Multiwalled

ARTICLE pubs.acs.org/JPCC

Preparation and Characterization of a Poly[Ni(salen)]/Multiwalled Carbon Nanotube Composite by in Situ Electropolymerization as a Capacitive Material Fei Gao,† Jianling Li,*,† Feiyu Kang,‡ Yakun Zhang,† Xindong Wang,† Feng Ye,§ and Jun Yang§ †

Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China Department of Material Science and Engineering, Tsinghua University, Beijing 100084, China § State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China ‡

ABSTRACT: A composite of poly[Ni(salen)] and multiwalled carbon nanotubes (MWCNTs) was synthesized by in situ electropolymerization. As-grown poly[Ni(salen)] filled gaps between MWCNTs and formed a ferroconcrete-like microstructure. It was found that the doping level and electrochemical reversibility of poly[Ni(salen)] were greatly improved because of the support of MWCNTs. Values of the charge diffusion transport coefficient estimated by cyclic voltammetry revealed the effect of the mass of poly[Ni(salen)] on the charge transport behavior of composites. Both the direct ohmic resistance and the alternating ohmic resistance of a composite with small mass ratios (0.29/1 and 0.63/1) were obviously lower than those of MWCNTs which was demonstrated by electrochemical impedance spectroscopy and galvanostatic charge/discharge tests. Galvanostatic charge/discharge tests also showed that the ability to store charge was improved significantly for composites by incorporating poly[Ni(salen)].

’ INTRODUCTION Transition metal complexes with N2O2 Schiff base ligand derived from salicylaldehyde, such as Ni(II)salen, have been extensively studied as homogeneous electrocatalysis over the past few decades.13 This type of transition metal complex can be anodically polymerized to generate electroactive films in solvents with low donor numbers. The redox switching of electroactive films in nonaqueous electrolyte system can reversibly achieved in the oxidation/neutral state at high potentials,4,5 and the neutral/reduction state at low potentials,6,7 so as to reserve and release charge repeatedly. Additionally, this type of electroactive polymer is generally considered to contain discrete redox sites (metal ion centers) and delocalized redox units (the extended conjugated system in ligand) so as to have both redox conductivity and electron conductivity,8,9 which meets the request of high rate of charge transfer. These characteristics indicate a potential application of this polymer in the field of energy storage and conversion, such as supercapacitor. The electrochemical activity of conducting polymer generally drops with the increase of polymer film thickness, which greatly limits its application as the active material of energy storage system. In order to effectively retain the high usable content of conducting polymer as well as the high electrochemical activity, the composites of conducting polymers and carbon materials with large specific surface area have been widely investigated.1012 The unusual conducting properties of multiwalled carbon r 2011 American Chemical Society

nanotubes (MWCNTs), their dominant mesoporous character, and high chemical stability are attractive criteria for using them as support of electroactive materials, such as metal nanoparticles,13,14 conducting polymer,15,16 and transition metal oxide.17,18 The combination of such two materials (i.e., MWCNTs and polymer) is particularly useful to integrate the properties of the two components in composite materials for use in catalysis, energy storage, and other fields. In the present work, we characterized the electrochemical properties of poly[Ni(salen)]/MWCNT composites and analyzed each function of MWCNTs and poly[Ni(salen] within composites. As the effective support of poly[Ni(salen)], MWCNTs not only provide a large surface to support the growth of large amount of poly[Ni(salen)] but also ensure high electrochemical activity of as-grown poly[Ni(salen)] by covalent interaction. The influences of content of as-grown poly[Ni(salen)] on the kinetics and conductivity of composites have been investigated by cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) tests. The forming process of the microstructure and the capacitive performances of composites are also presented. Received: December 13, 2010 Revised: April 24, 2011 Published: May 20, 2011 11822

dx.doi.org/10.1021/jp111831y | J. Phys. Chem. C 2011, 115, 11822–11829

The Journal of Physical Chemistry C

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Figure 1. Pore size distribution of MWCNTs.

’ EXPERIMENTAL SECTION Preparation of MWCNTs-Coated Pt Electrode. MWCNTs were purchased from Shenzhen NanotechPort Co., Ltd. According to supplier specifications, the surface area of MWCNTs was 40300 m2 g1 with main range of diameter 1020 nm and 515 μm length. These MWCNTs were then functionalized under reflux with concentrated HNO3 and H2SO4 (volume ratio = 1/3), stirred for 5 h at 60 °C, washed with distilled water until the pH value reached neutral, and dried overnight in a vacuum oven at 100 °C, in order to produce carboxylic acid groups at the defect sites and thus improve the solubility of MWCNTs in the ethanol slurry. With strong acid treatment, the specific surface area (or total pore volume) of MWCNTs increased from 84.9 m2 g1 (0.947 cm3 g1) to 194.9 m2 g1 (1.004 cm3 g1). The pore size distributions of MWCNTs before and after acid treatment are shown in Figure 1. The pores of MWCNTs after acid treatment had a wider pore size distribution with majority of pores between 2 and 4 nm. The slurry of desired concentration (2.5 mg mL1) was prepared by mixing processed MWCNTs (10 mg) and poly(vinylidene fluoride) (PVDF, 1 mg) as binder with 4 mL of ethanol, dispersing homogeneously by ultrasonic at 200 Hz for 1 h. The slurry was coated on polished Pt electrodes with the dosage of 200 μL. Then the MWCNTs-coated Pt electrodes were dried at 70 °C in vacuum for 12 h. Preparation of Poly[Ni(salen)]/MWCNT Composites. The Ni(salen) complex monomer was synthesized following the procedure in the literature19 and recrystallized from acetonitrile (AN). In situ electropolymerization was conducted in a closed three-electrode compartment cell. CV technology was adopted to electropolymerize Ni(salen) monomers on the surface of MWCNTs to prepare the poly[Ni(salen)]/MWCNT composite. The MWCNTs-coated Pt electrode was placed into polymerization electrolyte as working electrode for no less than 5 min to ensure that Ni(salen) monomer could sufficiently diffuse into the MWCNTs-coated film. A piece of activated carbon electrode with 8 cm2 surface area was used as the counter electrode. The reference electrode was a capillary Ag/AgCl wire electrode,8,20 and its potential was checked versus the ferrocene/ferrocenium couple. The polymerization solution was made with AN, 0.1 mM Ni(salen) monomer, and 0.1 M tetrabutylammonium perchlorate. Electropolymerization scan rate was 20 mV s1 with the potential sweep range of 0.01.2 V. Figure 2 shows that the continuous electropolymerization of Ni(salen) occurred on polished Pt electrode and MWCNTs-

Figure 2. CV plots for anodic polymerization of Ni(salen) on (a) polished Pt electrode and (b) MWCNTs-coated Pt electrode between 0.0 and 1.2 V for 1st, 5th, 10th, 15th, and 20th scans. Inset: the section of anodic 1st scan.

coated Pt electrode from 1st to 20th sequential scans. For electropolymerization of Ni(salen) on polished Pt electrode, well-fined anode and cathode peaks are obtained before 10th scan, but peak current intensities start to decrease gradually and the overall shape of CV plot alters after 10th scan; finally, the anode and cathode peaks disappear at 20th scan. As for the electropolymerization of Ni(salen) on the MWCNTs-coated polished Pt electrode (Figure 2b), there are three discernible points compared with Figure 2a. First, the starting electorpolymerized potential of Ni(salen) on MWCNTs is 0.85 V, which derives from the inflection of the first scan; it is lower than that on polished Pt electrode (0.93 V). Second, the current response rises when the scan potential exceeds 1.10 V in the first scan and the phenomena gradually disappears in subsequent cycles, which may relate with irreversible structure reorganization processes of as-grown polymer film (cross-linkage between pendant salen moieties).21 Third, the anodic and cathodic peak current intensities continue to increase as polymerization scans for MWCNTs as support. The distortion of CV plot for Figure 2a shows that the electrochemical activity gradually deteriorates with the increasing content of poly[Ni(salen)]. Comparatively, the growth of poly[Ni(salen)] on MWCNTs possesses not only more massive in scale but also higher electrochemical reversibility, which can be reflected from the large current response and the adjacent pair of redox peak potentials. The doping level can be used for investigating quantitatively the electrochemical activity of conducting polymer, which means the relative charge stored per monomer unit inside conducting polymer. According to the classical analysis of salen-type polymer,9,2124 the number of electrons per monomer expended during electropolymerization of Ni(salen) contains three parts: 2 electrons for polymerization, 2y electrons for ligand-based redox 11823

dx.doi.org/10.1021/jp111831y |J. Phys. Chem. C 2011, 115, 11822–11829


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Figure 3. Doping level of poly[Ni(salen)] grown on (9) polished Pt electrode and (b) MWCNTs-coated Pt electrode vs polymerization scan number.

switching, and z electrons for metal-based redox switching. The ratio of polymerization charge (Qpoly) and redox charge (Qredox) is Qpoly/Qredox = (2 þ 2y þ z)/(2y þ z), where the doping level can be estimated from the sum of 2y and z. Figure 3 shows the evolutions of the doping level of poly[Ni(salen)] grown on polished Pt electrode and MWCNTs-coated Pt electrode. The doping levels of poly[Ni(salen)] with MWCNTs as support have higher values than those grown on polished Pt electrode, which reflects that MWCNTs plays a positive role in electropolymerization of Ni(salen). The masses of poly[Ni(salen)] grown on MWCNTs are evaluated by the calculation of charge expended during electropolymerization and eventually the corresponding mass ratios of poly[Ni(salen)] and MWCNTs in composites are as follows: 0.28/1, 0.63/1, 0.93/1, and 1.18/1 for 5th, 10th, 15th, and 20th scans, respectively. The samples with mass ratio of 0.28/1 and 0.93/1 are received as two typical composites and used for following characterizations. Characterization. Nitrogen adsorption was performed at 77.3 K and was carried out using a Quantachrome instrument. The specific surface area and total pore volume were determined using the BrunauerEmmetTeller method. The pore size distribution was calculated using BarrettJoinerHalenda methods. When the preparation of poly[Ni(salen)]/MWCNT composites finished, they were rinsed with AN in order to remove any soluble species from the composites before following characterizations and electrochemical measurements. Fourier transform infrared spectra (FTIR) of poly[Ni(salen)]/ MWCNT composites were recorded on an IR spectrometer (Shimadzu, FTIR-8400S). The microstructures of poly[Ni(salen)]/MWCNT composites were characterized by field emission scanning electron microscopy (FESEM, Zeiss SuprA 55 microscope) and transition electron microscopy (TEM, JEOL JEM2011). Electrochemical measurements were carried out in the monomer-free AN solution containing 1.0 M triethylmethylammonium tetrafluoroborate in a closed three-electrode compartment cell at room temperature. CV, EIS, and galvanostatic charge/ discharge tests were measured by a VMP2 instrument with ECLab software (version 9.30). CV tests were done between 0.0 and 1.2 V at different scan rates in the range 201000 mV s1. EIS tests were measured with applied potential 0.0, 0.9, and 1.2 V in the frequency range at 100 kHz50 mHz. Galvanostatic charge/ discharge tests were measured at different current densities in the range 0.0520 mA cm2.

Figure 4. (a) FTIR spectra of commercial MWCNTs (A) and functionalized MWCNTs (B) and (b) MWCNTs, composite with mass ratio of 0.29/1 (C) and 0.93/1 (D), and poly[Ni(salen)] grown on polished Pt electrode (E).

’ RESULTS AND DISCUSSION Figure 4a shows the FTIR spectra of MWCNTs before and after strong acid process. Assignments of vibration peaks of MWCNTs are based on previous spectroscopic works of MWCNTs.2527 The bands in the range of 30002835, 17901512, and 1210970 cm1 are assigned to CHn, CdO, and CO groups, respectively. The obviously change in FTIR indicates that the strong acid process makes MWCNTs with carboxyl functional groups. Figure 4b shows the FTIR spectra of MWCNTs, poly[Ni(salen)] grown on polished Pt electrode, and composites with mass ratio of 0.29/1 and 0.93/1, respectively. The broad absorption peak at about 3500 cm1 indicates the presence of OH group which may be caused by absorbed water or H-bonded OH functionalities in carboxylic acid groups of functionalized MWCNTs.27,28 For poly[Ni(salen)] grown on polished Pt electrode (Figure 4b-E), the absorption band in the range of 3000500 cm1 is in accordance with poly[Ni(salen)] in the literature.29,30 However, for the composite with mass ratio of 0.29/1 (Figure 4b-C), the absorption peaks at 1744, 1620, 1532, 1446, 1383, and 1060 cm1 almost disappear, which are characteristic of CdN stretch of the salen ligand, phenyl ring inplane vibrations, and CO stretch vibrations, respectively. The variation in the band of 2000500 cm1 reveals that the interaction between poly[Ni(salen)] and MWCNTs hinders the vibrations of radicals in poly[Ni(salen)]. For the composite with mass ratio of 0.93/1 (Figure 4b-D), the absorption peaks at 1744, 1620, 1446, 1383, and 1060 cm1 in the band range of 2000500 cm1 appear, which reveals the presence of poly[Ni(salen)] structure. The appearance of absorption peaks in the band range of 2000500 cm1 for the composite with mass ratio of 0.93/1 means that the influence of interaction between 11824



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Figure 5. FESEM images of (a, b) MWCNTs, (c, d) composite with mass ratio of 0.29/1, and (e, f) composite with mass ratio of 0.93/1. (a, b, c, e) are obtained at the surfaces of samples (50K, except for b (200K)); (d, f) are obtained at the inner faultages of samples (200K).

poly[Ni(salen)] and MWCNTs gradually diminishes with the increasing content of poly[Ni(salen)] in composites. Figure 5 shows FESEM images of MWCNTs and composites with mass ratio of 0.29/1 and 0.93/1, respectively. The morphologies and inner structures of samples are clearly observed from the surface and faultage section. Figure 5a shows the tubular intertwist structure of MWCNTs, which is more evident in larger magnification in Figure 5b and is characteristic of plenty of gaps between MWCNTs. For composites, the differences are caused by the mass content of as-grown poly[Ni(salen)], as seen in Figure 5cf. Compared with Figure 5a,c, the surface carbon nanotubes are thicker than pure MWCNTs due to the adhesive growth of poly[Ni(salen)] on MWCNTs. Meanwhile, lots of gaps between MWCNTs have been filled by poly[Ni(salen)] which connects numerous MWCNTs to be an integral, except for some large gaps, as seen in Figure 5d. With the increasing content of poly[Ni(salen)] in composites, the morphology and inner structure of composites go on varying. In Figure 5e, it can be seen that as-grown poly[Ni(salen)] on MWCNTs not only connects MWCNTs to be a piece but also blocks gaps between MWCNTs, and the growth of poly[Ni(salen)] completely

Figure 6. CV plots of composites at the scan rate of (a) 40 and (b) 400 mV s1. Arrows indicate the peak current trend with increasing mass ratio of poly[Ni(salen)] in composites: 0.29/1 < 0.63/1 < 1.18/1 < 0.93/1.

fills the inner space of composites so as to form the ferroconcrete-like structure. The difference of inner microstructure of composites with different contents of poly[Ni(salen)] may influence the kinetic behavior and electrochemical response of composites. 11825



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Figure 7. Plots of ipa, ipc vs v1/2 of composites. Symbols: composites with mass ratio of (9) 0.29/1, (b) 0.63/1, (2) 0.93/1, and (1) 1.18/1.

Table 1. Values of D, Obtained from CV by Using the RandlesSevcik Equation

Figure 8. Nyquist plots of MWCNTs and composites with mass ratio of (a) 0.29/1, (b) 0.63/1, (c) 0.93/1, and (d) 1.18/1. Symbols: (9) MWCNTs at 0.0 V, (b) composite at 0.0 V, (2) composite at 0.9 V, and (1) composite at 1.2 V.

D (108 cm2 s1, 201000 mV s1) mpoly/mMWCNTs

n

anode

cathode

0.29/1

0.665

4.68

4.80

0.63/1

0.403

6.91

4.21

0.93/1

0.390

3.49

3.77

1.18/1

0.340

2.85

2.43

The CV plots of composites possess a couple of characteristic redox peaks of poly[Ni(salen)] in Figure 6 as well as the electric double-layer current attributed to MWCNTs. Two discernible parts are presented: first, the redox peak currents rise with the increasing content of poly[Ni(salen)] but drop when the content of poly[Ni(salen)] increases from 0.93/1 to 1.18/1, which reflects that the reasonable content range of poly[Ni(salen)] grown on MWCNTs to keep preferable electrochemical activity. Second, the electric double-layer current responses of composites within the potential range of 0.00.6 V are larger than those of MWCNTs, which are specifically obvious in low scan rates (Figure 6a), and the rise of electric double-layer current for composites compared with MWCNTs may be caused by the electron-withdrawing/donating interaction of the salen ligand in poly[Ni(salen)]. Under semi-infinite diffusion conditions, the peak current (ip) for a reversible electrode action is given by the RandlesSevick equation ip = 0.4463nFSC(nF/RT)1/2D1/2v1/2,31 which can be used to estimated the charge transport diffusion coefficient (D). In the RandlesSevick equation, n means the charge transfer number equivalent with the doping level according to its definition. Figure 7 shows the ip ∼ v1/2 plots of composites present approximately linear dependence. Therefore, values of D can be estimated and are listed in Table 1. The value of D consists of two parts: one is attributed to the redox charge transport of poly[Ni(salen)], and the other is attributed to the electron charge of MWCNTs. The values of D reflect the influence of mass content of as-grown poly[Ni(salen)] on the kinetics of composites. Plots of ip vs v1/2 in Figure 7 reveal that the slope sequence of cathode section is as follows: 0.29/1 < 0.63/1 ≈ 1.18/1 < 0.93/1. However, the doping level is in inverse proportional to the mass content of as-grown poly[Ni(salen)], so the value of D decreases in the order 0.29/1 > 0.63/1 > 0.93/1 > 1.18/1, as listed in Table 1. This order indicates that the composite with mass ratio

Figure 9. High-frequency (100 kHz100 Hz) data of MWCNTs and composites: (a) 0.29/1, (b) 0.63/1, (c) 0.93/1, and (d) 1.18/1. Symbols: (9) MWCNTs at 0.0 V, (b) composite at 0.0 V, (2) composite at 0.9 V, and (1) composite at 1.2 V.

of 0.63/1 possesses better kinetics than the others (0.29/1, 0.93/ 1, and 1.18/1). Figure 8 shows Nyquist plots for MWCNTs at 0.0 V and composites at neutral (0.0 V), oxidized (0.9 V), and overoxidized (1.2 V) states. At the neutral state (0.0 V), the low-frequency data of both MWCNTs and composites lie on almost vertical lines which correspond to the behavior of capacitive materials. And for MWCNTs and composites, the phase angles of low-frequency data gradually decrease in the order MWCNTs (85.4°) > composite with 0.29/1 (82.3°) > composite with 0.63/1 (78.3°) > composite with 0.93/1 (74.5°) > composite with 1.18/1 (71.4°). This order reflects that the as-grown poly[Ni(salen)] on MWCNTs limits the migration process of counterions within composites and thus increases the ionic resistance. When applied potential of composites increases to 0.9 and 1.2 V, the same variations of Nyquist plots for all composites can be observed. The low-frequency (100 kHz100 Hz) data remain almost line shapes at the oxidized state (0.9 V). However, it tends to be arc shape at the overoxidized state (1.2 V) which indicates that a new reaction happened at overoxidized state. That may be caused by the cross-linkage of molecule within poly[Ni(salen)], as Aubert proposed about the ladder-shape polymer.21 Nyquist plots are also used to evaluate alternating ohmic resistance and electrochemical reaction resistance according to high-frequency behaviors of composites. Figure 9 shows the variation of high-frequency section for composites with different 11826



Figure 10. Capacitance vs frequency dependence of MWCNTs and composites with mass ratio of (a) 0.29/1, (b) 0.63/1, (c) 0.93/1, and (d) 1.18/1. Symbols: (9) MWCNTs at 0.0 V, (b) composite at 0.0 V, (2) composite at 0.9 V, and (1) composite at 1.2 V.

Figure 11. Ohmic resistance variation of composites relative to MWCNTs. Symbols: composites with mass content of (9) 0.29/1, (b) 0.63/1, (2) 0.93/1, and (1) 1.18/1.

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mass ratios. When the ratio is smaller than 0.63/1, the as-grown poly[Ni(salen)] improves the conductivity of MWCNTs, as shown in Figure 9a,b. Contrarily, ohmic resistances of composites rise when the ratio exceeds 0.93/1 compared with MWCNTs. Additionally, the semicircles in high-frequency section are enlarged with the increasing applied potential, which are the most obvious for the composite with ratio of 1.18/1, and this implies the electrochemical resistance increases with the increasing applied potential. Figure 10 shows the capacitancefrequency plots for MWCNTs and composites under different applied potentials (0.0, 0.9, and 1.2 V). The capacitances are calculated from imaginary components of Nyquist plots in Figure 9, as functions of frequencies according to the equation C(f) = 1/(2π img(Z)). It can be seen that capacitances of composites at neutral state (0.0 V) have appreciable elevation at low-frequency section compared with those of MWCNTs at 0.0 V, and the elevation range is in accordance with the increasing content of poly[Ni(salen)] within composites, which corresponds to current responses in CV tests. When the applied potential increases from 0.0 to 0.9 to 1.2 V, it can be clearly seen that the capacitances of all composites increase, which indicates that the poly[Ni(salen)] offers additional charge store ability on the basis of electric double-layer capacitance formed by MWCNTs. Figure 11 shows the relative change of direct ohmic resistance between composites and MWCNTs, which were calculated by IR drop during galvanostatic charge/discharge tests. For composites with mass ratio of 0.29/1 and 0.63/1, ohmic resistance apparently decreases compared with MWCNTs, and the average range approaches 20%. For composite with mass ratio of 0.93/1, ohmic resistance rises mildly. However, for 1.18/1, the ohmic resistance rises obviously, and the average range approaches 50%. By coupling FESEM and EIS analyses, the results clearly demonstrate the influence of mass content of as-grown poly[Ni(salen)] on conductivities of composites. The formation course of poly[Ni(salen)]/MWCNT composite is schematically depicted in Figure 12. At the early stage of

Figure 12. Schematic view for the formation of a poly[Ni(salen)]/MWCNT composite. Inset exhibits high-magnification TEM image. 11827



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’ CONCLUSION In this study, a poly[Ni(salen)]/MWCNT composite was synthesized by in situ electropolymerization. MWCNTs not only supply a large active surface to electropolymerize huge amounts of Ni(salen) but also significantly improve the doping level of poly[Ni(salen)] by the covalent interaction. The growth of poly[Ni(salen)] fills gaps between MWCNTs and forms a ferroconcrete-like microstructure. The mass content of as-grown poly[Ni(salen)] influences both the kinetics and conductivity of the poly[Ni(salen)]/MWCNT composite. The composite with mass ratio of 0.63/1 owns lower resistance and higher capacitance per area than those of MWCNTs. The improvement of capacitance is due to the reversible redox switching of poly[Ni(salen)] between neutral and oxidative states. These features make it a potential electrode material for supercapacitors. ’ AUTHOR INFORMATION Corresponding Author

*Fax: þ86 10 62332651. E-mail: [email protected].

Figure 13. (a) Galvanostatic charge/discharge plots for MWCNTs and composites (2 mA cm2) and (b) capacitancecurrent relationship calculated from charge/discharge tests. Symbols: composites with mass ratio of (9) MWCNTs, (b) 0.29/1, (2) 0.63/1, (1) 0.93/1, and ([) 1.18/1.

electropolymerization, the growth of poly[Ni(salen)] covers the surface of carbon nanotubes and connects with each other; meanwhile, appropriate gaps remain, which not only increase the conductivity but also meet the quick movement of counterions (ClO4, BF4). When the growth of poly[Ni(salen)] reaches to some extent, the gaps between MWCNTs have been completely filled by poly[Ni(salen)], and the poly[Ni(salen)] continues growing on the surface of composites, which conversely caused the rise of electroresistance and ionic resistance as supported by the results of EIS and galvanostatic charge/discharge tests. Figure 13 shows the variation of the potential with time during constant current per area (2 mA cm2). MWCNTs have symmetric change for charge/discharge course which is the characteristic of high Coulombic efficiency. Composites possess longer charge/discharge time than MWCNTs, which demonstrates that poly[Ni(salen)] contributes the pseudocapacitance caused by the reversible redox switching between neutral and oxidative states on the basis of electric double-layer capacitance of MWCNTs. However, the improvement of charge storage ability is restricted by several factors, such as the pristine properties of conducting polymer, the effective active surface area of MWCNTs, or the loading mass of polymer. From Figure 13b, it can be clearly seen that the capacitance per area (mF cm2) is improved by the growth of poly[Ni(salen)] on MWCNTs. However, when the mass ratio increases from 0.29/1 to 0.63/1 and 1.18/1, the total capacitance per area has no obvious rise, which indicates that when the content of poly[Ni(salen)] raises to some extent, the number of total electrochemical activity sites for storing charge will cease to rise. Correspondingly, Coulombic efficiency gradually drops with the increasing content of poly[Ni(salen)].

’ ACKNOWLEDGMENT Financial support of this work by Beijing Natural Science Foundation of China (No. 2093039), Program for New Century Excellent Talents in University (NECT), and State Key Laboratory of Multiphase Complex Systems (MPCS-2011-D-08) is gratefully acknowledged. ’ REFERENCES (1) Gosden, C.; Kerr, J. B.; Pletcher, D.; Rosas, R. J. Electroanal. Chem. 1981, 117, 101. (2) Isse, A. A.; Gennaro, A.; Elio, V. Electrochim. Acta 1997, 42, 2065. (3) Bianchini, G.; Cavarzan, A.; Scarso, A.; Strukul, G. Green Chem. 2009, 11, 1517. (4) Hoferkamp, L. A.; Goldsby, K. A. Chem. Mater. 1989, 1, 348. (5) Audebert, P.; Hapiot, P.; Capdevielle, P.; Maumy, M. J. Electroanal. Chem. 1992, 338, 269. (6) Dahm, C. E.; Peters, D. G. Anal. Chem. 1994, 66, 3117. (7) Dahm, C. E.; Peters, D. G.; Simonet, J. J. Electroanal. Chem. 1996, 410, 163. (8) Tchepournaya, I.; Vasilieva, S.; Logvinov, S.; Timonov, A.; Amadelli, R.; Bartak, D. Langmuir 2003, 19, 005. (9) Vilas-Boas, M.; Freire, C.; de Castro, B.; Hillman, A. R. J. Phys. Chem. B 1998, 102, 8533. (10) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282. (11) Liu, T.; Sreekumar, T. V.; Kumar, S.; Hauge, R. H.; Smalley, R. E. Carbon 2003, 41, 2440. (12) Peng, C.; Jin, J.; Chen, G. Z. Electrochim. Acta 2007, 53, 525. (13) Wildgoose, G.; Banks, C.; Compton, R. Small 2006, 2, 182. (14) Georgakilas, V.; Gournis, D.; Tzitzios, V.; Pasquato, L.; Guldi, D. M.; Prato, M. J. Mater. Chem. 2007, 17, 2679. (15) Song, Y. J.; Lee, J. U.; Jo, W. H. Carbon 2010, 48, 389. (16) Lu, Y.; Li, T.; Zhao, X. Q.; Li, M.; Cao, Y. L.; Yang, H. X.; Duan, Y. Y. Biomaterials 2010, 31, 5169. (17) Zheng, S. F.; Hu, J. S.; Zhong, L. S.; Song, W. G.; Wan, L. J.; Guo, Y. G. Chem. Mater. 2008, 20, 3617. (18) Chen, P. C.; Shen, G. Z.; Shi, Y.; Chen, H. T.; Zhou, C. W. ACS Nano 2010, 4, 4403. (19) Holm, R. H.; Everett, G. W.; Chakravorty, A. Prog. Inorg. Chem. 1966, 7, 83. 11828



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