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carbon nanotubes nanocomposites for supercapacitors
Journal of Power Sources 246 (2014) 402e408
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A three-dimensional ordered mesoporous carbon/carbon nanotubes nanocomposites for supercapacitors Zhengju Zhu, Yanjie Hu, Hao Jiang*, Chunzhong Li* Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A three-dimensional ordered mesoporous carbon (OMC)/carbon nanotubes (CNTs) nanocomposite was synthesized via a facile route. The introduction of CNTs into OMC constructs a 3D conductive network, greatly improving the rate performance. The optical nanocomposite, when used as supercapacitor electrodes, exhibited an excellent electrochemical performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2013 Received in revised form 19 July 2013 Accepted 24 July 2013 Available online 6 August 2013
A three-dimensional ordered mesoporous carbon (OMC)/carbon nanotubes (CNTs) nanocomposite is prepared via a two-step procedure. Firstly, OMC is synthesized through a co-assembly strategy associated with the incorporation of Ni nanoparticles. Then Ni nanoparticles are used as catalyst for the growth of CNTs. The introduction of CNTs into OMC can construct a 3D conductive network, greatly improving the rate performance of the nanocomposites. The nanocomposite with optimal CNTs content, when applied as supercapacitor electrodes, exhibits a high specific capacitance (338.1 F g1 at 1 A g1), excellent rate capability (130.2 F g1 at 50 A g1) and high cycling stability (91.6% capacity retention after 4000 cycles) in 6 M KOH aqueous solution. Such intriguing electrochemical performance is mainly attributed to the synergistic effects between OMC and CNTs. It is reckoned that the present 3D OMC/CNTs nanocomposite can serve as a promising electrode material for supercapacitors. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Ordered mesoporous carbon Carbon nanotube Nanocomposite Supercapacitor
1. Introduction In recent years, supercapacitors, also known as electrochemical capacitors or ultracapacitors, have attracted significant attention as they can provide a higher power density than batteries, fast chargee discharge processes, and long cycle life [1e4]. They have been widely applied in consumer electronics, energy management, memory back-up systems, industrial power and mobile electrical systems
* Corresponding authors. Tel.: þ86 21 64250949; fax: þ86 21 64250624. E-mail addresses: [email protected] (H. Jiang), [email protected] (C. Li).
[2,5]. In these applications, low energy density and high production cost have been identified as major challenges for the furtherance of supercapacitors technologies [6]. To overcome these obstacles, one of the most intensive approaches is to develop new electrode materials for supercapacitors. Up to now, various transition metal oxides [7,8], carbon materials [9,10], and conducting polymers [11,12] have been explored. Among them, carbon materials are the most popular due to their high surface areas for charge storage, low cost, easy availability, stability and environmental benignity. Various carbon materials could be used as supercapacitors electrodes, typical examples include carbon nanotubes (CNTs) [13,14] and ordered mesoporous carbon (OMC) [15,16]. CNTs are
0378-7753/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.07.086
Z. Zhu et al. / Journal of Power Sources 246 (2014) 402e408
used as electrodes to increase the power density as a result of their unique tubular structures and superior electrical properties, which favor fast ions and electrons transportation. However, the high production cost and entanglement problem limit their wide spread applications [4,17,18]. OMC possesses excellent electrochemical performance due to its high surface area, narrow pore size distribution and uniform pore connection. But the conventional hard-template nanocasting method to prepare OMC is obviously high-cost [19]. Furthermore, the conductivity of the OMC is much lower than that of the CNTs. Therefore, it is important to construct novel architectures by coupling the advantages of different carbon materials to further improve the electrochemical performances. Recently, OMC/CNTs nanocomposites have aroused intense research interest in areas of energy storage systems due to their combined properties [20e22]. To be more detailed, OMC can provide an interconnected mesoporous structure with a large surface area, allowing for the facile transport of ions and electrons. In addition, CNTs that interconnect with the primary pieces of OMC can function as electrical bridges, thus lowering the interfacial resistance and generating fast electrical networks. However, some OMC/CNTs nanocomposites reported previously were synthesized via a nanocasting method using ordered mesoporous silica (OMS) particles as a template and Niephthalocyanine as carbon source, both of which are high cost, thus industrially unfeasible [20,21]. In other cases, OMC/CNTs nanocomposites were obtained by a modified organiceinorganic self-assembly route, and the results showed much lower specific surface area and limited improvement for power and energy densities [22]. Therefore, it is highly desirable to develop an approach to prepare OMC/CNTs nanocomposites with a high specific capacitance at high current densities while maintaining a low fabrication cost. In the present work, we have developed a simple and reproducible method to prepare OMC/CNTs nanocomposites with both high energy and power densities for supercapacitors and low fabrication cost. Our strategy can be briefly described as Fig. 1. The preparation process mainly involves: (1) the synthesis of OMC with Ni nanoparticles embedded in the carbon walls, which has high surface area and abundant mesoporous; (2) the CNTs growth on OMC channels using Ni nanoparticles as catalyst and C2H2 as carbon source, respectively by a modified CVD process [23,24]. The CNTs content can be easily controlled just by tuning the reaction time. The results indicated that the optimal OMC/CNTs nanocomposite exhibited a highest specific capacitance with intriguing rate performance and cycling stability.
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2. Experimental section 2.1. Synthesis of the OMC/Ni nanocomposites The resol precursor was prepared following synthesis procedure reported by Zhao et al. [25]. The OMC/Ni nanocomposites were synthesized via a co-assembly approach by using triblock copolymer F127, phenol resin, TEOS and nickel nitrate, followed by carbonization and silica removal [26]. In a typical preparation, 1.6 g of block copolymer F127 and 1.0 g of 0.2 M HCl were dissolved in 8.0 g of ethanol, followed by mixing with 0.13 g Ni(NO3)2$6H2O and stirred for 1 h at 40 C to obtain a clear solution. Then 2.08 g of TEOS and 5.0 g of 20 wt% resols’ ethanolic solution were added to the solution slowly in sequence. After stirring for 2 h, the solution was transferred into dishes. It took 5e8 h at room temperature to evaporate ethanol and 24 h at 100 C in an oven to thermopolymerize. The as-made flaxen and transparent products were peeled off and ground into fine powders. Calcination was carried out in a tubular furnace at 900 C for 2 h under N2 flow with a heating rate of 1 C min1 below 600 C and 5 C min1 above 600 C. The obtained carbon/silica/nickel products were stirred in 35 g of 1.0 M NaOH solution at 55 C for 24 h to remove the silica. The as-made carbon/nickel nanocomposite was denoted as OMC/Ni. 2.2. Synthesis of the OMC/Ni/CNTs nanocomposites The growth of CNTs on OMC/Ni nanocomposite was achieved using a modified CVD process according to procedure reported previously by our group [27]. Typically, 0.06 g OMC/Ni was spread evenly within a porcelain ark to form a powder bed. Then the ark was placed in the apparatus for the experiment, an electrical furnace equipped with a horizontal corundum tube, which is 42 mm in diameter, 700 mm in length, and has a 300 mm long reaction zone. The chamber was first heated to 650 C at a rate of 5 C min1 under an Ar flow, and then acetylene was introduced into the furnace under a gas feeding rate of 600/30 sccm Ar/C2H2 for growth of CNTs. After the growth step, the acetylene was turned off, and the samples were cooled to room temperature under flowing Ar. The final products were named as OMC/Ni/CNTs-xmin, wherein x represents the growth time of CNTs. The mass loadings of CNTs in these nanocomposites were evaluated by calculating the weight difference between raw materials and products. For comparison, OMC was synthesized by a solvent evaporation induced selfassembly (EISA) method [28].
Fig. 1. Schematic illustration of the preparation strategy.
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2.3. Characterizations
E ¼ C ðDVÞ2 =2
(3)
X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2550VB/PC X-ray diffractometer with Cu Ka radiation (40 kV, 100 mA). Transmission electron microscopy (TEM) experiments were conducted on a JEOL 2010 microscope operated at 200 kV. Scanning electron microscopy (SEM) images were conducted on a Hitachi S-4800 microscope operated at 15 kV. Nitrogen adsorptionedesorption isotherms were measured at 77 K with a Micromeritcs 2010 analyzer.
P ¼ E=Dt
(4)
The electrochemical measurements were conducted on an Autolab PGSTAT30 potentiostat using a three-electrode configuration in a 6 M KOH solution. The as-synthesized materials were mixed with acetylene black and polyvinylidene fluoride binder at a mass ratio of 80:10:10. A small amount of N-methyl-2-pyrrolidone was added to promote homogeneity [29]. The mixture was then coated onto one end of the graphite paper (w1 cm2) to form the working electrode after drying at 120 C, and the mass loading of all active materials was in the range of 0.90e1.2 mg). The reference and counter electrodes were Ag/AgCl electrode and platinum foil, respectively. Typical CV curves were measured between 0.8 and 0.2 V. The electrochemical impedance spectroscopy (EIS) test was conducted in the same electrolyte with a frequency loop from 100 kHz to 10 mHz using a perturbation amplitude of 5 mV at the open circuit potential. The specific capacitance was calculated from the CV curves according to the following equation [13]:
(1)
where C (F ge1) is the specific capacitance, m (g) is the mass of the active materials in the work electrode, qa and qc represent the anodic and cathodic voltammetric charges on the positive and negative sweeps, and DV is the potential window. The discharge specific capacitance could also be calculated from the discharge curves by the following equation [8]:
C ¼ I Dt=ðm DVÞ
3. Results and discussion 3.1. Morphology characterization and structure analysis
2.4. Electrochemical measurements
C ¼ ðqa þ jqc jÞ=ð2 m DVÞ
where C is the specific capacitance of the active materials, and DV is the potential window of discharge.
(2)
where I (A), Dt (s), m (g) and DV (V) are the discharge current, discharge time consumed in the potential range, mass of the active materials in the work electrode, and the potential windows, respectively. The energy density (E) and power density (P) were calculated by the following equations [8]:
Wide-angle XRD pattern of the OMC/Ni nanocomposite is shown in Fig. 2A. The XRD pattern shows three resolved diffraction peaks attributable to the (111), (200), and (220) reflections of metallic nickel with the face-centered cubic structure according to the JCPDS Card No. 04-0850, which suggests that Ni(II) from Ni(NO3)2$6H2O was completely reduced to metallic nickel during the carbonization at 900 C. In addition, a broad diffraction peak near 2q ca. 20e30 is also observed in the wide-angle XRD pattern, and this broad peak is characteristic of amorphous carbon contributed from OMC. TEM image of the OMC/Ni nanocomposite (Fig. 2B) shows an ordered 2-D hexagonal mesostructure with an estimated cell parameter (a0) of 10.4 nm. The Ni nanoparticles have been welllocated in the carbon matrix with a size of w20.0 nm. Then, the Ni nanoparticles were used to catalyze C2H2 for the realization of the growth of CNTs by a CVD process. As shown in Fig. 3A, trace of CNTs can be observed even by a 2 min reaction (OMC/Ni/CNTs2 min). With the extension of the growth time, CNTs keep growing and many well-defined CNTs are obtained. The OMC/Ni/CNTs-5 min demonstrates that CNTs are uniformly distributed on the surface of the OMC (Fig. 3B). The interconnected construction between OMC and CNTs is beneficial for the rapid transportation of the electrons and ions. The detailed structure of the OMC/Ni/CNTs-5 min was further investigated by TEM. As shown in Fig. 3E. An oriented stripe-like pattern similar to OMC/Ni was still well-maintained after growth of the CNTs. It is noted that the CNTs are grown by Ni catalysis, indicating a good connection by an OMC-Ni-CNTs way, which can construct a three dimensional conductive network for the rapid electron transportation, ensuing a high rate performance. The EDS image of OMC/Ni/CNTs-5 min was shown in the inset of Fig. 3E. The content Ni catalyst is estimated to about 4.8 w%. When the growth time was further prolonged to 10 and 30 min, the entire surfaces of OMC were almost fully covered with amounts of CNTs (Fig. 3C, D). The CNTs content of each sample is summarized in Table 1.
Fig. 2. (A) Wide-angle XRD pattern and (B) TEM image of the OMC/Ni.
Z. Zhu et al. / Journal of Power Sources 246 (2014) 402e408
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Fig. 3. (A) SEM image of the OMC/Ni/CNTs-2 min, (B) SEM image and (E) TEM image of the OMC/Ni/CNTs-5 min, (C) SEM images of the OMC/Ni/CNTs-10 min, (D) SEM image of the OMC/Ni/CNTs-30 min. (The inset of E is EDX image of OMC/Ni/CNTs-5 min.)
The small-angle XRD patterns (Fig. 4A) of the OMC/Ni/CNTsxmin nanocomposites mainly show two well-resolved diffraction peaks attributed to the (10), (11) reflections of two-dimensional hexagonal mesostructure symmetry with the space group of p6m [30]. The calculated cell parameters of these nanocomposites ranged from 10.9 to 11.7 nm, which were similar to that of the pristine sample OMC/Ni. The results suggested that the ordered hexagonal mesostructure of OMC/Ni has been well maintained in OMC/Ni/CNTs nanocomposites. It is noted that the intensity of the reflection peaks of OMC/Ni/CNTs-10 min and OMC/Ni/CNTs-30 min nanocomposites is relative weak, mainly due to the structural defects of amounts of CNTs. In wide-angle XRD patterns (Fig. 4B), all the samples exhibit a very sharp peak centered at 26.3 corresponding to the (002) diffraction of graphite. We suggest that the graphitic character of these samples may originate from the highly graphitic CNTs [20]. N2 adsorption and desorption isotherms and the pore size distribution curves of OMC/Ni/CNTs-xmin nanocomposites are shown in Fig. 4 and also summarized in Table 1. All nanocomposites exhibited type-IV curves with H1 hysteresis loops. Distinct capillary condensation steps occurred approximately at P/P0 ¼ 0.40e0.70, suggesting a high uniformity of mesoporous sizes. An increased sorption in the isotherm curves at relative pressure of P/P0 ¼ 0.1e0.3 is observed for all samples (Fig. 4C), which indicates that abundant small pores below 4.0 nm were generated. These small pores are
Table 1 Textural properties of OMC/Ni/CNTs-xmin nanocomposites. Sample
CNTs content (wt%)
a0 (nm)
SBET (m2 g-1)
Vt (cm3 g-1)
Dp (nm)
OMC/Ni/CNTs-2 min OMC/Ni/CNTs-5 min OMC/Ni/CNTs-10 min OMC/Ni/CNTs-30 min
10.3 23.3 36.4 50.1
10.9 11.1 10.9 11.7
1269.1 1197.3 850.1 539.9
1.22 1.13 0.79 0.62
6.7 6.5 6.4 6.6
CNTs content is evaluated by calculating the weight difference between raw materials and products; a0, the unit cell parameter, is calculated by using the formula a0 ¼ 2d10/O3; SBET is the BET surface area; Vt is the total pore volume; Dp is the primary mesoporous diameter.
caused by the removal of silica located inside the pore walls from the carbon/silica/nickel products [26,31]. But the absorption volume related to the small pores gradually disappears with the increase of CNTs loadings. This is because that the small pores inside the carbon walls were gradually destroyed with increasing the loadings amount of CNTs. The BET surface areas and pore volumes of OMC/Ni/ CNTs-2 min and OMC/Ni/CNTs-5 min are as high as 1269.1 m2 g1, 1197.3 m2 g1 and 1.22 cm3 g1, 1.11 cm3 g1 (Table 1), respectively. The high BET surface areas and mesoporous structures provide the possibility of efficient transport of electrons and ions in the electrodes. But these values decreased sharply in samples OMC/Ni/ CNTs-10 min and OMC/Ni/CNTs-30 min, associated with higher content of CNTs (Table 1) and mesostructural degradation. The pore size distribution curves (Fig. 4D) calculated from the adsorption branches clearly confirm the narrow pore size distributions centered at about 6.5 nm and multimodal pore structures for OMC/ Ni/CNTs-xmin nanocomposites. Interestingly, nanocomposites with different loadings of CNTs exhibit analogous narrow pore size distribution, however, the pore volumes related to the primary mesoporous, as well as the small pores decrease obviously. For samples OMC/Ni/CNTs-10 min and OMC/Ni/CNTs-30 min, small pores centered at 2.3 nm almost disappear. 3.2. Electrochemical behaviors of OMC/Ni/CNTs-xmin nanocomposites To explore the energy storage applications of these nanocomposites, cyclic voltammogram (CV) and galvanostatic chargee discharge (CD) measurements were carried out in 6 M KOH. Fig. 5A shows the CV curves of all OMC/Ni/CNTs-xmin nanocomposites at a scan rate of 50 mV s1. For comparison, the CV curve of OMC is also given. All nanocomposites present a typical capacitive behavior with rectangular and symmetric CV curves. Among them, the OMC/Ni/CNTs-5 min exhibits the highest specific capacitance of 135.9 F g1 at a scan rate of 50 mV s1. The pristine OMC possesses a specific capacitance of 99.9 F g1 at the same scan rate. As for CNTs, their specific capacitances cannot exceed 50 F g1 under a low current generally [32,33]. Even
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Fig. 4. (A) Small-angle XRD patterns, (B) wide-angle XRD patterns, (C) N2 sorption isotherms and (D) pore size distributions curves of the OMC/Ni/CNTs-xmin nanocomposites: (a) OMC/Ni/CNTs-2 min, (b) OMC/Ni/CNTs-5 min, (c) OMC/Ni/CNTs-10 min, (d) OMC/Ni/CNTs-30 min.
though the performance of CNTs electrode could be enhanced by chemical activation, functionalization and purification, the most values of specific capacitances are still around 100 F g1 [34,35]. It is clear that there is an unexpectedly large increase in specific capacitance due to the growth of CNTs on OMC. This demonstrates the strong synergistic effect between OMC and CNTs. Furthermore, the OMC/Ni/CNTs-xmin nanocomposites are also reckoned to possess some pseudocapacitance from the reduction peaks observed at the range of 0.2e0.2 V in the CV curves, which are caused by nickel oxide originated from Ni nanoparticles
embedded in the carbon walls. This phenomenon is similar to some reports previously [36]. It is well known that electrochemical oxidation of nano-sized Ni to its corresponding nickel oxide can be realized by potential scan at the Ni oxide region (range from 0.1 to 0.5 V vs. SCE) [37]. And no reduction peak can be observed for OMC containing no Ni nanoparticles at the same electric potential range. These results indicate that the capacitive responses of the OMC/Ni/CNTs-xmin nanocomposites come from the combination of electrical double layer capacitances (EDLCs) and redox reactions [38].
Fig. 5. (A) CV curves at a scan rate of 50 mV s1 and (C) the specific capacitance as a function of different current densities of the (a) OMC and OMC/Ni/CNTs-xmin nanocomposites: (b) OMC/Ni/CNTs-2 min, (c) the OMC/Ni/CNTs-5 min, (d) the OMC/Ni/CNTs-10 min and (e) OMC/Ni/CNTs-30 min. (B) chargeedischarge curves at 1e50 A g1 of the OMC/Ni/CNTs5 min, (D) Ragone plot (energy density vs. power density) of the OMC and the OMC/Ni/CNTs-5 min, respectively.
Z. Zhu et al. / Journal of Power Sources 246 (2014) 402e408
In order to further evaluate the electrochemical performance of the OMC/Ni/CNTs-5 min nanocomposites, the galvanostatic chargeedischarge curves were measured at 1e50 A g1, as shown in Fig. 5B. It can be seen that the charging time is almost equal to their corresponding discharge counterpart. The specific capacitance is calculated from the discharge curves. It is noted that the specific capacitance of the OMC/Ni/CNTs-5 min can reach as high as 338.1 F g1 at a current density of 1 A g1, which is significantly higher than that of the OMC (208.6 F g1). This result is in good agreement with that of the CV characterization. The rate capability is an important factor for supercapacitors in high power applications. The relationships between specific capacitance and charge/ discharge current density for all OMC/Ni/CNTs-xmin nanocomposites and OMC are illustrated in Fig. 5C. Obviously, as the growth time of the CNTs increases from 2 to 5 min, the corresponding nanocomposites show better rate capability than that of the OMC. However, when the growth time of CNTs further increases to 10 min and 30 min, the rate capability of the corresponding nanocomposites is poorer than that of the OMC. It can be seen that OMC/Ni/CNTs-5 min nanocomposite not only delivers the highest specific capacitance, but also maintains it at a high current density compared to the other materials. Even at 50 A g1, the specific capacitance of OMC/Ni/CNTs-5 min (c line) still remains 130.2 F g1. Its excellent capability is contributed to the hybrid nanostructure which consists of highly ordered mesostructure, well-dispersed Ni nanoparticles in the carbon walls and uniformly distributed CNTs. The hybrid nanostructure not only takes full advantage of the EDLCs from the carbon layer and pseudocapacitance from the oxidation of Ni nanoparticles, but also possesses the superior electrical properties with CNTs structure, which favors fast ions and electrons transportation [14]. Besides, the hybrid electrode consistently shows higher specific capacitance than that of the pristine OMC electrode, further indicating that the CNTs indeed facilitated the harvest of the EDLCs, nickel oxide pseudocapacitance and the synergetic effect of three components. Although the specific capacitance of sample OMC/Ni/CNTs-2 min is also higher than that of the OMC, the less and uneven CNTs could not interconnect all the primary pieces of the OMC (shown in Fig. 3A), thus its specific capacitance improvement is limited. For OMC/Ni/CNTs-10 min and OMC/Ni/CNTs-30 min nanocomposites, there are many entangled CNTs observed on the outer surfaces as shown in SEM images (shown in Fig. 3C, D). These entangled CNTs failed to contact with the host OMC/Ni thoroughly and their interaction with OMC/Ni was weak, so these CNTs made few contribution to charge accumulation. Moreover, the entangled CNTs on the outer surfaces would plug up the pores (shown in Table 1) and block the penetration of electrolyte, which inhibited the contact of active materials with the electrolyte. These shortcomings will increase resistance effects and hence decrease the specific capacitance. Ragone plots (power density vs. energy density) of the OMC and OMC/Ni/CNTs-5 min are presented in Fig. 5D. The energy and power densities were derived from chargeedischarge curves at different current densities. The energy density is estimated to be 46.9 Wh kg1 for OMC/Ni/CNTs-5 min at a power density of 500 W kg1, which is about 1.6 times larger than that of the OMC (28.9 Wh kg1). More significantly, the energy density is still as high as 18.1 Wh kg1 for OMC/Ni/CNTs-5 min even at a high power density of 25 kW kg1 while only 15.4 Wh kg1 for OMC at a power density of 15 kW kg1. The results have shown that the OMC/Ni/ CNTs-5 min nanocomposites can maintain a higher power density without a large sacrifice of energy density, which indicates that OMC/Ni/CNTs-5 min nanocomposite is a very promising electrode material for high performance supercapacitors, especially for highrate charge/discharge operations.
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The above-mentioned electrochemical properties were further proved by the EIS measurement. Fig. 6 is a Nyquist plot of the OMC (b) and OMC/Ni/CNTs-5 min (a) in the same electrolyte with an identical frequency loop. The intersection of the plots at the X-axis represents solution resistance (Rs), which includes the resistance of the electrolyte solution, the intrinsic resistance of the active materials and the contact resistance at the interface between active materials and current collectors [31]. The Rs of the OMC/Ni/CNTs5 min nanocomposite is 1.07 U. At the high-frequency region, the diameter of the semicircle on the real axis presents the charge transfer resistance (Rct) in the electrochemical system [39], which is approximated to be 0.15 U (insert of Fig. 6). However, the Rs and Rct for the OMC are 1.19 U and 0.30 U, respectively, which are both higher than that of the OMC/Ni/CNTs-5 min nanocomposite. This tendency is fairly consistent with the electrochemical activities demonstrated by the chargeedischarge measurements. These results further prove that CNTs with uniformly distributed growth on the surfaces of the OMC can help the electrolyte ions penetrate into the OMC and access the inner ordered mesopores easily. Therefore, OMC/Ni/CNTs-5 min nanocomposite could attain a high capacitance even at a high chargeedischarge rate. The long-term cycle stability of OMC/Ni/CNTs-5 min nanocomposite electrode was examined in 6 M KOH through constant chargeedischarge at a current density of 10 A g1 for 4000 cycles and the data is shown in Fig. 7. The initial specific capacitance of OMC/Ni/CNTs-5 min nanocomposite is 190.0 F g1 in the first cycle. It can be observed that the specific capacitance decreases gradually in the subsequent cycles and then reach a stable value. The loss in specific capacitance is possible ascribed to the pseudocapacitance degradation of the nickel oxide. After 4000 consecutive cycles, the specific capacitance (174.0 F g1) still remains at 91.6% of the initial specific capacitance. The results confirm that the as-synthesized OMC/Ni/CNTs-5 min nanocomposite exhibits an outstanding electrochemical stability. The results above have shown that the present OMC/Ni/CNTs5 min nanocomposite exhibited excellent electrochemical properties and good cycling stability, which make it a very promising electrode material for high performance supercapacitors. As mentioned earlier, the as-synthesized OMC/Ni/CNTs-5 min material has several advantages. Firstly, to improve the conductivity of the OMC, the OMC was uniformly covered and interconnected with a layer of CNTs with good electrical conductivity, forming a hybrid nanostructure network, which could greatly improve the conductivity of the electrode. Secondly, the CNTs were highly graphitic
Fig. 6. Nyquist plots for the (a) OMC/Ni/CNTs-5 min and the (b) OMC in the same electrolyte, the inset is the magnification image at the high-frequency region.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (21206043, 21236003), the Basic Research Program of Shanghai (11JC1403000), the Special Research Fund for the Docoral Program of Higher Education of China (2011007 4110010, 20120074120004), Program for New Century Excellent Talents in University (NCET-11-0641), Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (YD0130506), the Shanghai Pujiang Program (12PJ1401900), the Fundamental Research Funds for the Central Universities. References
Fig. 7. Variation of the specific capacitance with cycle number for the OMC/Ni/CNTs5 min nanocomposite in 6 M KOH at a current density of 10 A g1.
(Fig. 4B) and further prompted an excellent rate capability of this nanostructure. Highly graphitic CNTs will deliver the stored energy under a high current density. Above two points contribute to excellent conductivity and large specific capacitance under a high current density. However, the growth time of CNTs is very important. If the growth time is too short, the generated CNTs are less and uneven, the interconnected network is hardly formed. If the growth time is too long, entangled CNTs with high density almost fully cover the entire surfaces of the OMC and plug up the pores of the walls, which make few contributions to charge accumulation. These factors result in poor electrochemical properties. Thirdly, the nickel nanoparticles embedded in the carbon walls gradually underwent an electrochemical oxidation into nickel oxide in 6 M KOH aqueous solution during the electrochemical test. This process provides extra pseudocapacitance which will greatly improve the specific capacitance of entire nanocomposite at a low current density [40e 43]. 4. Conclusions In conclusion, we have demonstrated a rational and facile strategy to synthesize a novel ordered mesoporous carbon/nickel/ carbon nanotubes nanocomposite for high rate performance supercapacitors applications. The as-prepared nanocomposite with 5-min growth time of CNTs exhibits a high specific surface area (1197.2 m2 g1) and uniform pore size distribution (w6.4 nm and 2.2 nm). At the same time, the CNTs are uniformly distributed on the surfaces of the OMC. As predicted, when applied as supercapacitor electrode, OMC/Ni/CNTs-5 min nanocomposite delivers a specific capacitance as high as 338.1 F g1 at a current density of 1 A g1, which is about 1.6 times than that of the OMC (208.6 F g1). Even at a current density of as high as 50 A g1, the specific capacitance still remains at 130.2 F g1, much higher than that of the OMC and other reported carbon materials. The energy density is estimated to be as high as 18.1 Wh kg1 at a power density of 25 kW kg1. Last but not least, supercapacitors based on this nanomaterial are cost effective and possess excellent electrochemical performance, making them a very promising electrode material for practical applications.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A three-dimensional ordered mesoporous carbon/carbon nanotubes nanocomposites for supercapacitors Zhengju Zhu, Yanjie Hu, Hao Jiang*, Chunzhong Li* Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A three-dimensional ordered mesoporous carbon (OMC)/carbon nanotubes (CNTs) nanocomposite was synthesized via a facile route. The introduction of CNTs into OMC constructs a 3D conductive network, greatly improving the rate performance. The optical nanocomposite, when used as supercapacitor electrodes, exhibited an excellent electrochemical performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2013 Received in revised form 19 July 2013 Accepted 24 July 2013 Available online 6 August 2013
A three-dimensional ordered mesoporous carbon (OMC)/carbon nanotubes (CNTs) nanocomposite is prepared via a two-step procedure. Firstly, OMC is synthesized through a co-assembly strategy associated with the incorporation of Ni nanoparticles. Then Ni nanoparticles are used as catalyst for the growth of CNTs. The introduction of CNTs into OMC can construct a 3D conductive network, greatly improving the rate performance of the nanocomposites. The nanocomposite with optimal CNTs content, when applied as supercapacitor electrodes, exhibits a high specific capacitance (338.1 F g1 at 1 A g1), excellent rate capability (130.2 F g1 at 50 A g1) and high cycling stability (91.6% capacity retention after 4000 cycles) in 6 M KOH aqueous solution. Such intriguing electrochemical performance is mainly attributed to the synergistic effects between OMC and CNTs. It is reckoned that the present 3D OMC/CNTs nanocomposite can serve as a promising electrode material for supercapacitors. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Ordered mesoporous carbon Carbon nanotube Nanocomposite Supercapacitor
1. Introduction In recent years, supercapacitors, also known as electrochemical capacitors or ultracapacitors, have attracted significant attention as they can provide a higher power density than batteries, fast chargee discharge processes, and long cycle life [1e4]. They have been widely applied in consumer electronics, energy management, memory back-up systems, industrial power and mobile electrical systems
* Corresponding authors. Tel.: þ86 21 64250949; fax: þ86 21 64250624. E-mail addresses: [email protected] (H. Jiang), [email protected] (C. Li).
[2,5]. In these applications, low energy density and high production cost have been identified as major challenges for the furtherance of supercapacitors technologies [6]. To overcome these obstacles, one of the most intensive approaches is to develop new electrode materials for supercapacitors. Up to now, various transition metal oxides [7,8], carbon materials [9,10], and conducting polymers [11,12] have been explored. Among them, carbon materials are the most popular due to their high surface areas for charge storage, low cost, easy availability, stability and environmental benignity. Various carbon materials could be used as supercapacitors electrodes, typical examples include carbon nanotubes (CNTs) [13,14] and ordered mesoporous carbon (OMC) [15,16]. CNTs are
0378-7753/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.07.086
Z. Zhu et al. / Journal of Power Sources 246 (2014) 402e408
used as electrodes to increase the power density as a result of their unique tubular structures and superior electrical properties, which favor fast ions and electrons transportation. However, the high production cost and entanglement problem limit their wide spread applications [4,17,18]. OMC possesses excellent electrochemical performance due to its high surface area, narrow pore size distribution and uniform pore connection. But the conventional hard-template nanocasting method to prepare OMC is obviously high-cost [19]. Furthermore, the conductivity of the OMC is much lower than that of the CNTs. Therefore, it is important to construct novel architectures by coupling the advantages of different carbon materials to further improve the electrochemical performances. Recently, OMC/CNTs nanocomposites have aroused intense research interest in areas of energy storage systems due to their combined properties [20e22]. To be more detailed, OMC can provide an interconnected mesoporous structure with a large surface area, allowing for the facile transport of ions and electrons. In addition, CNTs that interconnect with the primary pieces of OMC can function as electrical bridges, thus lowering the interfacial resistance and generating fast electrical networks. However, some OMC/CNTs nanocomposites reported previously were synthesized via a nanocasting method using ordered mesoporous silica (OMS) particles as a template and Niephthalocyanine as carbon source, both of which are high cost, thus industrially unfeasible [20,21]. In other cases, OMC/CNTs nanocomposites were obtained by a modified organiceinorganic self-assembly route, and the results showed much lower specific surface area and limited improvement for power and energy densities [22]. Therefore, it is highly desirable to develop an approach to prepare OMC/CNTs nanocomposites with a high specific capacitance at high current densities while maintaining a low fabrication cost. In the present work, we have developed a simple and reproducible method to prepare OMC/CNTs nanocomposites with both high energy and power densities for supercapacitors and low fabrication cost. Our strategy can be briefly described as Fig. 1. The preparation process mainly involves: (1) the synthesis of OMC with Ni nanoparticles embedded in the carbon walls, which has high surface area and abundant mesoporous; (2) the CNTs growth on OMC channels using Ni nanoparticles as catalyst and C2H2 as carbon source, respectively by a modified CVD process [23,24]. The CNTs content can be easily controlled just by tuning the reaction time. The results indicated that the optimal OMC/CNTs nanocomposite exhibited a highest specific capacitance with intriguing rate performance and cycling stability.
403
2. Experimental section 2.1. Synthesis of the OMC/Ni nanocomposites The resol precursor was prepared following synthesis procedure reported by Zhao et al. [25]. The OMC/Ni nanocomposites were synthesized via a co-assembly approach by using triblock copolymer F127, phenol resin, TEOS and nickel nitrate, followed by carbonization and silica removal [26]. In a typical preparation, 1.6 g of block copolymer F127 and 1.0 g of 0.2 M HCl were dissolved in 8.0 g of ethanol, followed by mixing with 0.13 g Ni(NO3)2$6H2O and stirred for 1 h at 40 C to obtain a clear solution. Then 2.08 g of TEOS and 5.0 g of 20 wt% resols’ ethanolic solution were added to the solution slowly in sequence. After stirring for 2 h, the solution was transferred into dishes. It took 5e8 h at room temperature to evaporate ethanol and 24 h at 100 C in an oven to thermopolymerize. The as-made flaxen and transparent products were peeled off and ground into fine powders. Calcination was carried out in a tubular furnace at 900 C for 2 h under N2 flow with a heating rate of 1 C min1 below 600 C and 5 C min1 above 600 C. The obtained carbon/silica/nickel products were stirred in 35 g of 1.0 M NaOH solution at 55 C for 24 h to remove the silica. The as-made carbon/nickel nanocomposite was denoted as OMC/Ni. 2.2. Synthesis of the OMC/Ni/CNTs nanocomposites The growth of CNTs on OMC/Ni nanocomposite was achieved using a modified CVD process according to procedure reported previously by our group [27]. Typically, 0.06 g OMC/Ni was spread evenly within a porcelain ark to form a powder bed. Then the ark was placed in the apparatus for the experiment, an electrical furnace equipped with a horizontal corundum tube, which is 42 mm in diameter, 700 mm in length, and has a 300 mm long reaction zone. The chamber was first heated to 650 C at a rate of 5 C min1 under an Ar flow, and then acetylene was introduced into the furnace under a gas feeding rate of 600/30 sccm Ar/C2H2 for growth of CNTs. After the growth step, the acetylene was turned off, and the samples were cooled to room temperature under flowing Ar. The final products were named as OMC/Ni/CNTs-xmin, wherein x represents the growth time of CNTs. The mass loadings of CNTs in these nanocomposites were evaluated by calculating the weight difference between raw materials and products. For comparison, OMC was synthesized by a solvent evaporation induced selfassembly (EISA) method [28].
Fig. 1. Schematic illustration of the preparation strategy.
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2.3. Characterizations
E ¼ C ðDVÞ2 =2
(3)
X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2550VB/PC X-ray diffractometer with Cu Ka radiation (40 kV, 100 mA). Transmission electron microscopy (TEM) experiments were conducted on a JEOL 2010 microscope operated at 200 kV. Scanning electron microscopy (SEM) images were conducted on a Hitachi S-4800 microscope operated at 15 kV. Nitrogen adsorptionedesorption isotherms were measured at 77 K with a Micromeritcs 2010 analyzer.
P ¼ E=Dt
(4)
The electrochemical measurements were conducted on an Autolab PGSTAT30 potentiostat using a three-electrode configuration in a 6 M KOH solution. The as-synthesized materials were mixed with acetylene black and polyvinylidene fluoride binder at a mass ratio of 80:10:10. A small amount of N-methyl-2-pyrrolidone was added to promote homogeneity [29]. The mixture was then coated onto one end of the graphite paper (w1 cm2) to form the working electrode after drying at 120 C, and the mass loading of all active materials was in the range of 0.90e1.2 mg). The reference and counter electrodes were Ag/AgCl electrode and platinum foil, respectively. Typical CV curves were measured between 0.8 and 0.2 V. The electrochemical impedance spectroscopy (EIS) test was conducted in the same electrolyte with a frequency loop from 100 kHz to 10 mHz using a perturbation amplitude of 5 mV at the open circuit potential. The specific capacitance was calculated from the CV curves according to the following equation [13]:
(1)
where C (F ge1) is the specific capacitance, m (g) is the mass of the active materials in the work electrode, qa and qc represent the anodic and cathodic voltammetric charges on the positive and negative sweeps, and DV is the potential window. The discharge specific capacitance could also be calculated from the discharge curves by the following equation [8]:
C ¼ I Dt=ðm DVÞ
3. Results and discussion 3.1. Morphology characterization and structure analysis
2.4. Electrochemical measurements
C ¼ ðqa þ jqc jÞ=ð2 m DVÞ
where C is the specific capacitance of the active materials, and DV is the potential window of discharge.
(2)
where I (A), Dt (s), m (g) and DV (V) are the discharge current, discharge time consumed in the potential range, mass of the active materials in the work electrode, and the potential windows, respectively. The energy density (E) and power density (P) were calculated by the following equations [8]:
Wide-angle XRD pattern of the OMC/Ni nanocomposite is shown in Fig. 2A. The XRD pattern shows three resolved diffraction peaks attributable to the (111), (200), and (220) reflections of metallic nickel with the face-centered cubic structure according to the JCPDS Card No. 04-0850, which suggests that Ni(II) from Ni(NO3)2$6H2O was completely reduced to metallic nickel during the carbonization at 900 C. In addition, a broad diffraction peak near 2q ca. 20e30 is also observed in the wide-angle XRD pattern, and this broad peak is characteristic of amorphous carbon contributed from OMC. TEM image of the OMC/Ni nanocomposite (Fig. 2B) shows an ordered 2-D hexagonal mesostructure with an estimated cell parameter (a0) of 10.4 nm. The Ni nanoparticles have been welllocated in the carbon matrix with a size of w20.0 nm. Then, the Ni nanoparticles were used to catalyze C2H2 for the realization of the growth of CNTs by a CVD process. As shown in Fig. 3A, trace of CNTs can be observed even by a 2 min reaction (OMC/Ni/CNTs2 min). With the extension of the growth time, CNTs keep growing and many well-defined CNTs are obtained. The OMC/Ni/CNTs-5 min demonstrates that CNTs are uniformly distributed on the surface of the OMC (Fig. 3B). The interconnected construction between OMC and CNTs is beneficial for the rapid transportation of the electrons and ions. The detailed structure of the OMC/Ni/CNTs-5 min was further investigated by TEM. As shown in Fig. 3E. An oriented stripe-like pattern similar to OMC/Ni was still well-maintained after growth of the CNTs. It is noted that the CNTs are grown by Ni catalysis, indicating a good connection by an OMC-Ni-CNTs way, which can construct a three dimensional conductive network for the rapid electron transportation, ensuing a high rate performance. The EDS image of OMC/Ni/CNTs-5 min was shown in the inset of Fig. 3E. The content Ni catalyst is estimated to about 4.8 w%. When the growth time was further prolonged to 10 and 30 min, the entire surfaces of OMC were almost fully covered with amounts of CNTs (Fig. 3C, D). The CNTs content of each sample is summarized in Table 1.
Fig. 2. (A) Wide-angle XRD pattern and (B) TEM image of the OMC/Ni.
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Fig. 3. (A) SEM image of the OMC/Ni/CNTs-2 min, (B) SEM image and (E) TEM image of the OMC/Ni/CNTs-5 min, (C) SEM images of the OMC/Ni/CNTs-10 min, (D) SEM image of the OMC/Ni/CNTs-30 min. (The inset of E is EDX image of OMC/Ni/CNTs-5 min.)
The small-angle XRD patterns (Fig. 4A) of the OMC/Ni/CNTsxmin nanocomposites mainly show two well-resolved diffraction peaks attributed to the (10), (11) reflections of two-dimensional hexagonal mesostructure symmetry with the space group of p6m [30]. The calculated cell parameters of these nanocomposites ranged from 10.9 to 11.7 nm, which were similar to that of the pristine sample OMC/Ni. The results suggested that the ordered hexagonal mesostructure of OMC/Ni has been well maintained in OMC/Ni/CNTs nanocomposites. It is noted that the intensity of the reflection peaks of OMC/Ni/CNTs-10 min and OMC/Ni/CNTs-30 min nanocomposites is relative weak, mainly due to the structural defects of amounts of CNTs. In wide-angle XRD patterns (Fig. 4B), all the samples exhibit a very sharp peak centered at 26.3 corresponding to the (002) diffraction of graphite. We suggest that the graphitic character of these samples may originate from the highly graphitic CNTs [20]. N2 adsorption and desorption isotherms and the pore size distribution curves of OMC/Ni/CNTs-xmin nanocomposites are shown in Fig. 4 and also summarized in Table 1. All nanocomposites exhibited type-IV curves with H1 hysteresis loops. Distinct capillary condensation steps occurred approximately at P/P0 ¼ 0.40e0.70, suggesting a high uniformity of mesoporous sizes. An increased sorption in the isotherm curves at relative pressure of P/P0 ¼ 0.1e0.3 is observed for all samples (Fig. 4C), which indicates that abundant small pores below 4.0 nm were generated. These small pores are
Table 1 Textural properties of OMC/Ni/CNTs-xmin nanocomposites. Sample
CNTs content (wt%)
a0 (nm)
SBET (m2 g-1)
Vt (cm3 g-1)
Dp (nm)
OMC/Ni/CNTs-2 min OMC/Ni/CNTs-5 min OMC/Ni/CNTs-10 min OMC/Ni/CNTs-30 min
10.3 23.3 36.4 50.1
10.9 11.1 10.9 11.7
1269.1 1197.3 850.1 539.9
1.22 1.13 0.79 0.62
6.7 6.5 6.4 6.6
CNTs content is evaluated by calculating the weight difference between raw materials and products; a0, the unit cell parameter, is calculated by using the formula a0 ¼ 2d10/O3; SBET is the BET surface area; Vt is the total pore volume; Dp is the primary mesoporous diameter.
caused by the removal of silica located inside the pore walls from the carbon/silica/nickel products [26,31]. But the absorption volume related to the small pores gradually disappears with the increase of CNTs loadings. This is because that the small pores inside the carbon walls were gradually destroyed with increasing the loadings amount of CNTs. The BET surface areas and pore volumes of OMC/Ni/ CNTs-2 min and OMC/Ni/CNTs-5 min are as high as 1269.1 m2 g1, 1197.3 m2 g1 and 1.22 cm3 g1, 1.11 cm3 g1 (Table 1), respectively. The high BET surface areas and mesoporous structures provide the possibility of efficient transport of electrons and ions in the electrodes. But these values decreased sharply in samples OMC/Ni/ CNTs-10 min and OMC/Ni/CNTs-30 min, associated with higher content of CNTs (Table 1) and mesostructural degradation. The pore size distribution curves (Fig. 4D) calculated from the adsorption branches clearly confirm the narrow pore size distributions centered at about 6.5 nm and multimodal pore structures for OMC/ Ni/CNTs-xmin nanocomposites. Interestingly, nanocomposites with different loadings of CNTs exhibit analogous narrow pore size distribution, however, the pore volumes related to the primary mesoporous, as well as the small pores decrease obviously. For samples OMC/Ni/CNTs-10 min and OMC/Ni/CNTs-30 min, small pores centered at 2.3 nm almost disappear. 3.2. Electrochemical behaviors of OMC/Ni/CNTs-xmin nanocomposites To explore the energy storage applications of these nanocomposites, cyclic voltammogram (CV) and galvanostatic chargee discharge (CD) measurements were carried out in 6 M KOH. Fig. 5A shows the CV curves of all OMC/Ni/CNTs-xmin nanocomposites at a scan rate of 50 mV s1. For comparison, the CV curve of OMC is also given. All nanocomposites present a typical capacitive behavior with rectangular and symmetric CV curves. Among them, the OMC/Ni/CNTs-5 min exhibits the highest specific capacitance of 135.9 F g1 at a scan rate of 50 mV s1. The pristine OMC possesses a specific capacitance of 99.9 F g1 at the same scan rate. As for CNTs, their specific capacitances cannot exceed 50 F g1 under a low current generally [32,33]. Even
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Fig. 4. (A) Small-angle XRD patterns, (B) wide-angle XRD patterns, (C) N2 sorption isotherms and (D) pore size distributions curves of the OMC/Ni/CNTs-xmin nanocomposites: (a) OMC/Ni/CNTs-2 min, (b) OMC/Ni/CNTs-5 min, (c) OMC/Ni/CNTs-10 min, (d) OMC/Ni/CNTs-30 min.
though the performance of CNTs electrode could be enhanced by chemical activation, functionalization and purification, the most values of specific capacitances are still around 100 F g1 [34,35]. It is clear that there is an unexpectedly large increase in specific capacitance due to the growth of CNTs on OMC. This demonstrates the strong synergistic effect between OMC and CNTs. Furthermore, the OMC/Ni/CNTs-xmin nanocomposites are also reckoned to possess some pseudocapacitance from the reduction peaks observed at the range of 0.2e0.2 V in the CV curves, which are caused by nickel oxide originated from Ni nanoparticles
embedded in the carbon walls. This phenomenon is similar to some reports previously [36]. It is well known that electrochemical oxidation of nano-sized Ni to its corresponding nickel oxide can be realized by potential scan at the Ni oxide region (range from 0.1 to 0.5 V vs. SCE) [37]. And no reduction peak can be observed for OMC containing no Ni nanoparticles at the same electric potential range. These results indicate that the capacitive responses of the OMC/Ni/CNTs-xmin nanocomposites come from the combination of electrical double layer capacitances (EDLCs) and redox reactions [38].
Fig. 5. (A) CV curves at a scan rate of 50 mV s1 and (C) the specific capacitance as a function of different current densities of the (a) OMC and OMC/Ni/CNTs-xmin nanocomposites: (b) OMC/Ni/CNTs-2 min, (c) the OMC/Ni/CNTs-5 min, (d) the OMC/Ni/CNTs-10 min and (e) OMC/Ni/CNTs-30 min. (B) chargeedischarge curves at 1e50 A g1 of the OMC/Ni/CNTs5 min, (D) Ragone plot (energy density vs. power density) of the OMC and the OMC/Ni/CNTs-5 min, respectively.
Z. Zhu et al. / Journal of Power Sources 246 (2014) 402e408
In order to further evaluate the electrochemical performance of the OMC/Ni/CNTs-5 min nanocomposites, the galvanostatic chargeedischarge curves were measured at 1e50 A g1, as shown in Fig. 5B. It can be seen that the charging time is almost equal to their corresponding discharge counterpart. The specific capacitance is calculated from the discharge curves. It is noted that the specific capacitance of the OMC/Ni/CNTs-5 min can reach as high as 338.1 F g1 at a current density of 1 A g1, which is significantly higher than that of the OMC (208.6 F g1). This result is in good agreement with that of the CV characterization. The rate capability is an important factor for supercapacitors in high power applications. The relationships between specific capacitance and charge/ discharge current density for all OMC/Ni/CNTs-xmin nanocomposites and OMC are illustrated in Fig. 5C. Obviously, as the growth time of the CNTs increases from 2 to 5 min, the corresponding nanocomposites show better rate capability than that of the OMC. However, when the growth time of CNTs further increases to 10 min and 30 min, the rate capability of the corresponding nanocomposites is poorer than that of the OMC. It can be seen that OMC/Ni/CNTs-5 min nanocomposite not only delivers the highest specific capacitance, but also maintains it at a high current density compared to the other materials. Even at 50 A g1, the specific capacitance of OMC/Ni/CNTs-5 min (c line) still remains 130.2 F g1. Its excellent capability is contributed to the hybrid nanostructure which consists of highly ordered mesostructure, well-dispersed Ni nanoparticles in the carbon walls and uniformly distributed CNTs. The hybrid nanostructure not only takes full advantage of the EDLCs from the carbon layer and pseudocapacitance from the oxidation of Ni nanoparticles, but also possesses the superior electrical properties with CNTs structure, which favors fast ions and electrons transportation [14]. Besides, the hybrid electrode consistently shows higher specific capacitance than that of the pristine OMC electrode, further indicating that the CNTs indeed facilitated the harvest of the EDLCs, nickel oxide pseudocapacitance and the synergetic effect of three components. Although the specific capacitance of sample OMC/Ni/CNTs-2 min is also higher than that of the OMC, the less and uneven CNTs could not interconnect all the primary pieces of the OMC (shown in Fig. 3A), thus its specific capacitance improvement is limited. For OMC/Ni/CNTs-10 min and OMC/Ni/CNTs-30 min nanocomposites, there are many entangled CNTs observed on the outer surfaces as shown in SEM images (shown in Fig. 3C, D). These entangled CNTs failed to contact with the host OMC/Ni thoroughly and their interaction with OMC/Ni was weak, so these CNTs made few contribution to charge accumulation. Moreover, the entangled CNTs on the outer surfaces would plug up the pores (shown in Table 1) and block the penetration of electrolyte, which inhibited the contact of active materials with the electrolyte. These shortcomings will increase resistance effects and hence decrease the specific capacitance. Ragone plots (power density vs. energy density) of the OMC and OMC/Ni/CNTs-5 min are presented in Fig. 5D. The energy and power densities were derived from chargeedischarge curves at different current densities. The energy density is estimated to be 46.9 Wh kg1 for OMC/Ni/CNTs-5 min at a power density of 500 W kg1, which is about 1.6 times larger than that of the OMC (28.9 Wh kg1). More significantly, the energy density is still as high as 18.1 Wh kg1 for OMC/Ni/CNTs-5 min even at a high power density of 25 kW kg1 while only 15.4 Wh kg1 for OMC at a power density of 15 kW kg1. The results have shown that the OMC/Ni/ CNTs-5 min nanocomposites can maintain a higher power density without a large sacrifice of energy density, which indicates that OMC/Ni/CNTs-5 min nanocomposite is a very promising electrode material for high performance supercapacitors, especially for highrate charge/discharge operations.
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The above-mentioned electrochemical properties were further proved by the EIS measurement. Fig. 6 is a Nyquist plot of the OMC (b) and OMC/Ni/CNTs-5 min (a) in the same electrolyte with an identical frequency loop. The intersection of the plots at the X-axis represents solution resistance (Rs), which includes the resistance of the electrolyte solution, the intrinsic resistance of the active materials and the contact resistance at the interface between active materials and current collectors [31]. The Rs of the OMC/Ni/CNTs5 min nanocomposite is 1.07 U. At the high-frequency region, the diameter of the semicircle on the real axis presents the charge transfer resistance (Rct) in the electrochemical system [39], which is approximated to be 0.15 U (insert of Fig. 6). However, the Rs and Rct for the OMC are 1.19 U and 0.30 U, respectively, which are both higher than that of the OMC/Ni/CNTs-5 min nanocomposite. This tendency is fairly consistent with the electrochemical activities demonstrated by the chargeedischarge measurements. These results further prove that CNTs with uniformly distributed growth on the surfaces of the OMC can help the electrolyte ions penetrate into the OMC and access the inner ordered mesopores easily. Therefore, OMC/Ni/CNTs-5 min nanocomposite could attain a high capacitance even at a high chargeedischarge rate. The long-term cycle stability of OMC/Ni/CNTs-5 min nanocomposite electrode was examined in 6 M KOH through constant chargeedischarge at a current density of 10 A g1 for 4000 cycles and the data is shown in Fig. 7. The initial specific capacitance of OMC/Ni/CNTs-5 min nanocomposite is 190.0 F g1 in the first cycle. It can be observed that the specific capacitance decreases gradually in the subsequent cycles and then reach a stable value. The loss in specific capacitance is possible ascribed to the pseudocapacitance degradation of the nickel oxide. After 4000 consecutive cycles, the specific capacitance (174.0 F g1) still remains at 91.6% of the initial specific capacitance. The results confirm that the as-synthesized OMC/Ni/CNTs-5 min nanocomposite exhibits an outstanding electrochemical stability. The results above have shown that the present OMC/Ni/CNTs5 min nanocomposite exhibited excellent electrochemical properties and good cycling stability, which make it a very promising electrode material for high performance supercapacitors. As mentioned earlier, the as-synthesized OMC/Ni/CNTs-5 min material has several advantages. Firstly, to improve the conductivity of the OMC, the OMC was uniformly covered and interconnected with a layer of CNTs with good electrical conductivity, forming a hybrid nanostructure network, which could greatly improve the conductivity of the electrode. Secondly, the CNTs were highly graphitic
Fig. 6. Nyquist plots for the (a) OMC/Ni/CNTs-5 min and the (b) OMC in the same electrolyte, the inset is the magnification image at the high-frequency region.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (21206043, 21236003), the Basic Research Program of Shanghai (11JC1403000), the Special Research Fund for the Docoral Program of Higher Education of China (2011007 4110010, 20120074120004), Program for New Century Excellent Talents in University (NCET-11-0641), Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (YD0130506), the Shanghai Pujiang Program (12PJ1401900), the Fundamental Research Funds for the Central Universities. References
Fig. 7. Variation of the specific capacitance with cycle number for the OMC/Ni/CNTs5 min nanocomposite in 6 M KOH at a current density of 10 A g1.
(Fig. 4B) and further prompted an excellent rate capability of this nanostructure. Highly graphitic CNTs will deliver the stored energy under a high current density. Above two points contribute to excellent conductivity and large specific capacitance under a high current density. However, the growth time of CNTs is very important. If the growth time is too short, the generated CNTs are less and uneven, the interconnected network is hardly formed. If the growth time is too long, entangled CNTs with high density almost fully cover the entire surfaces of the OMC and plug up the pores of the walls, which make few contributions to charge accumulation. These factors result in poor electrochemical properties. Thirdly, the nickel nanoparticles embedded in the carbon walls gradually underwent an electrochemical oxidation into nickel oxide in 6 M KOH aqueous solution during the electrochemical test. This process provides extra pseudocapacitance which will greatly improve the specific capacitance of entire nanocomposite at a low current density [40e 43]. 4. Conclusions In conclusion, we have demonstrated a rational and facile strategy to synthesize a novel ordered mesoporous carbon/nickel/ carbon nanotubes nanocomposite for high rate performance supercapacitors applications. The as-prepared nanocomposite with 5-min growth time of CNTs exhibits a high specific surface area (1197.2 m2 g1) and uniform pore size distribution (w6.4 nm and 2.2 nm). At the same time, the CNTs are uniformly distributed on the surfaces of the OMC. As predicted, when applied as supercapacitor electrode, OMC/Ni/CNTs-5 min nanocomposite delivers a specific capacitance as high as 338.1 F g1 at a current density of 1 A g1, which is about 1.6 times than that of the OMC (208.6 F g1). Even at a current density of as high as 50 A g1, the specific capacitance still remains at 130.2 F g1, much higher than that of the OMC and other reported carbon materials. The energy density is estimated to be as high as 18.1 Wh kg1 at a power density of 25 kW kg1. Last but not least, supercapacitors based on this nanomaterial are cost effective and possess excellent electrochemical performance, making them a very promising electrode material for practical applications.
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