ at 10 mV s⁻¹. Li et al.²⁹ demonstrated the preparation of RuO₂/Co₃O₄ thin films for SCs with a specific capacitance of 690 ± 14 F g⁻¹ at 0.2 A g⁻¹. Liu et al.³⁰ reported Co₃O₄/RuO₂·xH₂O composites for electrochemical capacitors with a specific capacitance of 642 F g⁻¹. Apparently, the combination of RuO₂ with the appropriate metal oxide as an electrode material lowers the cost of the electrode. Besides, compared with single-phase oxides, multiple composite oxides have the advantages of high surface area and fast electron/electrolyte diffusion paths for charge storage and delivery.^31,32 Among the various metal oxides, because of the outstanding electrochemical behavior, good corrosion stability and abundance, Co₃O₄ is considered as a promising electrode material in SCs.^33–35 RuO₂@Co₃O₄ electrodes prepared by the spray pyrolysis method and the thermal decomposition method were studied in alkaline solutions and showed good electrochemical behavior.^29,30

Electrospinning is known to be a simple and facile technique, and one-dimensional (1D) nanostructures, such as nanofibers (NFs) and nanotubes (NTs) with controllable diameters and morphologies can be easily fabricated.^36,37 The electrospun nano materials have a lot of unique features, such as high surface area, extraordinary length, and porous structure, which are important determining factors of the performance of SCs.^38,39 Thus, we designed and rationally synthesized RuO₂@Co₃O₄ heterogeneous nanofibers (HNFs), based on the electrospinning method, and expected to provide a promising candidate for electrode materials in the SCs.

In this paper, we report a simple electrospinning strategy to prepare the novel RuO₂@Co₃O₄ HNFs and investigate their electrochemical properties as electrode materials for SCs. Additionally, a possible formation mechanism of RuO₂@Co₃O₄ HNFs is proposed, based on a series of temperature-dependent experiments during the process of calcination (350, 450 and 500 °C). Compared with previous RuO₂@Co₃O₄ composites for SCs, our prepared RuO₂@Co₃O₄ HNFs exhibit a remarkable cycling ability of 970.5 F g⁻¹ after 5000 cycles at the current density of 10 A g⁻¹ (88.0% of the initial capacitance). Even when the current density reached 100 A g⁻¹, the reversible capacity of the RuO₂@Co₃O₄ HNFs can still maintain 500.0 F g⁻¹. Because of the synergistic effect between RuO₂ and Co₃O₄, as well as the unique feature of heterostructure, the electrochemical performance of the electrode has been greatly enhanced.

2. Experimental section

All the chemicals and solvents were of analytical grade and were used without further purification. Ruthenium chloride hydrate (RuCl₃·3H₂O) was obtained from Beijing HWRK Chemical Company Limited, and cobaltous nitrate (Co(NO₃)₂·6H₂O) was obtained from Tianjin Chemical Corp., China. Polyvinylpyrrolidone (PVP; M_w ≈ 1 300 000) was purchased from Sigma-Aldrich.

2.1. Preparation of 1D RuO₂@Co₃O₄ HNFs by electrospinning

In a typical synthesis, 3.3 g absolute ethanol and 3.3 g N,N-dimethyl formamide were mixed as solvent. Then, 0.104 g ruthenium chloride hydrate (RuCl₃·3H₂O) and 0.291 g cobaltous nitrate (Co(NO₃)₂·6H₂O) were dissolved in the as-prepared solvent by magnetic stirring for 1 h at room temperature. The atomic ratio (Ru : Co) was about 1 : 2.5. After complete dissolution of RuCl₃·3H₂O and Co(NO₃)₂·6H₂O, 0.6 g polyvinyl pyrrolidone (PVP; M_w ≈ 1 300 000) was added to the resulting solution and stirred for 3 h at room temperature. The obtained solution was then loaded into a glass syringe with a pinhead. The distance between the pinhead and the collector was 18 cm and the applied voltage was 20 kV. The as-electrospun RuCl₃@Co(NO₃)₂@PVP composite NFs were placed in a vacuum oven for 24 h at 70 °C. The spun fibers were then annealed at 500 °C for 2 h with a heating rate of 3 °C min⁻¹ in air.

2.2. Synthesis of the RuO₂@Co₃O₄ HNFs electrode

The RuO₂@Co₃O₄ HNFs electrode was prepared by the following process. 80 wt% of RuO₂@Co₃O₄ HNFs, 10 wt% of carbon black and 10 wt% of polytetrafluoroethylene (PTFE, Aldrich) were mixed together. We used the Ni foam as an electrode and pressed the slurry onto it; finally, the RuO₂@Co₃O₄ HNFs electrode was dried at 70 °C in a vacuum oven for 24 h.

2.3. Characterization

The morphologies and the energy dispersive spectroscopy (EDS) mapping of the obtained samples were characterized by a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800) equipped with an X-ray energy dispersive spectrometer. The crystal structures of the samples were characterized by X-ray diffraction (XRD, Philips, X'pert pro, Cu Ka, 0.154056 nm). Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) investigations were carried out by a JEOL JEM-2100F microscope. In addition, X-ray photoelectric spectroscopy (XPS, ESCALAB 220i-XL, VG Scientific) was employed to analyze the surface chemical structures of the RuO₂@Co₃O₄ HNFs.

2.4. Electrochemical measurements

The electrochemical measurements were carried out in a three electrode electrochemical workstation with 2 M KOH aqueous solution as the electrolyte. A standard calomel electrode was used as the reference electrode and a Pt foil as the counter electrode. The RuO₂@Co₃O₄ HNFs electrode was used as the work electrode. The mass of the active materials in the electrode was about 1 mg. The cyclic voltammetry (CV) measurements, galvanostatic charge/discharge tests and electrochemical impedance spectroscopy (EIS) were carried out using the CHI660D electrochemical workstation (Chenghua, Shanghai). Electrochemical impedance spectroscopy (EIS) measurements were performed at a frequency range of 0.01 Hz to 100 kHz. The specific capacitance (C) was calculated according to the following formula:
where I is the discharge current, Δt is the discharge time, ΔV refers to the discharge range, and m represents the mass of the active electrode materials within the electrode.

3. Results and discussion

Fig. 1a shows the typical FE-SEM image of the as-spun RuCl₃@Co(NO₃)₂@PVP NFs before annealing, which have a regular diameter size in the range of 80–160 nm. Fig. 1b shows the FE-SEM image of the as-obtained RuO₂@Co₃O₄ HNFs after thermal treatment at 500 °C for 2 h, with a heating rate of 3 °C min⁻¹ in air. It is obvious that the diameter of the nanofibers decreased and lots of particles were formed on the NFs. The average diameter of the NFs after annealing was 50 nm. Fig. S1† shows the FE-SEM image and corresponding EDS mapping of the RuO₂@Co₃O₄ HNFs, revealing that mainly Ru element was distributed in the particles, and Co element was mainly distributed in the nanofibers; O element was distributed throughout the entire area. According to the EDS mapping, we confirmed that most of the RuO₂ crystals formed the particles and loaded on the surface of the nanofibers (mainly consisting of Co₃O₄). For comparison, Fig. S2† shows the FE-SEM image of pure Co₃O₄ NFs, which are continuous and have a regular diameter size in the range of 70–100 nm. To further study the morphology and structure of the as-prepared RuO₂@Co₃O₄ HNFs, the samples were examined by TEM and XRD. Fig. 1c is the TEM image of an individual RuO₂@Co₃O₄ heterogeneous nanofiber (HNF). As shown in Fig. 1c the particle is deposited on the fiber, which agrees well with the FE-SEM observation. Insets in Fig. 1c show the HRTEM images of the RuO₂@Co₃O₄ HNFs, which provide deep insight into the structural features of the as-prepared products. Fig. 1d is a HRTEM image of the particles on the fiber. The lattice fringes show the structural characteristics of the RuO₂ crystal, in which the d-spacing of 0.32 nm corresponds to the distance of the (110) plane. Fig. 1e and f are HRTEM images of the fiber. The lattice fringes with interplane spacing of 0.47 and 0.32 nm correspond to the (111) plane of Co₃O₄ and (110) plane of RuO₂, respectively. Fig. 1g shows the XRD patterns of the as-prepared RuO₂@Co₃O₄ HNFs. The diffraction lines are in good agreement with Co₃O₄ (JCPDS No. 43-1003) and RuO₂ phases (JCPDS No. 43-1027), indicating that the crystalline structures of Co₃O₄ and RuO₂ were formed after annealing. The diffraction values at 2 theta of 19.00, 31.27, 36.85, 38.55, 44.81, 55.66, 59.35, 65.23 and 77.33° correspond to (111), (220), (311), (222), (400), (422), (511), (440) and (533) crystal plane reflections of Co₃O₄, respectively. The diffraction values at 2 theta of 28.02, 35.07, 40.04, 40.55, 54.27, 57.92, 59.45, 65.56, 66.98, 69.54 and 74.10° are in good agreement with (110), (101), (200), (111), (211), (220), (002), (310), (112), (301) and (202) crystal plane reflections of RuO₂, respectively. The XRD patterns of pure Co₃O₄ NFs are shown in Fig. S3† and all the peaks were readily indexed to the Co₃O₄ (JCPDS No. 43-1003). Fig. S4a† is the XPS general spectra of the RuO₂@Co₃O₄ HNFs. As shown in Fig. S4b,† there are two XPS signals of Ru 3p at binding energies around 462.5 eV (Ru 3p_3/2) and 485.2 eV (Ru 3p_1/2), indicating the Ru⁴⁺ bonding in RuO₂.⁴⁰ The peaks at 780.2 and 795.6 eV, correspond to the Co 2p_3/2 and Co 2p_1/2 spin–orbit peaks of Co₃O₄ (Fig. S4c†).⁴¹ In addition, there are two peaks can be fitted to the O 1s (Fig. S4d†), attributed to the Co–O and Ru–O bonding, respectively.

Fig. 1 (a) FE-SEM image of the RuCl₃@Co(NO₃)₂@PVP NFs; (b) FE-SEM image of the RuO₂@Co₃O₄ HNFs after annealing; (c) TEM image of the RuO₂@Co₃O₄ HNFs; (d–f) HRTEM images of the RuO₂@Co₃O₄ HNFs; (g) XRD patterns of the as-prepared RuO₂@Co₃O₄ HNFs.

In order to further understand the formation mechanism of the RuO₂@Co₃O₄ HNFs (Fig. 2e), a series of temperature-dependent experiments during the process of calcination (350, 450 and 500 °C) was conducted and observed by FE-SEM (Fig. 2a–d). Fig. 2a shows the FE-SEM image of the as spun fibers before annealing, revealing their morphology of ordinary NFs. When the temperature increased to 350 °C, the diameter of the NFs was decreased and particle-like structures were formed on each nanofiber (Fig. 2b). This was probably due to the decomposition of PVP and crystallization of RuO₂ (oxidation of RuCl₃), deposited on the NFs; the NFs mainly consisted of Co₃O₄ (oxidation of Co(NO₃)₂).⁴² Fig. S5† shows the XRD of the spun fibers calcined at 350 and 450 °C, respectively, indicating the formation of RuO₂ and Co₃O₄. As the temperature continued to increase, the diameter of the NFs got smaller and the particle sizes got bigger (Fig. 2c and d). This was attributed to the coalescence of the small crystal grains of RuO₂. After the final calcination at 500 °C for 2 h, most of the crystal grains of RuO₂ deposited on the NFs, which were composed of Co₃O₄ and the rest of RuO₂, confirmed by EDS mapping and HRTEM images (Fig. S1† and 1d–f). To probe how the concentration of the precursor solution affected the morphology of the RuO₂@Co₃O₄ HNFs, we prepared the RuO₂@Co₃O₄ HNFs under the same conditions, except the high atomic ratio of Ru : Co in the precursor solution (about 1 : 1). Fig. S6† gives the FE-SEM image of the RuO₂@Co₃O₄ HNFs (high atomic ratio of Ru : Co), which shows larger particles, compared to the RuO₂@Co₃O₄ HNFs of low atomic ratio of Ru : Co.

Fig. 2 Formation mechanism of RuO₂@Co₃O₄ HNFs. FE-SEM images showing the morphology evolution during the process of calcination at three different temperatures: (a) the spun fibers; (b) calcined at 350 °C; (c) calcined at 450 °C; (d) calcined at 500 °C. (e) Schematic of the formation process of RuO₂@Co₃O₄ HNFs.

To investigate the electrochemical capacitive performance of the RuO₂@Co₃O₄ HNFs, a series of electrochemical measurements were carried out in a three-electrode cell with 2 M KOH as the electrolyte. The electrode using Co₃O₄ nanofibers was used for comparison. Fig. 3a shows the cyclic voltammograms (CVs) of the RuO₂@Co₃O₄ HNFs from 0 to 0.6 V (vs. SCE) at different scan rates of 5, 10, 20, 40, 60, 80 and 100 mV s⁻¹. The shapes of the CV curves are considerably different from an ideal rectangular shape. Besides, the distinct oxidation peak at about 0.52 V and the reduction peak at 0.38 V are observed when the scan rate is 5 mV s⁻¹, indicating that the capacitance characteristics are mainly governed by pseudocapacitance, which is caused by the faradaic redox reactions of the electroactive material.^43,44 With the increase of the scan rate, the anodic peaks shifted towards positive potential and the cathodic peaks shifted towards negative potential which was caused by the polarization of the electrode under high scan rates.^45,46

Fig. 3 Electrochemical characterizations of RuO₂@Co₃O₄ HNFs: (a) CV curves of RuO₂@Co₃O₄ HNFs at various scan rates; (b) galvanostatic charge/discharge curves of the RuO₂@Co₃O₄ electrode at various discharge current densities; (c) galvanostatic charge/discharge curves of the Co₃O₄ electrode at various discharge current densities; (d) specific capacitance of the RuO₂@Co₃O₄ and Co₃O₄ electrode at different current densities.

Fig. 3b displays the galvanostatic charge and discharge curves of the RuO₂@Co₃O₄ HNFs between 0.2 and 0.55 V in 2 M KOH solution at current densities ranging from 5 to 100 A g⁻¹. Fig. 3c displays the galvanostatic charge and discharge curves of the Co₃O₄ NFs. The specific capacitances of the RuO₂@Co₃O₄ HNFs calculated according to the equation above are 1375.0, 1167.9, 1007.1, 757.1, 664.3, 571.4, 500.0 F g⁻¹ at the current densities of 5, 10, 20, 40, 60, 80, 100 A g⁻¹, respectively, as presented in Fig. 3d. However, the specific capacitances of Co₃O₄ nanofibers are 788.2, 636.2, 540.4, 454.5, 392.6, 340.1, 312.2 F g⁻¹ at the current densities of 5, 10, 20, 40, 60, 80, 100 A g⁻¹, respectively (shown in Fig. 3d).

The cycling stability of the RuO₂@Co₃O₄ HNFs and pure Co₃O₄ electrodes were investigated at current density of 10 A g⁻¹. As shown in Fig. 4a, the specific capacitance was about 1103.6 F g⁻¹ in the beginning and it decreased to 970.6 F g⁻¹, with 88.0% retention after 5000 cycles. In addition, the FE-SEM image of the RuO₂@Co₃O₄ HNFs after the cycling test (Fig. S7†) shows that there was little change in morphology before and after the cycling test. Fig. 4b shows the high rate capability of the RuO₂@Co₃O₄ HNFs. The specific capacitances of the electrodes were calculated for 500 cycles at the current densities of 10, 20, 40, 60, 80 and 100 A g⁻¹, as 1078.6, 978.6, 728.6, 664.3, 571.4 and 500.0 F g⁻¹, respectively. A recovered specific capacitance of 1021.4 F g⁻¹ (94.7% of the initial capacitance) was calculated for the following 500 cycles, without a noticeable decrease when the current density decreased to 10 A g⁻¹. On the other hand, the specific capacitance of the Co₃O₄ nanofibers were only maintained at 636.2, 540.0, 443.2, 392.5, 340.1 and 312.5 F g⁻¹ at the current densities of 10, 20, 40, 60, 80 and 100 A g⁻¹, respectively (Fig. 4b). The results imply that the combination of RuO₂ and Co₃O₄ obtained by electrospinning greatly enhanced the electrochemical properties of SCs.

Fig. 4 (a) Cycling performance of the RuO₂@Co₃O₄ HNFs and Co₃O₄ nanofibers at a current density of 10 A g⁻¹; (b) cycling performance of the RuO₂@Co₃O₄ HNFs and Co₃O₄ nanofibers at gradually increasing current densities.

The improved performance of RuO₂@Co₃O₄ HNFs electrodes can be attributed to the following aspects. First, the unique feature of the heterostructure of the RuO₂@Co₃O₄ HNFs leads to a high specific surface area, which increases the electrolyte/electrode contact areas and reaction surface area. Thus, the electrodes can generate more active sites for fast faradaic redox reactions. Secondly, the enhanced capacity of the RuO₂@Co₃O₄ HNFs compared with the bare Co₃O₄ nanofibers can be interpretated by the addition of a new capacitive component of RuO₂ and the synergistic effect between RuO₂ and Co₃O₄. The as-formed Co₃O₄ NFs provide a scaffold for the growth of RuO₂ particles, which can avoid conventional aggregation and ensure sufficient ion diffusion. On the other hand, the deposited RuO₂ possesses high specific capacitance, leading to the high specific capacitance of the RuO₂@Co₃O₄ composite material.

Fig. 5 shows the electrochemical impedance spectra (EIS) for the first and the 5000th cycle of the RuO₂@Co₃O₄ HNFs electrode. At the high frequency region, both of the curves display a negligible semicircle, which indicates the charge transfer resistance is low. Apparently, there are two differences between the spectra. Firstly, in the high frequency area, the RuO₂@Co₃O₄ HNFs electrode has larger internal resistances after 5000 cycles, revealing that the interface charge transfer of the RuO₂@Co₃O₄ HNFs electrode shows a slight tendency to decline. Secondly, at lower frequencies, the diffusive resistance of the RuO₂@Co₃O₄ HNFs electrode after 5000 cycles is a little higher, which indicates that the electrolyte penetration and ion diffusion in the host materials have not reduced notably.

Fig. 5 Electrochemical impedance spectra for the first and the 5000th cycle of the RuO₂@Co₃O₄ HNFs electrode. Inset is the expanded view.

4. Conclusion

In summary, a facile strategy has been developed to prepare RuO₂@Co₃O₄ HNFs with high electrochemical performance for SCs. The as-prepared RuO₂@Co₃O₄ HNFs electrode shows a high specific capacitance of 1103.6 F g⁻¹ at 10 A g⁻¹, with good cycling performance (88.0% of the capacity retention after 5000 cycles) and desirable rate performance (500.0 F g⁻¹ at a current density of 100 A g⁻¹). Such excellent capacitive behavior is attributed to the synergetic effect between RuO₂ and Co₃O₄, as well as the unique feature of heterostructure. Our results demonstrate that the simple method of electrospinning could also be extended to preparing other noble metal oxides@transition metal oxides heterogeneous structures with promising applications in the field of electrochemical energy storage.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No. 21303046), the Research Fund for the Doctoral Program of Higher Education (No. 20130161120014), China Scholarship Council (File No. 201308430178), Hunan University Fund for Multidisciplinary Developing (No. 531107040762), Hunan Provincial Innovation Foundation for Postgraduate (No. 521293041).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04884f

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