Department of Chemistry BK-21 School of Chemical Materials Science, SKKU Advanced Institute of Nanotechnology, Research Institute of Advanced Nanomaterials,ungkyunkwan University, Suwon, Republic of KoreaBK 21 Physics Research Division, Department of Energy Science, Institute of Basic Science, Sungkyunkwan University, Suwon 440746, Republic of KoreaInterdisciplinary Research Centre in Biomedical Materials (IRCBM), COMSATS Institute of Information Technology (CIIT), Lahore, PakistanChemistry Department, Islamia University Bahawalpur, Bahawalpur 63100, PakistanSubsurface Technology, Petronas Research Sdn Bhd. (PRSB), Bangi 43300, Selangor, Malaysia
r t i c l e i n f o
rticle history:eceived 19 March 2012eceived in revised form 8 August 2012ccepted 16 August 2012
a b s t r a c t
Ultrathin nanoflakes of Ni(OH)2 were synthesized onto multi-walled carbon nanotubes (MWCNTs) bysimple low cost chemically precipitation method for high performance electrochemical supercapacitorapplications. The synthesized ultrathin Ni(OH)2 exhibit high specific capacitance of 1735 Fg−1 at a scanrate of 5 mV s−1 with excellent rate capability. This high performance of Ni(OH)2 nanoflakes was attributed
to its complete accessibility to the electrolyte and maximum utilization of metal hydroxides. Findings ofthis work suggest that synthesized electrodes offer low-cost and scalable solution for high-performanceenergy storage devices.
Electrochemical energy storage devices, in particular transitionetal (Ni, Co, Ru, etc.) oxides and hydroxides based supercapacitors
ave gained considerable interest because they can instanta-eously provide higher power density and long cycle life thanatteries and higher energy density than conventional electro-tatic capacitors [1–5]. Although, theoretical specific capacitancef these metal oxide and hydroxide is over 1000 F g−1 [6–8], butxperimentally observed values are very low due to their lowlectronic conductivity and lethargic redox kinetics. Among dif-erent metal hydroxide Ni(OH)2 as pseudocapacitor material hasained considerable attention due to its well defined electrochemi-al redox activity, low cost and high theoretical specific capacitance
9–12]. However, experimentally it turns out to be unsatisfactory
aterial for energy storage devices due to low operation volt-ge, poor electronic conductivity and rate capability. Therefore, a
strategy to overcome these intrinsic drawbacks of Ni(OH)2 energystorage devices while maintaining the high external-surface areais required to facilitate the maximum number of active sites forpseudocapacitance. In order to increase its utilization for elec-trode material in energy storage devices, various approaches havebeen used to synthesize Ni(OH)2 nanostructures with variousmorphologies such as nanorods, nanotubes, nanoparticle, plateletlike?, nanoflowers or flakes, microspheres [13–23]. Among these,nanoflakes of Ni(OH)2 have not only shown short diffusion pathlengths for both electrolyte ions and electrons as compared to othermorphologies but also facilitate the maximum electrochemicalutilization of Ni(OH)2 during the rapid charge/discharge process.Although, flowerlike morphology of Ni(OH)2 shorten the diffu-sion path lengths and increases its utilization but still it suffersfrom low conductivity and lower specific capacitance. In order toresolve these major technical challenges a convenient and scal-able solution based approach to deposit ultra-thin nanoflakes ofNi(OH)2 onto conducting MWCNTs is reported. The coating ofNi(OH)2 ultrathin nanoflakes onto MWCNTs facilitates the num-ber of active sites for pseudocapacitance generation, lowers the
polarization of electrolytes and increases the conductivity of elec-trode material. MWCNTs act as an excellent host material for thecoating of ultrathin nanoflakes of Ni(OH)2 which provides a goodelectrical conducting path for charge transfer, rate capability and
244 M. Shahid et al. / Electrochimica Acta 85 (2012) 243– 247
Fig. 1. (a) XRD patterns of (I) pure nickel hydroxide and (II) Ni(OH) /MWCNTs, C represent the carbon peak (b) The N adsorption–desorption isotherm of as preparedN i(OH)
rteiwt
2
2
flp3fttfiTpswNMfcssaww
3
NR�tfttI
2
i(OH)2 and Ni(OH)2/MWCNTs with insect showing the pore-size distributions of N
edox kinetics of the deposited layer. The obtained results showedhat the presence of MWCNTs in the nanocomposites improve thelectronic conductivity, homogeneous electrochemical accessibil-ty and high ionic conductivity by avoiding agglomerative binder
hich makes them a promising material for the fabrication of elec-rochemical energy storage devices.
. Experimental
.1. Synthesis of Ni(OH)2 and Ni(OH)2/MWCNTs nanocomposites
The MWCNTs are purchased from Nanokarbon (Korea) and wereunctionalized by attaching carboxylic groups on the surface by fol-owing the similar method developed by Gao et al. [24]. Brieflyristine MWCNTs (1.0 g), HNO3 (65%, 100 mL), and H2SO4 (98%,00 mL) were mixed in a flask and vigorously stirred under refluxor 100 min. The mixture was diluted with deionized water, fil-ered, and re-dispersed in water. This process was repeated untilhe pH of the filtrate was approximately neutral. Then, modi-ed MWCNTs were dried in a vacuum oven for 24 h at 80 ◦C.he Ni(OH)2/MWCNTs composite was synthesized by chemical co-recipitation method. NH4(OH) (2 mol/L) and Ni(NO3)2 aqueousolution (1 mol/L) were added drop wise into aqueous solutionith MWCNTs in the given ratio (mass ratio of Ni(OH)2 to MWC-Ts is 100:1) with constant stirring. To improve the dispersion ofWCNTs, the aqueous solution was treated by ultrasonic vibrations
or 1 h before addition of Ni(OH)2. The precipitation reaction wasarried out at 100 ◦C with constant stirring for 12 h. The obtaineduspension was kept in the mother solution for 24 h and allowed toettle. The precipitates were filtrated, washed with distilled water,nd dried at 80 ◦C for 10 h. For the sake of comparison, pure Ni(OH)2as also synthesized by the same chemical co-precipitation processithout the addition of MWCNTs.
. Results and discussion
The crystal structure of Ni(OH)2 and Ni(OH)2 coated MWC-Ts was investigated by X-ray diffraction (XRD) analysis usingigaku Rotaflex D/Max diffractometer with Cu K� of wavelength
= 1.5418 A and the XRD patterns are shown in Fig. 1. The XRD pat-erns of the samples are in good agreement with the standard peaks
or the hexagonal phase of typical �-Ni(OH)2 (JCPDS-14-0117) andhe crystalline MWCNTs. Compared to the pure Ni(OH)2, the diffrac-ion peaks of Ni(OH)2 coated MWCNTs are noticeably broadened.t is well known that this broadening of the diffraction peaks is due
2
2/MWCNTs.
to the increased crystal defects and the presence of other polymor-phic modification as interstratified phases [25,26]. The surface areaof Ni(OH)2 nanoflakes and Ni(OH)2 coated MWCNTs was estimatedby BET measurements (Coulter SA3100) using nitrogen adsorp-tion. Fig. 1(b) shows nitrogen sorption isotherms of the Ni(OH)2nanoflakes and Ni(OH)2 coated MWCNTs. The Ni(OH)2/MWCNTscomposite possess a surface area of 150 m2/g, pore volume of0.488 cm3/g. A pore size distribution with maximum at around4 nm is also shown in the inset of Fig. 1(b).
The Ni(OH)2 nanoflakes possess a low surface area of 83 m2/g,larger pore diameter of 26.7 nm, and a pore volume of 0.552 cm3/g.The well-developed pore structure and high surface area ofNi(OH)2/MWCNTs composite is advantageous to the transport anddiffusion of electrolyte ions during the rapid charge/discharge pro-cess. As a consequence, it is expected that Ni(OH)2/MWCNTs hasan excellent electrochemical performance as compared to pureNi(OH)2 nanoflakes. The morphological analysis of Ni(OH)2 coatedMWCNTs was performed by transmission-electron-microscopy(TEM) with the corresponding elemental mapping by energy-dispersive X-ray (EDX) spectroscopy and the obtained results arepresented in Fig. 2. The TEM analysis indicates that Ni(OH)2 is uni-formly coated on the entire surface of MWCNTs as shown in Fig. 2(aand b). Fig. 2(c and d) shows that TEM-EDS concentration profilesof C, Ni, and O measured in the Ni(OH)2 also indicates the uniformcoating of Ni(OH)2 on MWCNTs.
Electrochemical measurements were carried out by the three-electrode cells consisting of glassy carbon as a working electrode,Pt wire and Ag/AgCl (satd. KCl) electrodes as counter andreference electrodes, respectively. Fig. 3(a and b) shows thecyclic voltammograms (CVs) of the Ni(OH)2 nanoflakes, andNi(OH)2/MWCNTs composite electrodes at various sweep rates.The Ni(OH)2 nanoflakes electrode shows peaks at 0.4 and 0.3 V,which are attributed to the anodic oxidation and cathodic reduc-tion of the Ni(OH)2 nanoflakes, respectively [27,28]. The Ni(OH)2nanoflakes electrode shows an elliptical curve, indicating a faradaicreaction at the interface of electrodes with electrolyte ions, whichis a typical behavior of pseudocapacitors. The CV curve of Ni(OH)2coated MWCNTs electrode shows an elliptical curve with a muchlarger area indicating a much higher capacitance as compared toNi(OH)2 nanoflakes electrodes. This well-defined redox peaks in theNi(OH)2 coated MWCNTs electrode suggest a major contribution of
redox capacitance to the overall capacitance [29,30]. The presenceof these redox peaks in KOH aqueous electrolyte system is reportedto (e.g., K) associated with the insertion and de-insertion reactionswhich take place at different energy states [31,32]. It is assumed
M. Shahid et al. / Electrochimica Acta 85 (2012) 243– 247 245
F showm
tfloailmtaMeeasn
ig. 2. TEM images of: (a and b) Ni(OH)2-deposited fiber network on MWCNTs insetapping are shown in (d), (e) and (f), respectively.
hat charge storage mechanism in Ni(OH)2 occurred through theollowing steps: reaction of cations with electroactive material fol-owed by a redox reaction, electrochemical adsorption of cationsnto the electroactive material through a charge transfer processnd intercalation of ions into the vander Waals gaps of the Ni(OH)2n which layers are perfectly stacked along the c-axis with an inter-amellar distance of 4.6 A. On the other hand, the charge storage
echanism in Ni(OH)2 coated MWCNTs electrode is attributed tohe overlapping effect of the two different energy-storage mech-nisms, pseudo capacitance from Ni(OH)2 and double layer fromWCNTs. The current response of the Ni(OH)2 coated MWCNTs
lectrode was quite sharp at the vertex potentials indicating a lower
quivalent series resistance, which is an important parameter forchieving higher power density in supercapacitors. The measuredpecific capacitances were 870 and 1735 F g−1 for the Ni(OH)2anoflakes, and Ni(OH)2 coated MWCNTs electrodes, respectively,
one fiber with thickness of sub 10 nm. (c) TEM image of one fiber and its elemental
at the lowest scan rate of 5 mV s−1. The scan rate dependent elec-trochemical response of Ni(OH)2 nanoflakes, and Ni(OH)2 coatedMWCNTs electrodes was examined to understand the contribu-tion of each capacitance mechanism as shown in Fig. 3(c). At thehighest sweep potential rate of 100 mV s−1, measured capacitancesof Ni(OH)2 nanoflakes, and Ni(OH)2 coated MWCNTs electrodeswere 475 and 1430 F g−1, respectively. The specific capacitance ofNi(OH)2 coated MWCNTs electrode was still higher than that ofthe capacitor made from the Ni(OH)2 nanoflakes. This lower spe-cific capacitance at higher scan rates than the lower scan ratesis mainly due to the increase in ionic resistivity and the inac-cessibility of the electrode surface at high charge–discharge rates
[33,34].
The long-term chemical and electrochemical stability of Ni(OH)2coated MWCNTs electrode was examined by CV at a scan rateof 20 mV s−1 for 6000 cycles, and the corresponding results are
246 M. Shahid et al. / Electrochimica Acta 85 (2012) 243– 247
Fig. 3. Electrochemical properties of Ni(OH)2 and Ni(OH)2/MWCNTs in the 1 M KOH solution: (a) CV curves of Ni(OH)2 and (b) Ni(OH)2/MWCNT at different scan rateswithin a potential window of 0.0–0.5 V. (c) Specific capacitance of (I) Ni(OH)2 and (II) Ni(OH)2/MWCNTs at different scan rates in 1 M KOH solution (d) cyclic stability ofNi(OH)2/MWCNTs scan rate was 20 mV s−1.
Fig. 4. Galvanostatic charge/discharge curves of (a) Ni(OH)2, (b) Ni(OH)2/MWCNTs at a different current density. (c) Average specific capacitance of (I) Ni(OH)2 and (II)Ni(OH)2/MWCNTs at various discharge current densities and (d) electrochemical impedance spectra of pure nickel hydroxide electrode and Ni(OH)2/MWCNTs electrode.
imica
paptwsadcdctrbctfrod
owtdtdetdt0MniaaMt
iFfMtfipwocfC
4
andTc
[
[
[
[[[[
[
[
[[
[[
[
[[
[[[[[[[[
[
[
M. Shahid et al. / Electroch
resented in Fig. 3(d). The capacity decay was only 6%, evenfter 6000 cycles, indicating the excellent stability of the com-osite material in energy storage applications. To further quantifyhe specific capacitance, galvanostatic charge–discharge curvesere measured in same three electrode system. Fig. 4(a and b)
hows the charge–discharge curves of the Ni(OH)2 nanoflakes,nd Ni(OH)2 coated MWCNTs electrodes at different currentensity. The Ni(OH)2 nanoflakes electrode showed a non-linearharge–discharge curve, indicating that the nanoflakes are pseu-ocapacitive with a specific capacitance of 870 F g−1. However,harge–discharge plots (Fig. 4b) of Ni(OH)2 coated MWCNTs elec-rode display a distinctive transition between the two linearanges, indicating contributions to the capacitance from both dou-le layer and pseudocapacitance. Moreover, with increasing cycles,harge and discharge curves became more flattened compared tohe first cycle. This is an indication that the contribution fromaradaic reaction in Ni(OH)2 nanoflakes was decreased but not fullyemoved after first few cycles. However, non-faradaic contributionf composite survives for a long cycle test without an appreciableegradation.
The effect of the current density on the specific capacitancef Ni(OH)2 nanoflakes, and Ni(OH)2 coated MWCNTs electrodesas also measured as shown in Fig. 4(c). The specific capaci-
ance at low current densities was high due to the low ohmicrop, inner active sites or the full access of the pores of the elec-rode [35]. The decrease in capacitance with increasing currentensity has been attributed to slow redox reactions [36]. Thelectrochemical performance of the electrochemical cells was fur-her investigated by calculating the power density and energyensity of Ni(OH)2 nanoflakes, and Ni(OH)2 coated MWCNTs elec-rodes based on various charge/discharge currents between 0 and.5 V. The energy and power performance of this Ni(OH)2 coatedWCNTs electrode was improved significantly over the Ni(OH)2
anoflakes. The energy densities decreased slowly with increas-ng power density. The energy density reached 15.6 W h kg−1 at
power density of 600 W kg−1, and was still 7 W h kg−1, event a power density of 9 kW kg−1, suggesting that Ni(OH)2 coatedWCNTs is quite a promising electrode material for supercapaci-
ors.The electrochemical properties of the electrodes were exam-
ned further by electrochemical impedance spectroscopy (EIS).ig. 4(d) shows a Nyquist plot with a straight sloped line in the low-requency region. The almost ideal straight line of Ni(OH)2 coated
WCNTs along the imaginary axis at lower frequencies suggestshat the electrode has low diffusion resistance [37,38]. EIS con-rmed the excellent electrochemical capacitive properties of therepared composite electrode. The experimental impedance dataas further transformed to the specific capacitance and the results
f the specific capacitance of the Ni(OH)2 nanoflakes and Ni(OH)2oated MWCNTs has shown appreciable decrease with increasingrequency, which is in agreement with the decreasing trend in theV results with increasing scan rate.
. Conclusion
In summary, a simple and cost-effective solution basedpproach is developed to uniformly coat ultra-thin Ni(OH)2
anoflakes on MWCNTs onto conducting MWCNTs which showsrastic improvement in the performance of energy storage devices.he MWCNTs not only provide the high surface area for the uniformoating of ultrathin film of Ni(OH)2 nanoflakes but also improves
[[[
Acta 85 (2012) 243– 247 247
the electrical conductivity and the electrochemical stability of themetal hydroxide thin film. The 5 nm deposited Ni(OH)2 nanoflakessample exhibits very fast and reversible redox reaction, high spe-cific capacitance, cycling stability, ultrafast charge/discharge rate,and excellent energy and power density, making it one of the mostpromising electrode materials for high performance energy storageapplications.
Acknowledgements
This work was supported by the Korean Ministry of Education,Science and Technology under grants NRF-2010-0029700 (Prior-ity Research Centers Program) and R31-2008-000-10029-0 (WorldClass University Program).
References
[1] J.W. Lang, L.B. Kong, W.J. Wu, M. Liu, Y.C. Luo, L. Kang, Journal of Solid StateElectrochemistry 13 (2009) 333.
[2] J. Xub, L. Gao, J. Cao, W. Wang, Z. Chen, Electrochimica Acta 56 (2010) 732.[3] B.O. Park, C.D. Lokhande, H.S. Park, K.D. Jung, O.S. Joo, Journal of Power Sources
134 (2004) 148.[4] C. Du, N. Pan, Nanotechnology 17 (2006) 5314.[5] C. Zheng, Q. Yaochun, W. Ding, X. Qiangfeng, P. Yiting, W. Xiaolei, L. Hexing, W.
Fei, L. Yunfeng, Advance Functional Materials 19 (2009) 3420.[6] H. Kim, J.H. Kim, K.B. Kim, Electrochemical and Solid-State Letters 8 (2005)
A369.[7] X. Lang, A. Hirata, T. Fujita, M. Chen, Nature Nanotechnology 6 (2011) 232.[8] X. Zhang, W. Shi, J. Zhu, W. Zhao, J. Ma, S. Mhaisalkar, T.L. Maria, Y. Yang, H.
Zhang, H.H. Hng, Q. Yan, Nano Research 9 (2010) 643.[9] K.W. Nam, K.H. Kim, E.S. Lee, W.S. Yoon, X.Q. Yang, K.B. Kim, Journal of Power
Sources 182 (2008) 642.10] J. Liu, C. Cheng, W. Zhou, H. Lia, H.J. Fan, Chemical Communications 47 (2011)
3436.11] X. Wang, J. Yan, H. Yuan, Z. Zhou, D. Song, Y. Zhang, L. Zhu, Journal of Power
Sources 72 (1998) 221.12] U.M. Patil, K.V. Gurav, V.J. Fulari, C.D. Lokhande, O.S. Joo, Journal of Power
Sources 188 (2009) 338.13] Y. Li, B. Tan, Y. Wu, Chemistry of Materials 20 (2007) 567.14] D. Wang, C. Song, Z. Hu, X. Fu, Journal of Physical Chemistry B 109 (2004) 1125.15] B. Li, M. Ai, Z. Xu, Chemical Communications 46 (2010) 6267.16] F. Tao, M. Guan, Y. Zhou, L. Zhang, Z. Xu, J. Chen, Crystal Growth & Design 8
(2008) 2157.17] J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi, F. Wei, Advance
International Edition 43 (2004) 4212.19] S. Zhang, H.C. Zeng, Chemistry of Materials 21 (2009) 871.20] B.P. Bastakoti, H.-S. Huang, L.-C. Chen, K.C.W. Wu, Y. Yamauchi, Chemical Com-
munications (2012).21] J. Liang, Y. Li, Chemistry Letters 32 (2003) 1126.22] D. Yang, R. Wang, J. Zhang, Z. Liu, Journal of Physical Chemistry B 108 (2004)
7531.23] X. Ni, Q. Zhao, B. Li, J. Cheng, H. Zheng, Solid State Communication 137 (2006)
rous Metals Society of China 20 (2010) S249.26] G.P. Jin, Y.F. Ding, P.P. Zheng, Journal of Power Sources 166 (2007) 80.27] W. Visszher, E. Barenlrecht, Electrochimica Acta 25 (1980) 651.28] J.F. Wolf, L.S.R. Yeh, A. Damjanovic, Electrochimica Acta 26 (1981) 409.29] H. Pan, J. Li, Y.P. Feng, Nanoscale Research Letters 5 (2010) 654.30] C. Hui, C. Xuan, J. Dianzeng, Z. Wanyong, Rare Metals 30 (2011) 85.31] C.G. Liu, Y.S. Lee, Y.J. Kim, I.C. Song, J.H. Kim, Synthetic Metals 159 (2009) 2009.32] N.W. Duffy, W. Baldsing, A.G. Pandolfo, Electrochimica Acta 54 (2008) 535.33] T. Cottineau, M. Toupin, T. Delahaye, T. Brousse, D. Belanger, Applied Physics A
82 (2006) 599.34] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, S. Ferraris, Journal of Power Sources
47 (1994) 89.35] H. Jeong, M. Jin, E. Ra, K. Sheem, G.H. Han, S. Arepalli, Y.H. Lee, ACS Nano 4
(2010) 1162.36] Y. Hou, Y. Cheng, T. Hobson, J. Liu, Nano Letters 10 (2010) 2727.37] D. Yu, L. Dai, Journal of Physical Chemistry Letters 1 (2009) 467.38] H.C. Chang, C.S. Huan, C.C. Shih, J.Y. Shiang, C.C.T.L. YungYu, C. Hsin, R.Y. Tri, C.C.
Yen, R.Y. Shih, J.Y. Da, Nanotechnology 21 (2010) 485501.
