Hydrothermal synthesis of MnO2/CNT nanocomposite with a CNT core/porous MnO2 sheath hierarchy architecture for supercapacitors
© Xia et al.; licensee Springer. 2012
Received: 5 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
MnO2/carbon nanotube [CNT] nanocomposites with a CNT core/porous MnO2 sheath hierarchy architecture are synthesized by a simple hydrothermal treatment. X-ray diffraction and Raman spectroscopy analyses reveal that birnessite-type MnO2 is produced through the hydrothermal synthesis. Morphological characterization reveals that three-dimensional hierarchy architecture is built with a highly porous layer consisting of interconnected MnO2 nanoflakes uniformly coated on the CNT surface. The nanocomposite with a composition of 72 wt.% (K0.2MnO2·0.33 H2O)/28 wt.% CNT has a large specific surface area of 237.8 m2/g. Electrochemical properties of the CNT, the pure MnO2, and the MnO2/CNT nanocomposite electrodes are investigated by cyclic voltammetry and electrochemical impedance spectroscopy measurements. The MnO2/CNT nanocomposite electrode exhibits much larger specific capacitance compared with both the CNT electrode and the pure MnO2 electrode and significantly improves rate capability compared to the pure MnO2 electrode. The superior supercapacitive performance of the MnO2/CNT nancomposite electrode is due to its high specific surface area and unique hierarchy architecture which facilitate fast electron and ion transport.
where M represents protons (H+) and/or alkali cations such as K+, Na+, and Li+. The charge storage is based either on the adsorption of cations at the surface of the electrode material or on the intercalation of cations in the bulk of the electrode material. In order to achieve high capacitive performance, a large surface area and a fast ion/electron transport of the electrode material are required. Therefore, extensive research has been focused on the synthesis of nanostructured MnO2 as the nanoscale powder, which provides not only a high specific surface area, but also a fast ion and electron transport [16–25]. Various forms of MnO2 including one-dimensional (nanorods, nanowires, nanobelts, nanotubes) [16–22], two-dimensional [2-D] (nanosheets, nanoflakes) [23–25], and three-dimensional [3-D] (nanospheres, nanoflowers) [26–28] nanostructures have been synthesized. However, the reported specific capacitance values for the various nanostructured MnO2 electrodes are still far below the theoretical value (approximately 1,370 F/g) , which may be attributed to the intrinsically poor electronic conductivity of MnO2. To improve the capacitive performance of MnO2, the key is to add conductive additives to improve the electron transport . Due to their excellent electrical conductivity and high specific surface area, carbon nanotubes [CNTs] are now intensively used with MnO2 to make nanocomposites. Recently, MnO2/CNT nanocomposites have been prepared by various methods to improve the electrochemical utilization of MnO2 and electronic conductivity of the electrode [31–38]. In most studies, once the coated MnO2 layer becomes thick, it exhibits a dense structure, which is not beneficial for maximizing the utilization of MnO2 as only the surface area is involved in charge storage. However, if the coated MnO2 layer is too thin, the specific capacitance of the composite is difficult to be increased as the MnO2 loading becomes too low. In previous reports, although the MnO2 incorporation improves the capacitance of the CNT assembly, the overall specific capacitance remains typically less than 200 F/g. In order to increase the MnO2 loading in the composite while retaining the formation of a nanoscopic MnO2 phase, depositing a highly porous MnO2 layer on the CNTs could be a strategy to achieve this goal. However, a facile and fast synthesis of a uniformly distributed MnO2 porous layer on the CNTs is still a challenge. It could be a beneficial design if one of the nanostructures (nanowire, nanorod, nanoflake, etc.) of MnO2 could be transferred onto the CNTs as this hierarchy architecture may be able to provide a large specific surface area (due to the porous feature of the MnO2 sheath) and a fast electron and ion transport (due to the support of the CNT core and the formation of the nanoscopic MnO2 phase).
In the present work, a facile hydrothermal synthesis has been designed to deposit a uniform and highly porous MnO2 layer consisting of interconnected nanoflakes onto the surface of the CNTs. The structure, surface morphology, composition, and specific surface area of the as-prepared nanoflaky MnO2/CNT nanocomposites have been fully investigated. The capacitive behaviors of the CNTs, the pure MnO2, and the MnO2/CNT nanocomposite electrodes were also investigated and compared. The advantages of the present MnO2/CNT hierarchy architecture associated with its superior capacitive behaviors were discussed.
Commercial multiwalled CNTs (20 to 50 nm in diameter, Shenzhen Nanotech Port Co., Ltd., Shenzhen, China) were purified by refluxing the as-received sample in 10 wt.% nitric acid for 12 h. The acid-treated CNTs were then collected by filtration and dried at 120°C for 12 h in vacuum. A typical synthesis process of the MnO2/CNT nanocomposite is described as follows. Firstly, 0.1 g CNTs was dispersed in 25 ml deionized [DI] water by ultrasonic vibration for 2 h. Then, 0.3 g KMnO4 was added into the above suspension, and the mixed solution was stirred by a magnetic bar for 2 h. After that, the mixed solution was transferred to a 30-mL, Teflon-lined, stainless steel autoclave. The autoclave was sealed and put in an electric oven at 150°C for 6 h and then naturally cooled to room temperature. After the hydrothermal treatment, the resultant samples were collected by filtration and washed with DI water. MnO2/CNT nanocomposites were finally dried in an oven at 100°C for 12 h for further characterization. To prepare the MnO2 powders, 0.3 g KMnO4 and 0.2 mL H2SO4 (95 wt.%) were placed into 25 mL DI water to form the precursor. The precursor solution was then treated with a hydrothermal reaction in a 30-mL autoclave at 150°C for 4 h.
The crystallographic information of the products was investigated by X-ray diffraction [XRD] (Shimadzu X-ray diffractometer 6000, Cu Kα radiation, Kyoto, Japan) with a scan rate of 2°/min. Morphologies of the acid-treated CNTs, the MnO2 powders, and the MnO2/CNT nanocomposites were characterized by field emission scanning electron microscopy [FESEM] (Hitachi S4300, Tokyo, Japan). The morphology and structure of the MnO2/CNT nanocomposites were further investigated by transmission electron microscopy [TEM] and high-resolution transmission electron microscopy [HRTEM] (JEM-2010, JEOL, Tokyo, Japan). Compositional investigation of the samples was carried out with energy-dispersive X-ray [EDX] spectroscopy (Noran System SIX, Thermo Fisher Scientific, Shanghai, China). The contents of the interlayer water and CNTs of the nanocomposites were determined by thermogravimetric analysis [TGA] (Shimadzu DTG-60H, Kyoto, Japan). Nitrogen adsorption and desorption isotherms at 77.3 K were obtained with a Quantachrome Autosorb-6B (Beijing, China) surface area and a pore size analyzer.
Electrochemical measurements were carried out on three-electrode cells using a Solartron 1287 electrochemical interface combined with a Solartron 1260 frequency response analyzer (Hampshire, United Kingdom). To prepare the working electrode, 80 wt.% of the active material (CNTs, pure MnO2 powder, or MnO2/CNT nanocomposite), 15 wt.% carbon black, and 5 wt.% polyvinylidene difluoride dissolved in N-methylpyrrolidone were mixed to form a slurry. The slurry was pasted onto a Ti foil and dried for 12 h in a vacuum oven. The loading of the working electrode is typically in the range of 2 to 3 mg/cm2. A carbon rod was used as the counter electrode, an Ag/AgCl (saturated KCl) electrode was used as the reference electrode, and a 1-M Na2SO4 solution was used as the electrolyte. Cyclic voltammetry [CV] and electrochemical impedance spectroscopy [EIS] were utilized to evaluate the electrochemical behaviors of the different composite electrodes. CV measurements were carried out between 0 and 0.9 V (vs. Ag/AgCl) at different scan rates ranging from 10 to 100 mV/s. EIS measurements were carried out in a frequency range from 10 kHz to 0.01 Hz with an ac amplitude of 10 mV.
Results and discussion
It is speculated that in the present solution system, the decomposition of KMnO4 is much faster than the redox reaction between KMnO4 and CNTs. During the hydrothermal reaction, the preformed MnO2 nanocrystallines may serve as nucleation sites, where the newly formed MnO2 nucleus due to the KMnO4 decomposition could get deposited on. The flaky morphology is formed due to preferred growth along the ab plane of the layered birnessite-type MnO2[23, 24]. Consequently, the CNT core/porous MnO2 sheath hierarchy architecture could be easily produced using this simple hydrothermal method.
Figure 7b shows the capacitance retention as a function of frequency obtained by taking the real part of the complex capacitance C*(f) = 1/[i 2πf Z*(f)],where i, f, and Z* are the imaginary unit, ac frequency, and complex impedance at a frequency, respectively [30, 41, 44]. For the porous electrode, the frequency response of capacitance may be understood using the parameter 'penetration depth,' l' = 1/(fR' C')1/2, where R' and C' represent the pore resistance and pore capacitance per unit pore length, respectively . At low frequency, when the electrolyte penetration depth is larger than the pore length of the porous electrode, most of the pore surface is utilized, resulting in a maximum capacitance. On the contrary, at high frequency, when the penetration depth is smaller than the pore length, only limited electrode surface is utilized, resulting in a decreased capacitance. As shown in Figure 7b, the capacitance retention for all three electrodes reaches the maximum at very low frequency, starts to decrease as the frequency increases, and finally, goes down to zero at very high frequency. The CNT electrode exhibits an excellent rate capability with capacitance retention of 90% at a frequency of 1 Hz. The pure MnO2 electrode however exhibits a poor rate capability with capacitance retention of only 32% at 1 Hz. It can be seen that a significantly improved rate capability can be obtained by combining the MnO2 nanoporous sheath with the CNT core. The MnO2/CNT nanocomposite is able to retain 65% of its full capacitance at 1 Hz. The significantly improved rate capability of the MnO2/CNT nanocomposite electrode could be due to its small charge-transfer resistance and small diffusive resistance, indicating that the unique nanoarchitecture of CNT core/porous MnO2 sheath is able to provide fast transport for both ions and electrons.
MnO2/CNT nanocomposites with a unique nanoarchitecture consisting of a CNT core/porous MnO2 sheath have been successfully synthesized using a simple hydrothermal treatment. The nanoporous MnO2 sheath is composed of interconnected MnO2 nanoflakes directly grown from the surface of the CNTs. The birnessite-type MnO2 synthesized by the hydrothermal reaction contains 0.2 K+ and 0.3 H2O per formula. The nanoflaky MnO2/CNT nanocomposite containing 72 wt.% MnO2 exhibits a high specific surface area of 237 m2/g with a pore distribution of 2 to 8 nm. The MnO2/CNT nanocomposite electrode exhibits much higher specific capacitance compared with those of the CNT and the pure MnO2 electrodes and a significantly improved rate capability compared to that of the pure MnO2 electrode. The high specific capacitance of the MnO2/CNT nanocomposite electrode may be attributed to the highly porous structure of the MnO2 layer and its high specific surface area, resulting in high utilization of MnO2. The significantly improved rate capability of the MnO2/CNT nanocomposite electrode compared to that of the pure MnO2 electrode could be explained by its small charge-transfer resistance and diffusive resistance obtained from EIS measurements, resulting from its unique hierarchy architecture where the 3-D electron path network constructed by the CNT cores and the nanoporous sheath composed of tiny MnO2 nanoflakes facilitate faster electron and ion transport.
This research is supported by the National University of Singapore and the Agency for Science, Technology and Research through a research grant R-284-000-067-597 (072 133 0044). HX would like to thank Nanjing University of Science and Technology for the financial support through NUST Research Funding research grant (AB41385 and 2011ZDJH21).
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