Nickel Cobalt Oxide-Single Wall Carbon Nanotube Composite

Article pubs.acs.org/JPCC

Nickel Cobalt Oxide-Single Wall Carbon Nanotube Composite Material for Superior Cycling Stability and High-Performance Supercapacitor Application Xu Wang,† Xuanding Han,† Mengfang Lim,† Nandan Singh,† Chee Lip Gan,† Ma Jan,† and Pooi See Lee*,† †

School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 S Supporting Information *

ABSTRACT: The electron conductivity of electrode material has always been a problem that hinders the practical application of supercapacitor. In this contribution, we report a facile synthesis of highly conductive nickel cobalt oxide-single wall carbon nanotube (NiCo2O4−SWCNT) nanocomposite by controlled hydrolysis process in ethanol−water mixed solvent. Ultrafine NiCo2O4 nanocrystals with a diameter around 6−10 nm are formed on the functionalized SWCNT bundles. This novel material not only exhibits a high specific capacitance of 1642 F g−1 within a 0.45 V potential range but also shows an excellent cycling stability of 94.1% retention after 2000 cycles at high mass loading. Our method provides a promising facile and high-performance strategy for supercapacitor electrode application.

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INTRODUCTION Supercapacitors are a kind of important energy devices, which provide transient but high power output for various machines and devices.1,2 Early transition metal oxides such as MnO2,3 V2O5,4 NiO,5−7 and Co3O48 are a group of very promising supercapacitor electrode materials to replace expensive and toxic RuO2.9,10 However, because of the low conductivity of these metal oxides it leads to the problem of limited electrochemical activity and therefore an inefficient active material utilization for achieving high performance. Yan et al. reported a MnO2/SnO2 nanowire electrode material with enhanced capacitance and rate capability (loading mass 0.5 mg cm−2).3 Liu et al. also reported a similar strategy of nickel hydroxidenitrate/ZnO nanowire; however, the loading mass is still low (0.5 mg cm−2).11 Recently, Mao et al. reported a graphene−MnO2 composite material, which achieved a high capacitance of 631 F g−1, but the loading mass is around 0.1 mg on the glassy carbon electrode.12 Despite the achievements of these previous composites and hybrid materials, the loading masses of such electrode systems are far from realistic applications and the performance declines dramatically with the increasing loading mass, which leads to a low volumetric capacitance. To tackle this problem, one of the key strategies is to develop a composite material with high conductivity. A good dispersion of the active nanomaterials on the conducting scaffold is also favorable. A 1D nanostructure provides a rigid and porous scaffold for metal oxides growth. The facile electron conduction along the structure offers great potential to harness the anticipated electrochemical performance enhancements.3,11,13 © 2012 American Chemical Society

Carbon nanotube is the material of choice because of its versatility in making highly conductive network such as conductive paper14 and sponge.15 The merit of CNT network in supercapacitor applications has been well demonstrated, such as in CNT/MnO2 (246 F cm−3)16 and CNT/V2O5(450 F g−1),17 given its easy access to electrons and ions in the electrolyte. Nickel cobalt oxide (NiCo2O4) has 2 orders higher electron conductivity compared to pure NiO or Co3O4.18 The conductivity of NiCo2O4 nanoplate has been measured to be 62 S cm−1 by Hu, Fang and co-workers.19 NiCo2O4 has been studied as photodetector and elecctrocatalyst.20,25 However, there are limited literatures on the application of NiCo2O4 as supercapacitor electrode material to date. Until recently, there are several reports on the high performance of NiCo2O4 in alkaline electrolyte.21−23 Hu et al. reported a sol−gel synthesis route of NiCo2O4 with a high specific capacitance of 1532 F g−1(4 mg); however, the electrode encountered great capacity loss up to 50% after just 500 cycles.21 Wang et al. developed a NiCo2O4−RGO composite material with initial capacitance of 835 F g−1 (loading mass not given).22 Wei et al. demonstrated an epoxide-derived sol−gel method with a high capacitance of 719 F g−1 (loading mass 0.4 mg cm−2).23 Notably, little research has been done on nickel cobalt oxide- (NiCo2O4) based CNT composite material. To date, only Fan et al. reported an electrochemical deposition of Ni−Co mixed oxide Received: March 25, 2012 Revised: May 17, 2012 Published: May 17, 2012 12448

dx.doi.org/10.1021/jp3028353 | J. Phys. Chem. C 2012, 116, 12448−12454

The Journal of Physical Chemistry C

Article

Figure 1. (a) FESEM image of w/e − 4/1 sample prepared with 0.16 mg mL−1 p3-SWCNT, (b) FESEM image of w/e − 1/1 sample with 0.16 mg mL−1 p3-SWCNT, (c) FESEM image of w/e − 1/4 sample with 0.16 mg mL−1 p3-SWCNT, (d) FESEM image of ethanol sample with 0.16 mg mL−1 p3-SWCNT, (e) FESEM image of pure NiCo2O4 prepared in w/e − 1/4 mixed solvent.

over 3 mg cm−2 of active material is well beyond those achievable in many electrode material systems and indicates that our method is promising for future practical application of supercapacitors.

on CNT films,24 in which a moderate specific capacitance of 569 F g−1 at a current density of 10 mA cm−2 can be achieved (loading mass 0.31 mg). On the basis of previous reports, there are several challenges in improving the electrochemical performance of NiCo 2 O 4 , namely, the capacitance of NiCo2O4 as well as the loading mass remains low during testing,23,24 which limits the further practical application. In addition, the cycling stability of some high capacitance NiCo2O4 is poor.21 Therefore, it is imperative and of great importance to design strategies to enhance the capacitance as well as stability of NiCo2O4. Herein, we report a facile synthesis of fine NiCo2O4 on functionalized single wall carbon nanotube (SWCNT) bundles by controlled hydrolysis reaction in organic/water mixed solvent system. This novel CNT/ternary oxide composite material exhibits an excellent electrochemical performance with high specific capacitance of 1642 F g−1. Meanwhile, the capacitance remains 94.1% after 2000 cycles, showing an excellent stability. The relationships between electrochemical performance and SWCNT loading ratio as well as the mixed solvent water−ethanol ratio have been studied to determine the best composition. Most remarkably, the high loading mass of

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EXPERIMENTAL SECTION

All of the chemicals used in the experiments were analytical grade and were used without further purification. Co(NO3)2·6H2O, NiCl2·6H2O, and urea were purchased from Sigma-Aldrich. Absolute ethanol was purchased from Merck. P3-SWCNT (with −COOH functional group) was purchased from Carbon Solutions Co. Water/Ethanol Ratio. A 20 mL suspension containing the 0.05 M metal ion (Co2+:Ni2+ = 2:1), 0.25 M urea, together with 0.16 mg mL−1 p3-SWCNT is made (Supporting Information for more detail). Then the solution was transferred into a 40 mL Teflon lined autoclave. The autoclave was kept at 80 °C for 14 h. Black precipitates were collected by centrifugation and dried at 60 °C for 6 h. The black products were then sintered to 400 °C at a ramp of 2 °C min−1 and maintained for 3 h. The water/ethanol ratios were adjusted to 4:1, 1:1, 1:4, and pure 12449



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ethanol. These samples were labeled as w/e − 4/1, w/e − 1/1, w/e − 1/4, and ethanol respectively. Mass Ratio Variation of SWCNT. Under the optimized water/ethanol ratio 1:4, 0.008, 0.08, 0.24, and 0.32 mg mL−1 of p3-SWCNT suspensions containing 0.05 M metal ion and 0.25 M urea were prepared. Then the suspensions were transferred into a 40 mL Teflon-lined autoclave. The autoclave was kept at 80 °C for 14 h. Black precipitates were collected by centrifugation and dried at 60 °C for 6 h. The black products were then sintered to 400 °C at a ramp of 2 °C min−1 and maintained for 3 h. Structure Characterizations. The products were characterized using X-ray powder diffractometry (XRD; Bruker D8 advance, Cu Ka radiation) at a scan rate of 2° min−1, Raman spectroscopy (Confocal Raman Spectrometer 633 nm), scanning electron microscopy (FESEM; JEOL, JSM-7600F), transmission electron microscopy (TEM; JEOL, JEM-2100F) and electron dispersive X-ray spectroscopy (EDS). Thermal Analysis. Evaluation of carbon content in different samples was carried out by thermal gravimetric analysis (TGA) test. It was carried out from 25 to 900 °C under constant air flow using Mettler Toledo TGA/SDTA 851e. Electrochemical Characterization. The electrochemical tests were conducted using a three electrode system in 2 M KOH using Autolab PGSTAT 30 potentiostat. The reference electrode was Ag/AgCl electrode and counter electrode was Pt plate. The working electrode was prepared by mixing 85 wt % composite material, 10 wt % carbon black, and 5 wt % polyvinylidene fluoride (PVDF) in ethanol. The mixture was then stirred overnight and the slurry was loaded on the nickel foam (1 × 1 cm2 in area) and dried in air at 80 °C for 4 h. The electrode was pressed under 40 MPa and dried overnight. The loading mass of active material was acquired by measuring electrode with a microbalance with accuracy of 0.01 mg. Typically, the loading mass of active material was around 3.0 mg cm−2.

Figure 2. (a) XRD patterns of the w/e − 1/4 NiCo2O4/CNT nanocomposite and pure NiCo2O4 prepared in w/e − 1/4 mixed solvent, (b) TEM EDS of the w/e − 1/4 sample, (c) highmagnification TEM image of the w/e − 1/4 sample, (d) HRTEM image of the w/e − 1/4 sample.

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RESULTS AND DISCUSSION Structural Characterization. Parts a−d of Figure 1 show the FESEM images of the products with different water/ethanol ratios. Distinct differences in the final product morphologies can be observed. CNTs are not discernible in w/e − 4/1 sample (part a of Figure 1) and short NiCo2O4 nanorods and nanoparticles are observed. In part b of Figure 1, nanowire bundles interwoven with thick nanoflakes are found in the w/e − 1/1 sample. The w/e − 1/4 sample in part c of Figure 1 shows the well separated nanowires with just a few nanoflakes. However, for the ethanol sample (without water), a severe agglomeration of nanowires with particles was found. These results suggest that the fast hydrolysis under high water content condition caused severe inhomogeneity of the final products, whereas the slow hydrolysis found in the w/e − 1/4 condition is beneficial for uniform coating on CNT walls. However, the extremely slow hydrolysis in pure ethanol led to a faster agglomeration of SWCNT than hydrolysis. This in return created serious inhomogeneity. The control sample was synthesized with a water/ethanol ratio of 1/4 but without p3SWCNT. In contrast to the sample w/e − 1/4, this pure NiCo2O4 sample consists of irregular shaped large particles of about 500 nm. To investigate the metal oxide crystalline structure of w/e − 1/4 sample, XRD patterns of w/e − 1/4 sample and pure NiCo2O4 sample are shown in part a of Figure 2. Both samples

Figure 3. (a) Raman spectrum of pristine p3-SWCNT, (b) Raman spectrum of w/e − 1/4 sample.

are composed of NiCo2O4 with crystalline spinel structure (PDF #200781: a = 8.11 Å). This is in agreement with the previous reports.23,25 The chemical nature of the composite 12450



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Figure 4. (a) CV curves of different water/ethanol ratio samples with 4 mL p3-SWCNT addition at a scan rate of 10 mV s−1 in 2 M KOH, (b) CV curves of different p3-SWCNT addition volumes prepared in w/e − 1/4 mixed solvent at a scan rate of 10 mV s−1 in 2 M KOH, (c) relationship of specific capacitance vs water content in mixed solvent at a discharge current of 0.5 A g−1 in 2 M KOH, (d) relationship between p3-SWCNT concentration, carbon mass ratio, and specific capacitance.

The Raman spectrum of pristine p3-SWCNT and w/e − 1/4 sample are shown in parts a and b of Figure 3, respectively. The D band and G band of CNT are clearly shown in part b of Figure 3 indicating the presence of SWCNT in the w/e − 1/4 sample. The Raman spectra of the two samples show no distinct shifts in the position of G band, indicating there is no doping in the SWCNT.30 Electrochemical Characterizations. The investigations of the electrochemical performances of different samples were carried out through cycling voltammetry (CV) test and galvanostatic charge−discharge tests. The CV curves of samples with various water/ethanol ratios are shown in part a of Figure 4. It is evident that the w/e − 1/4 sample has the highest current density and largest area within the CV curves showing the best capacitive behavior. It therefore indicates that water content during synthesis has great influence in the capacitive behavior of composite materials. Additionally, a water/ethanol ratio of 1/4 is the optimum condition. The CV curves of different SWCNT concentrations are presented in part b of Figure 4. It reveals that SWCNT concentration during synthesis also greatly influences the capacitive behavior of the final products. The sample with 0.16 mg mL−1 SWCNT addition shows the highest current density and CV curve area. It is noteworthy that variations in redox peaks positions can be observed from different samples in both parts a and b of Figure 4. This may be attributed to the difference in electrode polarization behaviors during CV tests.31 The electrode polarization behavior is closely related to the chemical composition and physical morphology of the electrode material.

material are also confirmed by TEM based EDS, as shown in part b of Figure 2. The Ni and Co peaks confirm the presence of the ternary oxide. The C peak is from the carbon film and p3-SWCNT, and the Cu peak is from the copper grid. Further structural evaluation of sample w/e − 1/4 is carried out by TEM, as shown in parts b−d of Figure 2. From the highmagnification TEM image in part c of Figure 2, NiCo2O4 nanocrystals are found to grow separately around SWCNT bundles without serious agglomeration. The diameters of NiCo2O4 nanocrystal are around 6−10 nm. Clear lattice fringes can be seen from the high-resolution TEM image in part d of Figure 2, which confirms the ultrafine crystalline nature of the NiCo2O4 nanocrystals. As nanosized material is beneficial for improving electrochemical performance,10,26 NiCo2O4 nanocrystals ensure a highly effective active material utilization. The SWCNT bundles, which provide electron conduction channels for the composite material, exhibit different diameters ranging from 5 to 10 nm. Raman Spectroscopy. Raman spectroscopy is widely used for the determination of SWCNT properties, such as the structural, dimensional, and electronic properties.27,28 Raman scattering in SWCNTs is dominated by a resonant process, which is associated with optical transitions between the 1D states in the electronic band structure.29 Typical Raman spectra of SWCNTs contain two main peaks. The first one is situated between 1530 and 1600 cm−1 and is related to the C−C stretching tangential mode (TM) in graphite. The most intense band at 1590 cm−1 is referred to as the graphitic (G) band. The second strong peak around 1300 cm−1 is D band due to the scattering from defects present in the SWCNTs. 12451



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Figure 5. (a) Relationship of specific capacitance vs discharge current densities, (b) discharge curves of w/e − 1/4 sample at various current densities in 2 M KOH, (c) relationship of capacity retention vs cycling number, (d) Nyquist plots of w/e − 1/4 sample and pure NiCo2O4 sample prepared in w/e − 1/4 solutions.

Table 1. Simulated Values of Rs, Rf, W, Cdl, Cf from the Equivalent Circuit in Part d of Figure 5 pure NiCo2O4 w/e − 1/4

Rs/Ω

Rf/Ω

W

Cdl/mF

Cf/F

1.048 0.95

0.085 0.0332

15 2.089

0.557 1.116

0.014 0.035

in faster agglomeration of SWCNT in the suspension reducing the contact area of SWCNT with solution. This limits the nanocrystal growth on the SWCNT walls. Second, higher water content leads to a faster hydrolysis process, which favors a faster nucleation of nanocrystals. Generally, heterogeneous nucleation occurs much easier than homogeneous nucleation.34 Heterogeneous nanocrystal growth occurs very quickly on the SWCNTs. This leads to the burying of SWCNTs under NiCo2O4 as shown in parts a and b of Figure 1. It is interesting to note that the carbon content in the composite material does not correspond to the variation of SWCNT mass in suspension. As shown in part d of Figure 4 and in Figure S2 of the Supporting Information, for the higher concentration of 0.24 mg mL−1 and 0.32 mg mL−1 SWCNT, the carbon contents in the composite material are 10.88 wt % and 8.33 wt %, respectively. They are much lower than 15.40 wt %, which is the carbon content of 0.16 mg mL−1 SWCNT addition. It strongly indicates that excess SWCNT addition has affected the growth of the nanocomposite. Higher NiCo2O4 mass ratio can be achieved with higher SWCNT content in solution under the same water/ethanol ratio. It is evident that high concentration SWCNT (>0.16 mg mL−1) has affected the nanocrystals growth process in the solution. SWCNTs agglomerate much easier due to stronger van der Waals attraction. The low surface energy of the agglomerated SWCNTs at high concentration is less favorable for heterogeneous nanocrystal nucleation. Thus, more nanocrystals will grow in the bulk solution. As shown in parts a and b of Figure S1 of the Supporting Information, lots of nanocrystals with diameter up to several hundred nanometers can be seen interwoven within SWCNT bundles. This is distinct from the

The redox reactions in the alkaline electrolyte are based on the following equations:21,32,33 NiCo2O4 + OH− + H 2O ↔ NiOOH + 2CoOOH + 2e− (1) −

CoOOH + OH ↔ CoO2 + H 2O + e

−

(2)

The specific capacitances of different samples are measured by galvanostatic charge−discharge tests. The Csp is calculated by eq 3 Csp = I Δt /mΔV

(3)

where I is the discharge current, Δt is the discharge time, m is the active material mass, and ΔV is the electrochemical window (0.45 V for all the samples). The specific capacitances at a discharge current of 0.5 A g−1 of various water/ethanol ratios are shown in part c of Figure 4. Low capacitance is observed when pure ethanol is used. Because of the trace water content in the precursors, the hydrolysis rate is extremely slow. This results in a low active material mass loading. However, it is clear that, with the increase of water in the mixed solvent during synthesis, the specific capacitances of the samples drop dramatically. The causes are supposed to stem from the following two aspects. First, the increasing water content results 12452



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well separated nanowire morphology of w/e − 1/4 sample in part c of Figure 1. Thus, the nanocrystal growth process is altered under high concentration SWCNTs. This leads to lower carbon weight content in composite material. However, more detailed study is required to reveal the complicated interaction between SWCNT and nanocrystals. The electrochemical performance of NiCo2O4-p3 SWCNT nanocomposite (w/e − 1/4 sample) material was further investigated under optimized conditions. As shown in part a of Figure 5, the capacitance of w/e − 1/4 sample retains 879 F g−1 at a discharge current density of 20 A g−1, which is 53.5% of 1642 F g−1 at 0.5 A g−1, indicating a good rate capability. Cycling stability is another critical parameter for practical application. This NiCo2O4-p3 SWCNT nanocomposite shows an excellent retention of 94.1% after 2000 cycles (part c of Figure 5) suggesting the good stability toward long time charge−discharge applications. Overall, such composite material shows an enhanced cycling stability compared to previous reports on NiCo2O4.21,22 To further elucidate the origin of high electrochemical performance, electrochemical impedance spectrum (EIS) is carried out to examine the effect of SWCNT on the nanocomposite material. The experiment data are fitted according to the equivalent circuit in part d of Figure 5, which contains Rs (the intrinsic resistance of substrate, contact of material with substrate, electrolyte resistance), Rf (the resistance of Faradic reaction), Cdl (the electric double layer capacitance), W (the Warburg impedance), and Cf (the limit capacitance). The simulated values of w/e − 1/4 sample and pure NiCo2O4 sample are shown in Table 1. The Warburg impedance of the NiCo2O4-p3 SWCNT nanocomposite (w/e − 1/4 sample) shows a much lower value compared to pure NiCo2O4, which corresponds to more vertical line leaning to imaginary axis at a low frequency region. This indicates a more facile electrolyte diffusion to the composite surface and a more ideal capacitor behavior compared with pure NiCo2O4 sample.35,36 The SWCNT in the composite material also helps to reduce the interfacial resistance Rs.35,37 In addition, lower Faradic resistance Rf in the composite material can also be found as shown in Table 1. In Nyquist plot, the composite material shows a smaller real axis intersection and negligible semicircle. This suggests a low interfacial resistance between current collector and active material, active material and electrolyte, as well as low charge transfer resistance, which agrees well with the calculation. Thus, the reduced active material internal resistance and charge transfer resistance of the nanocomposite contributes to the enhanced electrochemical performance.

Article

ASSOCIATED CONTENT

S Supporting Information *

Experimental procedure, SEM images of samples synthesized in water/ethanol = 1:4 mixed solvent with SWCNT addition of 6 and 8 mL, carbon mass ratios in different samples determined by TGA tests. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Zviad Tsakadze for technical support. This work is supported by Singapore National Research Foundation under CREATE program - Nanomaterials for Water and Energy Management. X.W., X.H., M.L., and N.S. acknowledge the research scholarship provided by Nanyang Technological University, Singapore.

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REFERENCES

(1) Liu, C.; Li, F.; Ma, L. P.; Cheng, H. M. Adv. Mater. 2010, 22, E28−E62. (2) Du Pasquier, A.; Plitz, I.; Menocal, S.; Amatucci, G. J. Power Sources 2003, 115, 171−178. (3) Wang, X.; Yang, Y. L.; Fan, R.; Wang, Y.; Jiang, Z. H. J. Alloys Compd. 2010, 504, 32−36. (4) Khoo, E.; Wang, J. M.; Ma, J.; Lee, P. S. J. Mater. Chem. 2010, 20, 8368−8374. (5) Lee, J. Y.; Liang, K.; An, K. H.; Lee, Y. H. Synth. Met. 2005, 150, 153−157. (6) Liu, X. M.; Zhang, X. G.; Fu, S. Y. Mater. Res. Bull. 2006, 41, 620−627. (7) Wang, Y. G.; Xia, Y. Y. Electrochim. Acta 2006, 51, 3223−3227. (8) Wei, T. Y.; Chen, C. H.; Chang, K. H.; Lu, S. Y.; Hu, C. C. Chem. Mater. 2009, 21, 3228−3233. (9) Kotz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483−2498. (10) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845−854. (11) Liu, J. P.; Cheng, C. W.; Zhou, W. W.; Li, H. X.; Fan, H. J. Chem. Commun. 2011, 47, 3436−3438. (12) Mao, L.; Zhang, K.; Chan, H. S. O.; Wu, J. S. J. Mater. Chem. 2012, 22, 1845−1851. (13) Yan, C. Y.; Jiang, H.; Zhao, T.; Li, C. Z.; Ma, J.; Lee, P. S. J. Mater. Chem. 2011, 21, 10482−10488. (14) Hu, L. B.; Choi, J. W.; Yang, Y.; Jeong, S.; La Mantia, F.; Cui, L. F.; Cui, Y. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 21490−21494. (15) Chen, W.; Rakhi, R. B.; Hu, L. B.; Xie, X.; Cui, Y.; Alshareef, H. N. Nano Lett. 2011, 11, 5165−5172. (16) Lee, S. W.; Kim, J.; Chen, S.; Hammond, P. T.; Shao-Horn, Y. ACS Nano 2010, 4, 3889−3896. (17) Chen, Z.; Qin, Y. C.; Weng, D.; Xiao, Q. F.; Peng, Y. T.; Wang, X. L.; Li, H. X.; Wei, F.; Lu, Y. F. Adv. Funct. Mater. 2009, 19, 3420− 3426. (18) Tarasevich, M. R.; Efremov, B. N. In Electrodes of Conductive Metallic Oxides Part ATrasatti, S., Ed.; Elsevier: USA, 1982; p 227. (19) Hu, L. F.; Wu, L. M.; Liao, M. Y.; Hu, X. H.; Fang, X. S. Adv. Funct. Mater. 2012, 22, 998−1004. (20) Hu, L. F.; Wu, L. M.; Liao, M. Y.; Hu, X. H.; Fang, X. S. Adv. Mater. 2011, 23, 1998−1992. (21) Hu, G. X.; Tang, C. H.; Li, C. X.; Li, H. M.; Wang, Y.; Gong, H. J. Electrochem. Soc. 2011, 158, A695−A699. (22) Wang, H. W.; Hu, Z. A.; Chang, Y. Q.; Chen, Y. L.; Wu, H. Y.; Zhang, Z. Y.; Yang, Y. Y. J. Mater. Chem. 2011, 21, 10504−10511.

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CONCLUSIONS We have successfully prepared a NiCo2O4-p3 SWCNT nanocomposite with enhanced electrochemical performance. It is found that water/ethanol ratio in the solvent and CNT concentration strongly affects the performance of the nanocomposite. The optimized specific capacitance reaches 1642 F g−1 with high electrode material loading mass. In addition, the composite material shows good rate capability as well as excellent long time cycling stability. The enhanced electrochemical performance is mainly the contribution of reduced resistance of the nanocomposite. The ultrafine NiCo2O4 nanocrystals enable efficient utilization of the active materials. This method offers a promising design and synthetic protocol for future supercapacitor application. 12453



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(23) Wei, T. Y.; Chen, C. H.; Chien, H. C.; Lu, S. Y.; Hu, C. C. Adv. Mater. 2010, 22, 347−351. (24) Fan, Z.; Chen, J. H.; Cui, K. Z.; Sun, F.; Xu, Y.; Kuang, Y. F. Electrochim. Acta 2007, 52, 2959−2965. (25) Li, Y. G.; Hasin, P.; Wu, Y. Y. Adv. Mater. 2010, 22, 1926−1929. (26) Zhao, X.; Sanchez, B. M.; Dobson, P. J.; Grant, P. S. Nanoscale 2011, 3, 839−855. (27) Kneipp, K.; Kneipp, H.; Corio, P.; Brown, S. D. M.; Shafer, K.; Motz, J.; Perelman, L. T.; Hanlon, E. B.; Marucci, A.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rev. Lett. 2000, 84, 3470−3473. (28) Jorio, A.; Saito, R.; Hafner, J. H.; Lieber, C. M.; Hunter, M.; McClure, T.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rev. Lett. 2001, 86, 1118−1121. (29) Holzinger, M.; Abraha, J.; Whelan, P.; Graupner, R.; Ley, L.; Hennrich, F.; Kappes, M.; Hirsch, A. J. Am. Chem. Soc. 2003, 125, 8566−8580. (30) Kukovecz, A.; Pichler, T.; Pfeiffer, R.; Kramberger, C.; Kuzmany, H. Phys. Chem. Chem. Phys. 2003, 5, 582−587. (31) Conway, B.E. Electrochemical supercapacitors: Fundamentals and Technological Applications; KluwerAcademic/Plenum Publishers: New York, 1999. (32) Gupta, V.; Gupta, S.; Miura, N. J. Power Sources 2008, 175, 680− 685. (33) Hu, C. C.; Cheng, C. Y. Electrochem. Solid-State Lett. 2002, 5, A43−A46. (34) Bronic, J.; Subotic, B. Microporous Mater. 1995, 4, 239−242. (35) Gamby, J.; Taberna, P. L.; Simon, P.; Fauvarque, J. F.; Chesneau, M. J. Power Sources 2001, 101, 109−116. (36) Song, H. K.; Hwang, H. Y.; Lee, K. H.; Dao, L. H. Electrochim. Acta 2000, 45, 2241−2257. (37) Fan, Z. J.; Yan, J.; Wei, T.; Zhi, L. J.; Ning, G. Q.; Li, T. Y.; Wei, F. Adv. Funct. Mater. 2011, 21, 2366−2375.

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Nickel Cobalt Oxide-Single Wall Carbon Nanotube Composite

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