Functional Carbon Nanotube / Mesoporous Carbon / MnO 2 Hybrid Network for High-Performance Supercapacitors

A functional carbon nanotube/mesoporous carbon/MnO 2 hybrid network has been developed successfully through a facile route. The resulting composites exhibited a high specific capacitance of 351 F/g at 1 A g, with intriguing charge/discharge rate performance and cycling stability due to a synergistic combination of large surface area and excellent electron-transport capabilities of MnO 2 with the good conductivity of the carbon nanotube/mesoporous carbon networks. Such composite shows great potential to be used as electrodes for supercapacitors.


Introduction
Supercapacitors have attracted significant attention over the past decades due to their great advantages such as high power supply, long cycle life, and low cost and, therefore, offer a promising approach to meet the increasing power demands for portable devices and automotive applications [1][2][3].It is noteworthy that performances of supercapacitors depend strongly on the properties of the electrode materials they employ.As a type of good candidate, carbonaceous materials, transition metal oxides, and conducing polymers have always attracted great interest [4][5][6].Among them, manganese dioxide (MnO 2 ) has been intensively studied as electrode material due to its outstanding capacitive properties and environmentally benignity, in addition to being cheap and largely available [7][8][9].However, the poor electronic conductivity of MnO 2 has limited its application in highperformance supercapacitors.To enhance the desired properties, the combination of MnO 2 and other conductive electrode materials is reckoned to tackle this limitation.
On other hand, carbon materials, especially carbon nanotubes (CNTs), are promising materials as electrodes for supercapacitors because of their high conductivity and short diffusion path to ions and excitons.The supercapacitors based on CNTs exhibit excellent power density as testified in recent years.However, the specific capacitance and energy density of CNTs are always lower than other carbon materials such as activated carbon (typical in the range of 15-200 F g −1 for CNTs) due to the limited surface area (typically less than 200 m 2 g −1 ), which restricts the usage of CNTs as electrodes for supercapacitors [10][11][12].In comparison with CNTs, mesoporous carbon (MC) usually exhibits a better specific capacitance up to 200 F g −1 , because of their large surface area and suitable pore size distribution [13][14][15].And, in our previously work [16], we reported the fabrication of uniform CNT/MC networks via a facile organic sol-gel chemical route, which exhibited greatly increased specific capacitance due to the favorable balance between specific surface area and pore size distribution, and a 3D, well-connected throughpore structure.However, the specific capacitance of CNT/MC networks is still poor relative to transition metal oxides such as MnO 2 .Therefore, it has inspired attempts to develop novel electrode materials via the coupling of CNT/MC networks and MnO 2 as electrode for supercapacitor, which may exhibit huge potentials to combine the advantages of both of them.
Herein, we report a convenient preparation of welldesigned uniform CNT/MC/MnO 2 hybrid networks by growing MnO 2 along the backbone of the CNT/MC networks.The resulting CNT/MC/MnO 2 hybrid network electrode, under preferred conditions, exhibits an outstanding specific capacitance of 351 F g −1 at a scan rate of 10 mV s −1 and retains the high values of 195 F g −1 even at a high scan rate of 500 mV s −1 .All the evidence indicates that the rational combination of CNT/MC networks and MnO 2 will greatly improve the electrochemical performance, where MnO 2 on the surface of CNT/MC networks is essential for high energy storage and interconnected porous channels and high electrical conductivity of networks can accelerate the kinetic process of ion diffusion.

Experimental Section
2.1.Synthesis of CNT/MC/MnO 2 Hybrid Network.All the reagents used in the experiments were of analytical grade and used without further purification.Prior to use, purified MWCNTs (Alpha Nano Technology Co., Ltd., China) were treated with concentrated acids (H 2 SO 4 /HNO 3 = 1/1 (v/v)) for 3 hours under 60 ∘ C, washed with copious of water, and dried at 60 ∘ C. In a typical synthesis, the acidtreated CNTs (0.05 g) were suspended in deionized water and thoroughly dispersed using ultrasound (sonic power ∼ 100 W) for 2 hours.Once the CNTs were dispersed, P123 ((PEO-PPO-PEO), 0.248 g, 22.4 mmol), resorcinol (0.124 g, 11.2 mmol), formaldehyde (0.176 g, 22.1 mmol), and sodium carbonate catalyst (0.6 mg, 0.056 mmol) were added to the reaction solution.The P123 was chosen as the generation of mesoporous structures.After stirring for half an hour, carbon fiber papers (CFP) were put into the solution, and the solgel mixture was cured at 85 ∘ C for 72 hours.The CFP act as an electron collector, and CNT/MC networks directly grow on the CFP substrate to improve the contact between them.The resulting gels were washed with tert-butanol for 72 h to remove water from the pores of the gel network.The gels were subsequently dried with lyophilization (freeze-drying) and pyrolyzed at 950 ∘ C under N 2 atmosphere for 3 hours, and CNT/MC networks were obtained.Then, the obtained CNT/MC network was again dispersed in deionized water, and followed by adding KMnO 4 (CNT/MC and KMnO 4 mass ratio of 1 : 1) the suspension was stirred at room temperature for different times (10/20/30 min, resp.), filtrated and washed using deionized water, and dried in air at room temperature.CNT/MC/MnO 2 hybrid network was finally obtained and named as CNT/MC/MnO 2 -10, CNT/MC/MnO 2 -20, and CNT/MC/MnO 2 -30, respectively.

Characterization. The morphology of samples was characterized with scanning electron microscopy (FESEM;
Hitachi S-4800) operated at 5 kV.The samples were also analyzed by a powder X-ray diffraction system (XRD, D/MAX 2550 VB/PC) equipped with CuKa radiation.N 2 adsorption/desorption was determined by Brunauer-Emmett-Teller (BET) measurements using a Micromeritics ASAP 2100 surface area analyzer.

Electrochemical Measurements.
All electrochemical measurements were done on an Ametek PARSTAT 2273 potentiostat in a three-electrode setup: the as-prepared products as the working electrode, a platinum foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode.The electrochemical measurements included cyclic voltammograms (CVs) and galvanostatic charge/discharge (CD).All electrochemical measurements were carried out at room temperature in 1 M NaSO 4 aqueous electrolyte, under the potential range from −0.2 to 0.8 V.

Results and Discussion
Figure 1(a) shows the low magnification SEM image of the obtained CNT/MC networks.It can be seen that the CFP collector is covered with a layer of foam-like CNT/MC networks.We have reported that the network is comprised of random CNTs or CNTs bundles which interconnected by thin carbon sheets [5].Figures 1(b 2. It is clear that most MnO 2 is in amorphous nature, since the weak and broad peaks of MnO 2 in XRD analysis and the peaks at 2 around 12 ∘ (0 0 1), 37 ∘ (1 1 1) and 66 ∘ (3 1 2) are indexed to poor crystallized birnessite-type MnO 2 .It has been reported that the amorphous parts are beneficial to increase the specific capacitance for the supercapacitors because the highly amorphous structure should favor the electrolyte to insert into or to expel out from the oxide matrix, which can enhance contact between the electrolyte and the electrode material and then improve the utilization ratio of the material.Comparing the samples with different reaction times, it is clear that the uniform MnO 2 coating has grown thicker with the increasing of reaction time.And for CNT/MC/MnO 2 -30, there are too many MnO 2 nanoparticles on the CNT/MC network so that parts of the channel in the hybrid networks are blocked, which have influence on the electrochemical performance of the sample.
To examine the electrochemical performance of our present CNT/MC/MnO 2 hybrid network materials, the samples were fabricated into the supercapacitor electrode, and cyclic voltammogram (CV) and galvanostatic chargedischarge (CD) measurements were carried out in 1 M NaSO 4 aqueous electrolyte, under the potential range from −0.2 to 0.8 V to avoid the electrolysis of water.Figure 3 shows the CV curves of CNT/MC/MnO 2 hybrid networks at different scan rates (20,50,100,200, and 1000 mV/s, resp.).CV curves of all CNT/MC/MnO 2 hybrid networks are relatively rectangular in shape, proving the ideal pseudocapacitive nature at small scan rate.And under large scan rate (1000 mV/s), CV curves of CNT/MC/MnO 2 -10 and sample CNT/MC/MnO 2 -20 are still symmetrical, indicating excellent power capacitor behaviours; however, with MnO 2 loading increasing, the CV curves change into asymmetrical sharp.The low rate capability of CNT/MC/MnO 2 -30 is caused by the poor conductivity of MnO 2 nanoparticles.allows for easy accessibility of ions to the electrode/electrolyte interface and charging the capacitors.The energy density of CNT/MC/MnO 2 -20 can be estimated to be 25.5 Wh kg −1 at a power density of 1.1 kW kg −1 (Figure 4(c)) which is much higher than not only the CNT/MC electrode (15.6 Wh kg −1 ) [6], but also most of carbonaceous and MnO 2 -based electrodes, such as CNTs (<10 Wh kg −1 ), grapheme (3.15 Wh kg −1 ), and MnO 2 nanoparticles (<20 Wh kg −1 ) [17][18][19].More significantly, the energy density is still as high as 16.3 Wh kg −1 for CNT/MC/MnO 2 -20 even at a high power density of 17 kW kg −1 .The long-term cycle stability of the CNT/MC/MnO 2 -20 was also investigated.Figure 4(d) demonstrates the specific capacitance as a function of cycle number at a current density of 5 A g −1 , in galvanostatic charge-discharge measurements for up to 1000 cycles.
After the cycling test, there are about 86% of its initial capacities that are maintained, due to the loss of adhesion of MnO 2 with the current collector or the dissolution of MnO 2 nanoparticles into the solution [20].This cycling performance is also superior to most of other MnO 2 nanoparticles based supercapacitors.The present results have shown that the as-designed CNT/MC/MnO 2 hybrid network exhibits an excellent electrochemical performance, even for high-rate charge/discharge operations.
Based on the above discussions, the specific capacitance of CNT/MC/MnO 2 hybrid network is not only better than the reported carbonaceous electrodes, such as CNTs (180 F g −1 ), graphene nanosheets (175 F g −1 ) [21,22]  (219 F g −1 ) and grapheme/MnO 2 (310 F g −1 ) [23,24].Pure MnO 2 always exhibits high specific capacitance; however, the poor conductivity of MnO 2 results in low rate capability for high power performance.Xu et al. reported the synthesis of mesoporous MnO 2 nanowire array, which exhibited a specific capacitance as high as 493 F g −1 [25].However, at a high current density of 12 A g −1 , only 84 F g −1 was observed.The sustainable specific capacitance of our materials is mainly due to its ability to have short paths ion diffusion with the CNT/MC networks and effective utilization of active material due to its uniform structures.

Conclusions
In summary, functional carbon nanotube/mesoporous carbon/manganese dioxide (CNT/MC/MnO 2 ) hybrid networks have been designed and fabricated via a facile route for electrochemical energy storage.The obtained structures combined the advantages of large surface area and excellent electron-transport capabilities of MnO 2 with the good conductivity of carbon nanotube/mesoporous carbon networks and short transportation pathway of unique 3Dnetwork configuration.The CNT/MC/MnO 2 hybrid networks exhibit greatly increased specific capacitance (351 F g −1 at 1 A g −1 ) compared with CNT/MC networks as supercapacitor electrode in 1 M NaSO 4 aqueous electrolyte.The energy density of CNT/MC/MnO 2 -20 can be estimated to be 25.5 Wh kg −1 at a power density of 1.1 kW kg −1 , and the energy density is still as high as 16.3 Wh kg −1 even at a high power density of 17 kW kg −1 , indicating excellent electrochemical performance.It is reckoned that the present work also sheds light on the design of carbon and transition metal oxides 3D-networks for other applications.

Figure 4 (
Figure 4(a) shows the charge-discharge curves of all CNT/MC/MnO 2 hybrid networks and CNT/MC network at a current density of 1 A g −1 .The linear voltage-time profile and the highly symmetric charge-discharge characteristics also show good capacitive behavior of all samples.The discharge time for CNT/MC/MnO 2 hybrid networks is about two times larger than that of CNT/MC network, exhibiting an enhanced capacitance performance with the addition of MnO 2 .The galvanostatic charge-discharge measurement is assumed to be the most accurate technique to estimate the supercapacitive performance.The specific capacitances  in this paper are calculated from galvanostatic chargedischarge curves according to  / Δ, where  is the

1 )Figure 4 :
Figure 4: (a) charge-discharge curves of CNT/MC and CNT/MC/MnO 2 hybrid networks at 1 A g −1 ; (b) relationship of the specific capacitance with charge/discharge current density of CNT/MC/MnO 2 hybrid networks; (c) Ragone plots of CNT/MC/MnO 2 -20; (d) variation of specific capacitance with cycle number at a current density of 1 A g −1 of CNT/MC/MnO 2 -20.
) to 1(d) show the SEM image of the as-synthesized CNT/MC/MnO 2 hybrid networks with different reaction time.It can be obviously seen that the MnO 2 nanoparticles assembled on the surface of the CNT/MC networks, owning to the reaction of MnO 4 − with less active carbons.And most of the nanosheets of MC in the CNT/MC networks disappeared, due to the reaction.The XRD patterns of CNT/MC networks, MnO 2 , and CNT/MC/MnO 2 -10 sample are shown in Figure , and CNT/MC networks (210 F g −1 ), but also superior to most of other carbon/MnO 2 composites, such as MnO 2 /carbon aerogel