High-yield synthesis of carbon nanotube–porous nickel oxide nanosheet hybrid and its electrochemical capacitance performance

High-yield synthesis of carbon nanotube–porous nickel oxide nanosheet hybrid and its electrochemical capacitance performance

Materials Chemistry and Physics 143 (2014) 1344e1351 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www...

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Materials Chemistry and Physics 143 (2014) 1344e1351

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

High-yield synthesis of carbon nanotubeeporous nickel oxide nanosheet hybrid and its electrochemical capacitance performance Kai Dai a, *, Changhao Liang b, *, Jianming Dai b, Luhua Lu c, Guangping Zhu a, Zhongliang Liu a, Qinzhuang Liu a, Yongxing Zhang a a

College of Physics and Electronic Information, Huaibei Normal University, Huaibei 235000, PR China Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, PR China c State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China b

h i g h l i g h t s  CNTeNiO hybrid nanocomposites were large-scale fabricated.  CNTeNiO presented a high specific capacitance of 759 F g1.  The obtained CNTeNiO exhibit excellent cycling performance.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 March 2013 Received in revised form 12 October 2013 Accepted 24 November 2013

This study reports an easy chemical conversion route toward large-scale fabrication of carbon nanotube (CNT)eporous nickel oxide (NiO) hybrid nanocomposites as supercapacitor electrode materials. The electrocapacitive performance of CNTeporous NiO hybrids is evaluated by cyclic voltammetry and galvanostatic chargeedischarge measurements. The synthesized CNTeNiO hybrid nanocomposite electrode presents a high specific capacitance of 759 F g1 at 0.5 A g1 in 6 M KOH aqueous electrolyte, which is almost twice that of pure NiO nanoparticle (388 F g1) electrodes and nine times of that of commercial NiO particle (88.4 F g1) electrodes. Furthermore, good capacitance retention is achieved after 1000 cycles of galvanostatic chargeedischarge. The synergistic effects from the pseudocapacitance of porous NiO particles, good electrical conductivity, and open tip CNTs attribute to the high capacitance performance. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Nanostructures Chemical synthesis Electrochemical techniques Electrochemical properties

1. Introduction An ideal electrochemical electrode material for practical applications should have high electrochemical capacity and cyclic stability [1], large-scale processible material form (powder or slurry) for device fabrication [2], low cost, and environment friendly [3,4]. Nickel oxide (NiO) has been widely used as a battery electrode material because of its low cost [5], abundant natural resources, theoretical ultrahigh specific capacitance of 2584 F g1, and low environmental toxicity [6,7]. Similar to other metal oxide electrode materials, large specific volume change commonly occurs in the NiO host matrix during the cycling of the chargingedischarging

* Corresponding authors. Fax: þ86 561 3803256. E-mail addresses: [email protected], [email protected] [email protected] (C. Liang). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.11.045

(K.

Dai),

processes. This characteristic leads to the pulverization of the NiO electrode and rapid capacity decay. Moreover, the poor electrical conductivity of NiO reduces its bulk electrode charging speed and full utilization of its capacitance [8e10]. Thus, persistent involvement has been attempted in the recent years to resolve these problems. Several studies have shown that NiO nanoparticles (NPs) with an increased specific surface area are favorable to ion exchange with electrolyte environments, consequently increasing its specific capacitance [11e13]. Furthermore, compositing NiO with conductive fillers, such as carbon nanotube (CNT) and graphene, can effectively improve its cyclic stability and partially improve the specific capacitance of NiO composite electrodes to as high as 525 F g1 because of the high electron transfer rate of build in conducting network during faradaic charge transfer reactions [14e 17]. In addition, an appropriate approach toward large-scale fabrication of nanostructured NiO-based composite is still a great challenge for practical applications.

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In the current study, a hard template technique was developed to prepare CNTeporous NiO hybrid nanocomposites. The formation processes of the composite is illustrated in Fig. 1. The activated CNT acts as a template for polyethylene glycol (PEG) and ammonia to direct the nucleation and growth of Ni(OH)2. The as-obtained CNTe Ni(OH)2 hybrid was further calcined in a furnace to dehydrate Ni(OH)2 into NiO and decompose PEG. This treatment led to the formation of porous NiO nanoplates uniformly dispersed in the conducting CNT network. The nanostructured electrodes with CNTeNiO hybrid materials exhibited a 759 F g1 high specific capacitance because of the enhanced interfacial faradaic process and effective charge accession. Meanwhile, the CNT build in the network largely reinforced the reversibility of the electrochemical performance. The cyclic chargeedischarge process can preserve more than 95% of the initial specific capacitance over 1000 cycles. Furthermore, the present CNTeNiO nanocomposites can be synthesized in the level of tens of grams, which are critical for practical applications as electrode materials. 2. Experimental 2.1. Chemicals CNTs were purchased from Shenzhen Nano Tech Port Co., Ltd. Other A.R. grade chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. and were used as received without further purification. 2.2. Treatment of CNTs The raw materials were treated as previously reported [18]. CNTs were refluxed in 68 wt% HNO3 at 180  C for 12 h. The mixture was then washed several times with deionized water on a sintered glass filter. Finally, the CNTs were dried in an oven at 100  C for 5 h. 2.3. Synthesis of CNTeporous NiO hybrid nanocomposites Approximately 6.5 g of NiCl2$6H2O, 8 g of PEG6000, and 50 mL of ethanol were dissolved in 100 mL H2O. In addition, 2 g of treated CNTs were initially immersed in a mixed solution, and stirred at 60  C for 0.5 h. Approximately 1 L of 1% ammonium hydroxide was then dropped into the suspension. After filtering the suspension with deionized water and drying at 100  C for 3 h, the precursor of CNTeNi(OH)2 composites was obtained. The precursor was finally calcined at 380  C for 2 h to obtain CNTeporous NiO hybrid nanocomposites. NiO NPs were prepared via the aforementioned

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method without CNTs. All experiments were carried out under normal atmospheric environments. 2.4. Characterization X-ray diffraction (XRD) analysis of phases was carried out using a Rigaku D/MAX 24000 diffractometer with Cu Ka radiation (l ¼ 1.54056 A) and a scanning speed of 0.02 s1. The accelerating voltage and emission current were 40 kV and 40 mA, respectively. The sample morphology was checked using a scanning electron microscopy (SEM, JSM-6700F) at an accelerating voltage of 30 kV equipped with an Inca energy dispersive spectrometer (EDS). The fine structure of the CNTeNiO hybrid nanocomposites was further investigated on a Tecnai G2 F20 S-Twin high resolution transmission electron microscope (HRTEM) with an accelerating voltage of 200 kV. The thermal decomposition behavior of the precursor was analyzed using thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) analysis on a NetzscheSTA 409 thermal analysis device. TG-DSC determination was carried out in an air flow with a 10  C min1 temperature increasing rate from room temperature to 1000  C. Core level analysis was conducted on an X-ray photoelectron spectrum (XPS) using a Kratos AXIS Ultra DLD X-ray photoelectron spectrometer at room temperature with 1486.6 eV X-ray from the Al Ka line. 2.5. Electrochemical measurements All electrochemical experiments were carried out using a threeelectrode system at room temperature. Fabricating the working electrodes required mixing 80 wt% active materials with 15 wt% graphite and 5 wt% polyvinylidene difluoride (PVDF) binder. Afterward, the mixture was diluted with a small amount of deionized water to form homogeneous mixture slurry. After a short period of drying by evaporation, the resulting paste was pressed into a pretreated nickel grid under a pressure of 1.2  107 Pa. The electrode was then dried for 12 h at room temperature under ambient atmosphere. Each electrode has a geometric surface area of 10 mm  10 mm. Electrochemical measurements were carried out using 6 M KOH aqueous solution as electrolyte. Before the tests, the electrodes were soaked overnight in 6 M KOH aqueous solution. A platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic chargeedischarge processes were performed on a CHI 660D workstation. The galvanostatic chargeedischarge process was carried out with current densities ranging from 0.5 A g1 to 5.0 A g1. The specific capacitance (C [F g1]) of the electrode

Fig. 1. Schematics for the formation of CNTeporous NiO hybrid nanocomposites: (I) acid treatment; (II) formation of nuclei; (III) growth of Ni(OH)2; and (IV) calcination to CNTe porous NiO hybrid nanocomposites.

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material was calculated from the discharge curve according to the following equation [19,20]:

C ¼ I Dt=ðDVmÞ where: I is the discharge current (A); Dt is the discharge time (s); DV is the voltage change (V), excluding the IR drop in the discharge process; and m is the mass of the working electrode material (g), excluding the binder and conductive carbon black. 3. Results and discussion

(222)

(311)

(220)

(201)

(103)

CNT-NiO (111)

(012)

(110)

(111) (100) (011)

(001)

(002)

Intensity (arb. units)

CNT Ni(OH)2 NiO

(200)

Fig. 1 illustrates the procedures for the fabrication of CNTe porous NiO hybrid nanocomposites. In stage (I), CNTs were refluxed by acid; the oxygen-containing surface groups appeared as active sites of CNTs [18]. In stage (II), the hydrated positively charged Ni2þ ions were attracted and bound onto negatively charged CNT surfaces by electrostatic attraction and hydrogen bond with eOH and eOOH groups when the treated CNTs and NiCl2$6H2O were suspended in the deionized water and ethanol mixture. Thus, this process promotes the nucleation of Ni(OH)2 crystals. When a small amount of ammonium hydroxide was added, the formation of such intermediate complex makes the Ni2þ ion assemble around the outer CNT surfaces with the help of PEG. This treatment provides a higher potential for the precipitation of Ni(OH)2 nuclei on the surfaces of CNTs. In stage (III), Ni(OH)2 seeds agglomerate to form a planar nucleus in orientation corresponding to the formation of the Ni(OH)2 NPs when more ammonium hydroxide is added. Thus, the precursor of CNTeNi(OH)2 composites can be obtained. In stage (IV), CNTeporous NiO hybrid nanocomposites can be formed by decomposing Ni(OH)2 into NiO in the subsequent calcination process, together with the strong chemical interaction between the backbone of CNTs and porous NiO particles. Fig. 2 shows the XRD patterns of treated CNTs, porous NiO, CNTe Ni(OH)2 precursor and CNTeporous NiO hybrid nanocomposites, respectively. The CNTs showed a typical (002) reflection at 26.1. In CNTeNi(OH)2 precursor, Major diffraction peaks corresponding to Ni(OH)2 and CNTs were observed, and all of Ni(OH)2 diffraction peaks can be unambiguously indexed to the pure hexagonal structures (JCPDS No. 74-2075). During the calcination process, the hydroxyl group of Ni(OH)2 is decomposed and a large number of H2O are emitted, NiO are formed at last. As indicated in CNT-porous NiO hybrids, the CNTs still showed a typical (002) reflection at 26.1, whereas the rest of the reflections can be indexed to NiO in cubic structures (JCPDS No. 78-0643). No peaks due to Ni(OH)2 were found from XRD, indicating that Ni(OH)2 was completely

CNT-Ni(OH)2 NiO CNT 10

20

30

40

50

60

70

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2 Theta (degree) Fig. 2. XRD patterns of CNT, NiO, CNTeNi(OH)2 precursor and CNTeNiO hybrid nanocomposites.

decomposed to NiO at 380  C for 2 h, which is also confirmed by the TG measurement. Fig. 3(a), (b), and (c) shows the typical SEM images of treated CNTs, NiO NPs, and CNTeNiO hybrid nanocomposites, respectively. The open tip CNTs can be clearly observed, and the size of the assynthesized NiO NPs was 120 nme150 nm. The EDS spectrum (Fig. 3(d)) of the synthesized CNTeNiO hybrid indicated that the hybrid only contains elements of C, Ni, and O. The HRTEM images in Fig. 4(e) and (f) shows the sidewalls of the CNTs and the lattice fringes of the NiO phase that corresponds to the [111] and [200] planes of the NiO crystalline. The binding between NiO and CNT surfaces was tight enough to resist repeated rinsing and ultrasonication. To the best of our knowledge, such unique structure of porous NiO NPs on CNT surfaces has not been reported thus far. The thermal decomposition behaviors of the dried assynthesized precursor before calcination were investigated using the TG-DSC analysis to obtain the appropriate conditions for achieving CNT-porous NiO hybrid. As comparison, TG analysis of CNTs was also studied. As shown in Fig. 4(A), the thermal degradation of CNTs is a multistage process. The first stage, up to a temperature of 160  C, a weight loss of approximately 1% is detected for the acid-treated CNTs, which corresponds to the evaporation of the adsorbed water. The second stage from 160 to 380  C is attributed to the decarboxylation of the carboxylic groups present on the CNT walls. Thermal degradation in the range between the 380  C and 520  C may be explained by the elimination of hydroxyl functionalities of the CNT walls. Finally, at the temperatures higher than 520  C, the observed degradation corresponds to the thermal oxidation of the remaining disordered carbon. The results are according to the literature [21]. As indicated in Fig. 4(B), the endothermic process at the beginning of the decomposition was dominant and the weight loss in the temperature range of 20  Ce200  C can be attributed to the removal of water absorbed on the surfaces of the sample. During the process, the Ni(OH)2 was decomposed into NiO and H2O in the atmosphere, which led to the weight loss in the temperature range of 290  Ce 335  C. The exothermic peak at 325.5  C mainly resulted from the crystallization process. Moreover, the exothermic peaks at 378  C and 643.2  C mainly resulted from the burnout of a small portion of organic residues, dehydration and evaporation of chemisorbed water, and the burnout of CNTs, respectively. According to Fig. 4(B), the NiO weight ratio in CNTeNiO material is 58%. The detailed XPS spectra of CNTeNiO are shown in Fig. 5. Fig. 5(a) shows the high-resolution spectra of C1s, in which its peaks can be fitted as three peaks at binding energies of 284.90, 285.72, and 288.76 eV. This result implies three different chemical environments of carbon in the sample. The peak at 284.90 was assigned to the contributions of CeC (sp2) [22,23]. The peak at 285.72 eV was ascribed to the existence of CeOH bonds, whereas the peak at 288.76 eV was ascribed to the existence of CeOOH bonds [24,25]. Fig. 5(b) shows the high-resolution spectra of O 1s. For the CNTeNiO hybrid, the curve fitting of O 1s spectrum indicated three components centered at 529.77, 531.72, and 532.53 eV. The peak at 529.77 eV was due to the oxygen in the NiO crystal lattice [26,27]. The latter two peaks are commonly ascribed to the surface oxygen complexes of the carbon phase. The Ni 2p XPS spectra are shown in Fig. 5(c), where the binding energies of Ni 2p3/2 were centered at 853.40 eV, which was in agreement with the Ni 2p3/2 binding energy values reported in the NiO [28]. The specific capacitance is related to the specific surface area of electrode activated material. Fig. 6 shows the nitrogen adsorptione desorption isotherms for NiO particles, the as-prepared CNT/NiO hybrid, and treated and untreated CNT at 77 K. The data of BET surface area, pore specific volume of samples were listed in Table 1. As indicated in Table 1, the surface area and Pore specific volume of

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Fig. 3. SEM images of (a) treated CNTs, (b) NiO nanoparticles and (c) CNTeNiO, EDS spectrum of (d) CNTeNiO, HRTEM images of (e) CNTeNiO and (f) of selected area of (e).

treated CNTs were increased with their tips opened. The specific surface area of CNTeNiO is 5.4 and 36.2 time larger than that of NiO NPs and Commercial NiO particles, respectively. The pore size distribution curve of CNTeNiO exhibits a broad peak in 1.6e50 nm. The result indicates that there are many mesopores/micropores in the CNTeNiO hybrid which formed by CNTs, NiO nanosheets and the connection between CNTs and NiO nanosheets. These porous structures can be also directly observed from the SEM and TEM images. Fig. 7 shows the CV curves of CNT, commercial NiO, NiO NPs and CNTeporous NiO hybrid within the potential range of 0.2e0.5 V (vs. SCE) at various scan rates. As indicated in Fig. 7(a), The shapes of the CV reveal that the capacitance characteristic is electric double-layer capacitance in which the shape is normally close to an ideal rectangular shape. As indicated in Fig. 7(b), (c) and (d), the current CV density of NiO-based materials gradually increased with the increase of scan rate, whereas the capacitive character remained well. A pair of redox peaks was observed in the CV curves, which is attributed to the quasi-reversible redox processes occurring on the surfaces or near the surfaces of the NiO NPs, w0.26e w0.4 V and w0.02ew0.15 V for oxidation and reduction, respectively, as shown in the following [29,30],

NiO þ OH

charge

#

discharge

NiOOH þ e

This result indicates that the observed capacity mainly comes from the faradaic reaction pseudocapacitance based on the surface reversible redox mechanism. Furthermore, the shape of the current responses of CNTeNiO hybrid are essentially the same over the entire range of scan rates, indicative of rapid faradic reactions at high scan rates. The chargeedischarge process of the hybrid in the KOH aqueous electrolyte was mainly governed by the insertion of ions from the electrolyte into the hybrid material and its release to the electrolyte, which can be facilitated by the porous structure [6]. To understand the electrochemical performance characteristics of the CNTeporous NiO hybrids, we performed EIS measurements on these samples as well as NiO NPs, and commercial NiO particles sample. Fig. 8 shows the Nyquist plots obtained from the EIS measurements. The EIS data were fitted based on an equivalent circuit model that consisted of a bulk solution resistance RS, a charge-transfer resistance Rct, a capacitive element Cdl, and a Warburg element (W), as shown in the inset of Fig. 8. The bulk solution resistance RS and the charge-transfer resistance Rct can be estimated from the intercepts of the high-frequency semicircle

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(A)

100

TG (%)

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325.5

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643.2

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50

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TG (%)

1000

-2 0

200

400

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800

1000

Fig. 4. (A) TG curve of CNT and (B) (a) TG and (b) DSC curves of CNTeNi(OH)2 precursor.

with the real axis at RS and (RS þ Rct), respectively. In this way, the RS of the CNTeporous NiO hybrids, NiO NPs, and commercial NiO is 0.4, 2.4 and 2.7 U, respectively, whereas Rct was 0.08, 1.8, and 5.3 U in the same order. As is well known, the electrochemical capacitor performance is closely related to the charge-transfer resistance Rct. The higher the charge-transfer resistance, the lower specific capacitance. This clearly demonstrates that the CNTeporous NiO hybrids display much more favorable charge-transfer kinetics than NiO NPs, and commercial NiO. These data suggest that the conducting agent obviously affects the specific capacitance of the supercapacitor in the CNT-based electrode. Galvanostatic chargeedischarge measurements were also carried out to assess the potential of CNTeporous NiO hybrid as electrode materials for supercapacitors. Fig. 9(a), (b), and (c) shows the chargeedischarge curves for the synthesized CNTeporous NiO hybrid, NiO NPs, and commercial NiO particles at varied current densities (0.5 A g1e5 A g1) within a voltage range between 0.0 and 0.4 V, respectively. Fig. 9(d) shows the corresponding specific capacitances of CNT-porous NiO hybrid, NiO NPs, and commercially available NiO particles. The CNTeporous NiO hybrid exhibited higher specific capacitances at identical current densities than those of pure NiO NPs. CNTeporous NiO hybrid exhibited the maximum specific capacitance of 759 F g1 at 0.5 A g1, which is almost twice that of pure NiO NPs (388 F g1) and nine times that of commercial NiO particles (88.4 F g1). Thus, the mesoporous structure of NiO effectively attached to the surfaces of conductive CNT networks can effectively make its intrinsic electrochemical capacitance well embodied. Moreover, the treated CNTs with open

Fig. 5. XPS spectra for (a) C, (b) O, and (c) Ni of the CNTeporous NiO hybrid nanocomposites.

tips supplied sufficient electrochemically active sites for effective redox reactions on the NiO surfaces and provided porous channels for the deep intercalation or de-intercalation of ions, which is beneficial to the fast transfer of ions throughout the whole electrode, consequently improving electrochemical performance [30,31]. The CNTeporous NiO hybrid exhibited attractive high specific capacitance because of the synergistic effect of the pseudocapacitive porous NiO nanoplates and conductive CNTs. Fig. 10 shows the cyclic electrochemical performance of the electrodes. The cycle-life stability was evaluated by galvanostatic

K. Dai et al. / Materials Chemistry and Physics 143 (2014) 1344e1351

Fig. 6. Isotherms for nitrogen adsorptionedesorption of (a) commercial NiO particles, (b) NiO NPs, (c) untreated CNT, (d) CNTeporous NiO hybrid and (e) treated CNT. (Inset: BJH pore size distribution plot of CNTeporous NiO hybrid).

Table 1 Data of BET surface area and pore specific volume. 2

Sample

BET (m g

CNTeNiO Treated CNT Untreated CNT NiO NPs Commercial NiO particles

144.69 152.21 82.32 26.66 4.01

1

)

3

)

0.385 0.410 0.194 0.062 0.009

chargeedischarge at a current density of 0.5 A g1 within a voltage range between 0.0 V and 0.4 V for 1000 cycles. Approximately, 95% of the initial capacitance was preserved after 1000 cycles for the CNTeporous NiO hybrid, whereas pure NiO NPs preserved only

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4. Summary A promising route for easy and large scale synthesis of CNTe porous NiO hybrid was developed. The as-synthesized hybrid exhibited attractive specific capacitance as high as 759 F g1 at

40

(a) Current (A g-1)

50

Fig. 8. EIS Nyquist plots for CNTeporous NiO hybrids, NiO NPs, and commercial NiO. (Inset: equivalent circuit diagram proposed for EIS analysis of the data.)

79%. The commercial NiO particle totally failed after about 500 cycles. The distribution of porous NiO NPs on the CNT networks provided sufficient volume for reversible volume expansion compared with the internal stress-induced crack of neatly stacked NiO. The present CNTeporous NiO hybrid with high capacitance performance can be interesting for potential applications.

1

Pore specific volume (cm g

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Fig. 7. CV curves of (a) CNT, (b) commercial NiO, (c) NiO NPs and (d) CNTeNiO with different scan rates.

0.5

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Fig. 9. Galvanostatic chargeedischarge of (a) CNTeporous NiO hybrid electrode, (b) NiO NPs, and (c) commercial NiO particles at current densities of 0.5, 1.0, 3.0, and 5.0 A g1, respectively. (d) Variation of specific capacitance against current densities for CNTeporous NiO hybrid, NiO NPs, and commercial NiO particles.

0.5 A g1 and more than 95% maintenance effect after 1000 cycles of electrochemical chargeedischarge. These results are attributed to the synergistic effects of the pseudocapacitance of porous NiO NPs, good electrical conductivity, and the opened tips of selected CNTs. This simple, low cost, and large scale preparation strategy is not limited to NiO, rather, CNTs can be used as templates to prepare other metal oxide-based hybrid electrode materials for electrochemical energy conversion.

Acknowledgments This work was supported by the National Natural Science Foundation of China (51302101, 21303129, 11004071), the Key Foundation of Educational Commission of Anhui Province (KJ2012A250, KJ2010A306), and the Huaibei Science and Technology Development Funds (20110305). References

Fig. 10. Cycle performance of CNTeporous NiO hybrid, NiO NPs, and commercial NiO particles measured at a current density of 0.5 A g1.

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