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Abstract

In this article, the novel carbon materials (carbon nanotubes (CNTs) and graphene) with high-specific area and superior mechanical behavior are employed to strengthen the specific capacitance and cyclical stability of manganese dioxide (MnO₂) for supercapacitors. The electrode material, synthesized by the CNTs, reduced graphene oxide (RGO), and MnO₂ (CNTs-RGO-MnO₂ composite), is characterized by scanning electron microscope, transmission electron microscope, and X-ray powder diffraction. The results indicate that the MnO₂ particles are compactly distributed on the surface of the RGO and CNTs. The state of MnO₂ in the CNTs/RGO/MnO₂ composite is δ-MnO₂. The electrochemical test results indicate that the specific electric capacities of the CNTs/RGO/MnO₂ composite are 404 F/g, 255 F/g, and 82 F/g, respectively, at 1 A/g, 3 A/g, and 10 A/g ampere densities, which illustrate that the addition of the CNTs and RGO greatly hoists the specific capacitance of the MnO₂. Moreover, the long-time charge–discharge test results indicate that the specific capacitance of the CNTs/RGO/MnO₂ composite remains 70% after 5000 circles under the big current density of 30 A/g. The electrochemical impedance spectroscopy test results show that the RGO and CNTs can remarkably reduce the electric charge shifting resistance; at the same time, the electrolyte resistance and electric charge shifting resistance after the charge–discharge test are scarcely increasing, showing that the CNTs/RGO/MnO₂ composite is a kind of supercapacitor electrode material with stable structure, which has the prospect of industrialization.

Keywords

Graphene CNTs supercapacitor

Introduction

Supercapacitor is a new type of energy storage device researched extensively at present, which is widely used in electric energy storage and conversion because of its excellent properties such as high-specific capacitance, good cyclical stability, fast charge–discharge process, and so on.^1
–3 The electrode material of the supercapacitor is the key factor to determine the performance of the supercapacitor. At present, the electrode material of the supercapacitor mainly includes carbon-based electrode material, metal oxide electrode material, and conductive polymer electrode material.^{4

–13} Simple carbon-based electrode materials often have stable cycle performance and high coulomb efficiency, but the specific capacitance is low. The transition metal oxide and conducting polymer usually have high-specific capacitance, poor cycling stability, and low coulomb efficiency. Numerous researchers compound the carbon materials that have double-layer capacitor characteristics with the pseudocapacitive materials that have the faradaic pseudocapacitor characteristics (such as transition metal oxide and conductive polymer), to obtain the composite materials, which can effectively use the excellent properties of carbon materials theirselves, showing good electrochemical performance.^14

–17

As a kind of new carbon materials with high conductivity, large surface area, and good chemical stability, graphene has been widely studied in the preparation research for the composite electrode materials of supercapacitors.^18

–24 At present, a large number of research works use graphene as a carrier, which loads oxide semiconductor materials (such as tin oxide (SnO₂), zinc oxide (ZnO), etc.), metal and metal oxides (such as platinum (Pt), MnO₂, ruthenium oxide (RuO₂), etc.) on its surface, or compounds with conductive polymer materials to synthesize good performance materials.^18

–24 In these electrode materials, the composites prepared by graphene and manganese dioxide (G/MnO₂) with low-cost, low-pollution, and high-specific capacitance are considered to be the electrode material which has a good prospect of industrialization. Usually the graphene in the G/MnO₂ composite is employed as the skeleton material, which can effectively enhance the conductivity and the specific surface area of the G/MnO₂ composite, thus speeding up the charge–discharge proceed of the faradaic pseudocapacitor electrode materials and ensuring full contact between it and the electrolyte. However, practical research results show that the surface areas of the G/MnO₂ composite materials reunite largely in the process of preparation.^25
–27 Therefore, their specific surface areas are not very large and the excellent properties of graphene cannot be fully utilized. In addition, the mechanical properties of the prepared electrode materials are still poor.^25
–27

Carbon nanotubes (CNTs) have small density, high mechanical properties, good electrical properties, large specific surface area, and the hollow one-dimensional structure, which can be used as an ideal electrode material to form a large electrical double-layer capacitor.^28,29 Moreover, the CNTs can also be used as the carrier material of the composite electrode material, which increases the specific capacitance, the electric conductivity, and the mechanical strength of the electrode material.^30
–32 Therefore, we propose to use the composite synthesized by the CNTs, reduced graphene oxide (RGO) and MnO₂ (CNTs/RGO/MnO₂) using a hydrothermal method as a nano electrode material. The stable structure constructed by carbon materials close combined with MnO₂ to form a high pseudocapacitor channel, which can accelerate the speed of the ion/electron exchange between the material and the electrolyte, resulting in improving the specific capacitance and the cycling stability of the CNTs/RGO/MnO₂ material. In this article, the surface morphology and structure of the CNTs/RGO/MnO₂ composite are characterized by a scanning electron microscope (SEM), a transmission electron microscope (TEM), and X-ray powder diffraction (XRD); and the electrochemical performance of the CNTs/RGO/MnO₂ composite is analyzed using an electrochemical workstation.

Experimental

Preparation of the RGO

The graphene oxide is prepared by a modified Hummers method. Typical procedures are as follows: first, 50 g expandable graphite, 25 g sodium nitrate, and 500 mL sulfuric acid are added to a 2-L beaker. The beaker is then put in a water bath, in which the temperature is fixed at 0^oC, to prevent the reaction temperature from reaching 10^oC. In 1 h, 300 g potassium permanganate is added 6 times to the mixture in under gentle stirring. After that, the temperature of the mixture is increased to 35^oC. After holding for 30 min, 920 mL distilled water is slowly dropped in the beaker. After further stirring for 1 h, the solution is set quietly for 12 h. After the mixture is maintained at 98^oC for 1 h, it is filtrated. A graphene oxide can be obtained, after the filter residue is dried.

In a flask, 0.65 g graphene oxide prepared above is dispersed in 500 mL distilled water by ultrasonic agitation for 30 min. Then, the pH of the mixture is adjusted to 9 by ammonia water. After that, tiny amounts of hydrazine hydrate are added to the flask. Afterward, the suspension is boiled for 1 h. Then, the mixture is directly neutralized with the proper amount of hydrochloric acid (HCl) solution. Afterward, the mixture is filtrated. Then, the filter residue is dried in a vacuum oven for 12 h at 80°C. The RGO is obtained after the dry black solid powder is annealed at 350^oC in Argon atmosphere for 90 min.

Preparation of the CNTs/RGO/MnO₂ composite

The CNTs/RGO/MnO₂ composite is prepared according to the following procedures: first, 25 mg RGO prepared above is dispersed in 80 mL potassium permanganate solution (1 M). Then, 50 mg CNTs (XFM07) is added into the suspension. After 5 min, it is dispersed by ultrasonic agitation for 5 min. Thereafter, the mixture is transferred to a hydrothermal reaction kettle, which is placed in an oven at 160^oC for 5 h. After cooling, the obtained solution is filtrated and washed fully. The CNTs/RGO/MnO₂ composite is obtained after the filter residue is dried in a vacuum oven at 80^oC for 12 h.

Characterization of electrode materials

The morphology of the RGO is investigated using a HITACHI S-4800 SEM and a Tecnai G2 F20 TEM. The TEM image of the CNTs/RGO/MnO₂ composite is also obtained using a Tecnai G2 F20 TEM. XRD data of the RGO, MnO₂, and CNTs/RGO/MnO₂ composite are collected using a Rigaku Dmax 2400 X-ray diffractometer.

Electrochemical test

The electrochemical performance of the CNTs/RGO/MnO₂ composite for supercapacitor is investigated in 2 M Na₂SO₄ solution using a CHI 760E electrochemical workstation (Shanghai, China) at the ambient temperature (∼25^oC). The pH of the electrolyte is adjusted to 9–10 by potassium hydroxide (KOH) solution. The working electrode is fabricated according to the method described in literature.^33
–35 In brief, 20 μL composite ink, prepared by mixing 20 mg tested electrode material, 5 mg carbon black, 4 mL ethanol, 30 μL PTFE emulsion, and 30 μL Nafion solution (5 wt%, DuPont Corp,. USA), is placed on a polished glassy carbon working electrode (Φ = 5 mm) using a microsyringe. A silver/silver chloride (Ag|AgCl) electrode and a sheet of Pt (2 cm²) are, respectively, employed as the reference electrode and counterelectrode.

Results and discussion

Electrode materials characterization

The SEM and TEM images of the RGO are shown in Figure 1a and b. Figure 1c displays the TEM image of the CNTs/RGO/MnO₂ composite. It can be seen from the SEM image that the prepared graphene is composed of several layers and the chip thickness is about 5–50 nm. According to Figure 1b, there is a large amount of folding on the surface of the RGO, because the overlap between graphene layers or the transformation from the two-dimensional (2D) to three-dimensional (3D) structures of graphene with few layers can decrease the surface energy.^36,37 As the result of the formation of a large amount of folding, the specific surface area of the RGO is decreased and the good physical and chemical properties of the RGO are weakened. Figure 1c indicates that the microstructure of the CNTs/RGO/MnO₂ composite is regular, and the MnO₂ nanoparticles closely load on the surface of the 3D carbon material constructed by the RGO and CNTs.

Figure 1.

SEM (a) and TEM (b) images of the RGO. (c) The TEM image of the CNTs/RGO/MnO₂ composite. SEM: scanning electron microscope; TEM: transmission electron microscope; RGO: reduced graphene oxide; CNTs: carbon nanotubes; MnO₂: manganese dioxide.

Figure 2 displays the XRD patterns of the pure MnO₂, RGO, and CNTs/RGO/MnO₂ composite. A wide diffraction peak appears between 24° and 26° (corresponding layer spacing is 3.4 Å–3.7 Å) in the RGO sample, among which the strong peak at 2θ = 26° is [002] diffraction peaks of graphite emerges. Meanwhile, the [001] diffraction peaks of the graphite oxide do not arise, showing clearly that the content of the oxygen groups has been significantly reduced in the RGO. Other miscellaneous peaks are considered as the residual intercalation compounds. Moreover, the strong diffraction peaks of the CNTs/RGO/MnO₂ composite and pure MnO₂ are located at 2θ = 12°, 25.7°, 36.8°, and 66°, which are corresponding to that of the MnO₂ standard card (JCPDS No. 18-0802). According to Figure 2, the MnO₂ in the CNTs/RGO/MnO₂ composite exists as δ-MnO₂ state of the birnessite structure, which has low crystallinity.

Figure 2.

XRD patterns of the pure MnO₂, RGO, and CNTs/RGO/MnO₂ composite. XRD: X-ray powder diffraction; MnO₂: manganese dioxide; CNTs: carbon nanotubes; RGO: reduced graphene oxide.

Electrochemical study

Figure 3a shows the cyclic voltammograms (CVs) of the CNTs/RGO/MnO₂ composite at different scanning rates. The rectangle presented in the CV at 2 mv/s scanning rate indicates that the CNTs/RGO/MnO₂ composite has the characteristics of the double-layer capacitor. Rectangular structures in CVs continue to expand along with the increase in the scanning rate. Meanwhile, their shapes and symmetries are deteriorated, mainly because of the increased influence of the internal resistance for the electrode material on the CV, resulting in the rectangular structure continuing to expand and deform, which indicates that the charge–discharge reversibility of the CNTs/RGO/MnO₂ composite depresses at high sweep rate. The specific capacitance of the material is represented by the rectangular integral area in the CVs. Figure 3b represents the CVs of the pure MnO₂, RGO, and CNTs/RGO/MnO₂ composite at 200 mv/s. It can be seen from Figure 3b that the rectangular area of the pure MnO₂ (19.3 A·V/g), RGO (18.6 A·V/g), and CNTs/RGO/MnO₂ composite (19.4 A·V/g) is nearly equal, which indicates that the specific capacitance of the CNTs/RGO/MnO₂ composite is hardly improved at the scan rate of 200 mv/s, compared to that of the pure MnO₂. However, the CV of the CNTs/RGO/MnO₂ composite has not an obvious redox peak, which indicates that the cycling stability of the CNTs/RGO/MnO₂ composite is good.

Figure 3.

(a) CVs of the CNTs/RGO/MnO₂ composite at different scanning rate. (b) CVs of the pure MnO₂, RGO, and CNTs/RGO/MnO₂ composite at 200 mv/s. CVs: cyclic voltammograms; CNTs: carbon nanotubes; RGO: reduced graphene oxide; MnO₂: manganese dioxide.

Figure 4a exhibits the galvanostatic charge–discharge (GCD) curves of the pure MnO₂, RGO, and CNTs/RGO/MnO₂ composite at 1 A/g current density. The GCD curves of the CNTs/RGO/MnO₂ composite at different current densities are shown in Figure 4b. Figure 4a represents that the specific electric capacities of the RGO, pure MnO₂, and the CNTs/RGO/MnO₂ composite reach 66 F/g, 282 F/g, and 404 F/g, respectively. The specific capacitance of the CNTs/RGO/MnO₂ composite is greater than that of the MnO₂ or RGO, because the 3D carbon structure constituted by the 2D planar structure of the graphene and the tubular structure of the CNTs can prevent the agglomeration of the RGO in liquid-phase reaction.³⁸ Simultaneously, the 3D carbon structure provides the load platform for MnO₂ without reducing the conductivity of the carbon materials, which greatly improves the efficiency of the Faraday charge transfer. The specific electric capacities of the CNTs/RGO/MnO₂ composite reach 404 F/g, 255 F/g, and 82 F/g, respectively, at 1 A/g, 3 A/g, and 10 A/g current densities, according to the calculation of GCD curves from Figure 3b. Generally, the end of discharge curve in the GCD curve of the pseudocapacitor material at low current density appears to have certain bending, which is mainly caused by the material internal oxidation-reduction reaction.³⁸ For the CNTs/RGO/MnO₂ composite, the GCD curve at low current density (0.3 A/g) still maintains good symmetry, suggesting that the material has good charge–discharge reversibility.

Figure 4.

(a) GCD curves of the pure MnO₂, RGO, and CNTs/RGO/MnO₂ composite at 1 A/g current density. (b) GCD curves of the CNTs/RGO/MnO₂ composite at different current densities. GCD: galvanostatic charge–discharge; CNTs: carbon nanotubes; RGO: reduced graphene oxide; MnO₂: manganese dioxide.

Figure 5 indicates the comparative cycling performance of the pure MnO₂ and CNTs/RGO/MnO₂ composite at the current density of 30 A/g for 5000 circles. Figure 5 displays that the specific capacitance retention rate of the pure MnO₂ is only 32%, while that of the CNTs/RGO/MnO₂ composite reaches 70%, which implies that the addition of the RGO and CNTs not only increases the capacitance of the composite but also greatly improves its cycling stability. The excellent cycling stability is attributed to the 3D carbon structure constituted by the RGO and CNTs. First, this structure provides a platform for the uniform loading MnO₂. Second, the high conductivity and stable chemical property of the carbon material can avoid the continuous accumulation of the Faraday charge in the composite materials, which increases its capacitance as well as enhances the electrochemical stability of materials.³⁹

Figure 5.

Cycle curves of the pure MnO₂ and CNTs/RGO/MnO₂ composite at the current density of 30 A/g for 5000 circles. CNTs: carbon nanotubes; RGO: reduced graphene oxide; MnO₂: manganese dioxide.

The electrochemical impedance spectroscopy (EIS) contrast curves of the CNTs/RGO/MnO₂ composite before and after 5000 charge–discharge cycles at 30 A/g are displayed in Figure 6. The Nyquist complex-plane impedance diagrams of the CNTs/RGO/MnO₂ composite are recorded from 100 KHz to 10 mHz. The EIS curves in the high-frequency zone are shown in Figure 6b, which shows that both semicircles are almost coincident, illustrating that the electrolyte resistance and charge transfer resistance of the CNTs/RGO/MnO₂ composite after 5000 charge–discharge cycles under high current density have no change. Moreover, the electrolyte resistance for the CNTs/RGO/MnO₂ composite before and after charge–discharge test is estimated at about 5.9 Ω, while the charge transfer resistance is about 3.7Ω. Meanwhile, the slope of the EIS curve in the low-frequency region is decreased after 5000 charge–discharge cycles under the large current density of 30 A/g, suggesting that the Warburg resistance of the CNTs/RGO/MnO₂ composite is increased, due to the collapse of the internal structure of the CNTs/RGO/MnO₂ composite during charge–discharge process.^40,41

Figure 6.

(a) EIS curves of the CNTs/RGO/MnO₂ composite before and after 5000 charge–discharge cycles at 30 A/g. (b) Enlarged EIS curves of the CNTs/RGO/MnO₂ composite in the high-frequency zone. EIS: electrochemical impedance spectroscopy; CNTs: carbon nanotubes; RGO: reduced graphene oxide; MnO₂: manganese dioxide.

Conclusion

In this article, the CNTs/RGO/MnO₂ electrode material for supercapacitor is prepared by compositing the CNTs, RGO, and MnO₂. The high conductivity of the RGO and the big specific surface area and mechanical strength of the CNTs are made full use of to improve the electrical conductivity, specific capacitance, and cycling stability of the MnO₂. It is found from SEM and TEM that MnO₂ nanoparticles are grown uniformly on the surface of the CNTs and RGO, closely integrating each other. The specific capacities of the CNTs/RGO/MnO₂ composite at 1 A/g, 3 A/g, and 10 A/g current densities reach 404 F/g, 255 F/g, and 82 F/g, respectively. Furthermore, the GCD curves display that CNTs/RGO/MnO₂ composite has good charge–discharge reversibility and that the RGO and CNTs have a reinforcing effect on the specific capacitance of MnO₂. The charge–discharge cycling test at the high current density of 30A/g for 5000 cycles exhibits that the CNTs/RGO/MnO₂ composite retains 70% of the initial specific capacitance, and the cycle stability of CNTs/RGO/MnO₂ composite is much higher than that of the pure MnO₂ under the same conditions. The addition of the RGO and CNTs can significantly decrease the charge transfer resistance and Warburg resistance of MnO₂. Moreover, the charge transfer resistance and electrolyte resistance for the CNTs/RGO/MnO₂ composite are not significantly increased after long period of charge–discharge test at high current density, which shows that the structure of the CNTs/RGO/MnO₂ composite is stable. The unique 3D structure of CNTs/RGO/MnO₂ composite could prevent the charges from accumulating, thereby decelerating the change in the structural conformation of MnO₂ with repeated ion exchange, which is highly desirable to improve the electrochemical cycling stability and achieve rapid charge−discharge characteristics at high discharge current densities. Therefore, the CNTs/RGO/MnO₂ composite has the potential to be used as a supercapacitor electrode material.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is supported by the Natural Science Foundation of Shanxi (2014011017-2) and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (2014113).

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Electrochemical performance of CNTs/RGO/MnO 2 composite material for supercapacitor

Abstract

Keywords

Introduction

Experimental

Preparation of the RGO

Preparation of the CNTs/RGO/MnO2 composite

Characterization of electrode materials

Electrochemical test

Results and discussion

Electrode materials characterization

Electrochemical study

Conclusion

Footnotes

Declaration of conflicting interests

Funding

References

Preparation of the CNTs/RGO/MnO₂ composite