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poly ortho aminophenol composites
Synthetic Metals 161 (2011) 1760–1765
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
An electrochemical study of the synthesis and properties of multi-walled carbon nanotube/poly ortho aminophenol composites A. Ehsani, M.G. Mahjani ∗ , M. Jafarian Department of Chemistry, Faculty of Science, K. N. Toosi University of Technology, P.O. Box 15875-4416, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 13 February 2011 Received in revised form 12 June 2011 Accepted 13 June 2011 Available online 12 July 2011 Keywords: POAP Composite CNTs Impedance
a b s t r a c t Composites of multi-walled carbon nanotubes (MWCNTs) and poly ortho aminophenol (POAP) with good uniformity for use as electrodes in electrochemical capacitors were prepared by electropolymerization by using the ionic surfactant as electrolyte, for dispersing CNTs within conducting polymer/carbon nanotube composite films. The capacitance properties were investigated with cyclic voltammetry (CV), discharge tests and ac impedance spectroscopy. The composite electrode shows much higher specific capacitance, better power characteristics and is more promising for application in the capacitor than a pure POAP electrode. The effect and role of MWCNT in the composite electrode are discussed in detail. In comparison with a Ni–POAP/glassy carbon (GC), a Ni–MWCNT–POAP/GC electrode shows a better catalytic performance for the electrocatalytic oxidation of methanol. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) have a novel structure, a narrow distribution size, highly accessible surface area, low resistivity, and high stability [1]. It has been shown experimentally that the introduction of CNTs into a polymer matrix improves the electric conductivity as well as the mechanical properties of the original polymer matrix [2–4]. Composites of carbon nanotube and polypyrrole were also prepared to enhance its electrochemical capacitance performance [5]. The high surface area and conductivity of CNTs may improve the redox properties of conducting polymers. Recently, it has been found that single-walled carbon nanotube and polyaniline (SWCNT/PANI) composites with good uniformity can be formed by the polymerization of aniline containing well-dissolved SWCNT [6]. The CNT in the composites adhere strongly to the PANI matrix by the formation of a charge-transfer complex rather than the weak van der Waals interactions between them, which improves dispersion of SWCNTs into the PANI matrix, and results in enhanced electric conductivity. CNTs have been aligned through different techniques such as template-directed fabrication, template-free chemical vapor deposition, self-assembly, mechanical stretching, electrophoresis, etc. [7–11]. However, when CNTs introduced into a polymeric matrix, disperse randomly, they lose their orientations. Because of the unique one-dimensional structure of nanotubes, a high anisotropy
∗ Corresponding author. Tel.: +98 21 22853551; fax: +98 21 22853650. E-mail address: [email protected] (M.G. Mahjani). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.020
is expected for nanotube–polymer composites. Hence, fabrication of high-performance nanotube-based composites should take into consideration the alignment of CNTs in a certain direction. Melt [12] or electrical spinning [13] of a composite through a narrow hole leads to nanotube alignment, but the method provides only fibers. The use of perpendicularly aligned CNTs for making composite materials with polymers should offer additional advantages to many applications with great ease [14]. Magnetic field assisted alignment of carbon nanotubes in a polymeric matrix is another very powerful strategy [15,16]. Aminophenols are interesting members of the class of substituted anilines. The hydroxyl group in the phenyl ring can be oxidized to quinine and quinine can be reduced again. POAP gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerized in acidic solution [17,18]. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7. Herein, we describe a simple strategy for aligning of disordered CNT within the conducting polymer matrix by in situ electropolymerization using an ionic surfactant as the supporting electrolyte. The CNTs were first dispersed in an aqueous solution containing an ionic surfactant similar to the procedure reported by Islam et al. [19]. Then electroactive monomer (OAP) was added into the above mixture and finally electrochemical reaction was preceded at the surface of the glassy carbon (GC) electrode. In the present work POAP/MWCNT composite was synthesized by normal pulse voltammetry (NPV) methods and electrochemical properties of film were investigated by using electrochemical
A. Ehsani et al. / Synthetic Metals 161 (2011) 1760–1765
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Fig. 1. The surface morphology (SEM) of (a) POAP films, (b, c) POAP film in presence of MWCNT in two magnification.
techniques, viz. CV and electrochemical impedance spectroscopy (EIS).
2. Experimental The chemicals used in this work were of Merck origin and used without further purification. Typical MWCNT were uniform with outer diameters in the range of 5–20 nm and lengths up to several hundreds of nanometers were used. Their hollow cores and multi-layer walls could be clearly seen by transmission electron microscopy (TEM) observation [20]. All electrochemical measurements were carried out in a conventional three electrodes cell, powered by a potentiosat/galvanostat (EG&G273A) and a frequency response analyzer (EG&G, 1025). The system was run by a PC through M270 and M398 software using GPIB interface. The frequency ranges of 100 kHz to 100 mHz and modulation amplitude of 5 mV were employed for impedance studies. POAP films electrodeposited on a GC disk of 0.0314 cm2 areas were employed as working electrode. Saturated calomel electrode (SCE) and a platinum wire were used as reference and counter electrodes respectively. POAP/MWCNT composites were prepared by NPV with 5 mV potential increment and plus width 0.005 s in a stirring solution containing 0.01 M monomer, 0.5 M HClO4 , 0.1 M LiCO4 , 5.0 × 10−3 M sodium dodecyl sulfate (SDS) and 8% of MWCNT in suspension. SDS was used as an additive in order to suspend MWCNT particles and improve the stability and electroactivity of the resulting films. The effect of SDS on the electropolymerization of conducting polymer in aqueous solutions was investigated [18]. The coated electrode with POAP/MWCNT composite was thoroughly washed with distilled water and dried. In order to incorporate Ni(II) ions into the POAP composite film, the freshly electropolymerized POAP/GC was placed at open circuit in a well stirred aqueous solution of 0.1 M nickel sulfate. The thickness of the deposited film on graphite was estimated from the charge consumed in reducing the polymer (Q) and the molar concentration of electro-active sites in the film, which is obtained by faraday law [21]. For a given Q the molar concentration of electro-active sites in the film and then the mass of deposited polymer was estimated, considering a value of 2 for electron transfer and 109 for OAP molecular weight. Assuming a value of 1.33 g cm−3 for POAP films density at 25 ◦ C and the geometric area of GC as 0.0314 cm2 , the polymer film thickness can easily be calculated. The microstructure of the obtained films was investigated by a VEGA/TESCAN scanning electron microscope. The transmission electron microscopy (TEM) was performed using a CEM 902A ZEISS transmission electron microscope, with an accelerating voltage of 80 kV. The impedance data was analyzed and fitted by Zplot/Zview software (Scribner Associated Inc.).
3. Results and discussion Fig. 1a and b shows the scanning electron microscopy (SEM) micrographs of the surface of the pure POAP and POAP/MWCNT composite films on the GC electrode respectively and Fig. 2 shows transmission electron microscopy (TEM) of POAP/MWCNT that obtained by NPV with potential scale of −0.2 to +0.9 V in an aqueous solution of 0.01 M monomer, 0.5 M HClO4 , 0.1 M LiCO4 , 5.0 × 10−3 M SDS and 8% of MWCNT in different magnification. These images confirm that the POAP has been wrapped on the surface of MWCNTs or MWCNTs have been dispersed in the polymer matrix. It was clearly observed that a uniform film was deposited at the surface of the GC electrode. There were so many one-dimensional nanostructures that stood almost vertically on the deposited films. Such nanostructures were of roughly uniform size with diameters in the range of several hundreds of nanometers. However, these one-dimensional nanostructures could not be seen in the absence of CNTs and by keeping other experimental parameters constant, which indicated that the growth of these nanostructures at the surface of the GC electrode has something to do with CNTs. It is expected that this difference in surface morphology will play an important role in subsequent processes, e.g., impedance spectra. From these investigations, it is evident that micelleencapsulated carbon nanotube composite nanostructures would form by using SDS to disperse carbon nanotubes in aqueous solution [17,22,23], and that OAP could enter the interiors of micelle-encapsulated carbon nanotubes and locate at the interfaces between surfactants and CNT [19,20]. These micelleencapsulated CNT composite nanostructures were with random orientation without an electric field existing. By exerting positive voltage to GC electrode, due to orientation of CNT in the presence of an electric field [24], these micelle-encapsulated CNT composite nanostructures at the vicinities of the GC electrode would orient towards the surface of the GC electrode. Meanwhile, due to the interaction between positive GC electrode and SDS negative ions, the micelle-encapsulated carbon nanotube composite nanostructures would move towards and as far as the surface of the GC electrode. Once micelle encapsulated CNT composite nanostructures ran into the GC electrode, if potentially high enough, OAP located at the interfaces between surfactants and carbon nanotubes would polymerize at the surfaces of both the GC electrode and CNT. Fig. 3 displays the FT-IR spectra of ortho-aminophenol (a), POAP (b), and POAP/MWCNTs (c). FT-IR spectrum for ortho-aminophenol shows a typical profile with two peaks at 3378 cm−1 and 3311 cm−1 due to the asymmetrical and symmetrical N–H stretching vibrations, two peaks at 1510 cm−1 and 1473 cm−1 are the characteristic bands of the C C stretching vibration mode for benzenoid rings,
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A. Ehsani et al. / Synthetic Metals 161 (2011) 1760–1765
Fig. 2. TEM images of POAP/MWCNT in different magnification.
and the bands at 1401 cm−1 and 1220 cm−1 can be attributed to the C–O–H deformation vibration and the C–O stretching vibration respectively. Spectrums b and c present POAP and POAP/MWCNT patterns which synthesized at different conditions. POAP/MWCNT has been synthesized in the presence of surfactant and MWCNT. The spectrum of POAP shows two peaks at 3380 cm−1 and 1602 cm−1 due to the characteristic bands of the N–H stretching vibrations and the axial stretching of the C O groups in the POAP structure. The peaks in the region of 1400–1600 cm−1 are attributed to the stretching of C–H and C C groups. The peak at 1384 cm−1 is clearly seen that could be assigned to the C–N stretching vibration of a secondary aromatic amine. The band at 1121 cm−1 is ascribed to the stretching of the C–O–C linkages and further support the o-aminophenol changed into POAP [25]. The spectrum of POAP/MWCNT (c) is similar in the main peaks to that of the POAP pattern however with different intensity, and it is well known that the peaks of pristine MWCNTs is very weak. These results confirm that the MWCNTs have been dispersed in the polymer matrix. To elucidate the effect of MWCNTs on the property of POAP films, electrochemical performance of composite films was evaluated by carrying out CV measurements in 0.5 M HClO4 , as shown in Fig. 4. The cyclic voltammograms of the MWCNT/POAP films
(Fig. 4(2)) showed a couple of strong and broad oxidation and reduction waves. Their wave currents were stronger than that of pure POAP films electrodes (Fig. 4(1)). The voltammetric behavior of both films is similar and the CV curves show capacitive-like responses approximately rectangular in shape. The voltammograms reveal the electrodes are stable in HClO4 solution within the sweeping potential range. Close comparisons of CV curves between MWCNT/POAP electrode and pure POAP electrode show that a MWCNT/POAP electrode not only displays a higher background current in the potential sweep but also there exist faradic currents, which are believed to arise from the contribution of the loaded MWCNT. Owing to the higher current in the voltammograms of the MWCNT/POAP electrode than a pure POAP electrode, a larger capacitance for capacitors equipped with MWCNT/POAP electrode can be anticipated. It is interesting to see that the supercapacitor made POAP/MWCNT show a wide-open rectangular shaped voltammogram with a charge density of 450 F g−1 . The specific capacitance, SC, can be obtained from the following equation for CV measurements: SC =
2i sm
(1)
where i denotes the average cathodic current, s is the applied potential sweep rate, and m is the mass of each active material on the electrode. To illustrate the performance of the resulting electrodes in supercapacitors, constant current discharge was performed to measure the capacitance in a 0.5 M HClO4 solution. Typical galvanostatic discharge curves at a current of 0.1 mA between 0 and 0.8 V are shown in Fig. 5. It can be clearly seen that the MWCNT/POAP electrode cell presents higher capacitance than the pure POAP and this
Fig. 3. FT-IR spectrum of (a) ortho-aminophenol, (b) POAP, and (c) POAP/MWCNTs.
Fig. 4. Comparative cyclic voltammograms for POAP and POAP/MWCNT electrodes with a potential sweep rate of 100 mV s−1 between −0.2 and 0.85 V in acidic solution.
A. Ehsani et al. / Synthetic Metals 161 (2011) 1760–1765
Fig. 5. Galvanostatic discharge curves of the POAP film at 0.1 mA, (1) in presence of MWCNT, (2) in absence of MWCNT.
has been considered as the contribution of faradic pseudo capacitance of MWCNT/POAP supercapacitor. In the case of electrochemical system, EIS can reveal information regarding processes occurring in the polymer matrix when it is doped. This may include kinetic values of the doping process and parameters of the diffusion of ions into the polymers [26]. EIS was analyzed for POAP films in two different synthesis conditions in acidic solution of HClO4 and LiClO4 . Fig. 6 shows the Nyquist diagrams of electrodes in open circuit potential (OCP). The plot in Fig. 6 depicts a single semi-circle in the high frequency region and a straight line in the low-frequency region for all spectra. The highfrequency arc is the overall contact impedance generated from the electrical connection between MWCNT/POAP composites and the backing plate as well as the charge transfer at the contact interface between the electrode and the electrolyte solution. In spite of the similar shape of the impedance spectra, there is an obvious difference between the diameters of the four semi-circles. That is, the diameters of the semicircles decline greatly with the increasing doping content of MWCNTs in POAP. In other words, the bulk-film transport of electrons and the charge transfer resistance (Rct ) of MWCNT/POAP composite films are much lower than that of the pure POAP films. This means that the MWCNTs inside the POAP matrix may lead to a faster electron transport in the bulk-film and charge transfer in the parallel POAP film/solution interface and MWCNTs/solution interface, compared to that in the originally single POAP film/solution interface. In conducting polymer/carbon
Fig. 6. Nyquist diagrams of POAP films in OCP in acidic solution in two magnification, (1) POAP, (2) POAP/MWCNT film.
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Fig. 7. C2 vs E dependences for the POAP film with (1) presence MWCNT, (2) absence MWCNT.
nanotubes (CP/CNTs) composites, it is suggested that either the polymer functionalizes the CNTs [27], or the CPs are doped with CNTs, i.e., a charge transfer occurs between the two constituents. This fact may suggest that the MWCNT has an obvious improvement effect, which makes the composites have more active sites for faradic reactions and a larger specific capacitance than pure POAP. Also, this result in enhanced electric conductivity, lowers the resistance, and facilitates the charge-transfer of the composites. According to the our previous works [17,18], the behavior of space charge capacitance vs E (potential) dependence and the observed capacitance values were typical of a thin film semiconductor electrode and suggest the formation of space charge or depletion region within the polymer film. This conclusion is consistent with numerous result of other author who demonstrated the formation of a space charge region (SCR) on conducting polymer modified electrodes. At the negative potentials, the space charge region extends over whole film. As the potential drops across the SCR decrease, its thickness becomes smaller than the film thickness, and a quasi-neutral region is formed and the electrode capacitance increases. The dependence of the electrode capacitance on the potential in the simplest case is given by the Mott–Schottky equation [17]: C −2 =
2 εε0 eNA2
E − Efb −
KT e
(2)
where C is the space charge capacitance, ε is the dielectric constant of the polymer, ε0 is the permittivity of free space, e is the elementary charge, k is Boltzmann’s constant, T is the absolute temperature, N is the carrier density that set up the space charge, (E − Efb ) is the absolute value of the potential drop across the SCR. The flat band potential Efb is the potential at which the thickness of the SCR is zero. From Eq. (2) it follow that C−2 vs E line are known as Mott–Schottky plots. Fig. 7 presents the C−2 vs E dependences for the POAP film in the presence and absence of MWCNTs, that obtained from Nyquist plots of films in different offset potential. One can see that each curve has a linear portion that can be described by the Mott–Schottky law to a first approximation. Extrapolating these linear portions, we can estimate the values 0.21 and 0.23 V for the flat band potential of film in presence and absence of CNTs respectively. The values of the carrier density in two cases can be also obtained 8.98 × 1019 and 2.28 × 1019 /cm3 for the film in the presence and absence of CNTs respectively from the slop of the plots. The higher density of carriers in the presence of CNTs than pure POAP film results in doping of CNTs into the polymer film. Furthermore, the negative slopes of Mott–Schottky plots of them show that we can categorize them as p-type semiconductors that space charge is established by counter anions. In this case, polarons or bipolarons act as holes in ordinary semiconductors.
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Fig. 8. Cyclic voltammograms of Ni–MWNT–POAP/GC and Ni–POAP/GC electrode in 0.1 M NaOH solution in the presence of 3 mM methanol at a potential sweep rate of 10 mV s−1 .
Fig. 8 shows cyclic voltammograms of Ni–MWCNT–POAP/GC electrode in 0.1 M NaOH solution in the presence of 0.1 M methanol at a potential sweep rate of 10 mV s−1 . The larger methanol response at the Ni–MWCNT–POAP/GC in respect to Ni–POAP/GC electrode is proposed to be the Ni–MWCNT–POAP/GC enhances the catalytic properties of nickel oxide through fine dispersion of the catalyst particles into the conductive polymer matrix to result in a drastic increase in surface area. Fig. 9 shows cyclic voltammograms of Ni–MWCNT–POAP/GC electrode in 0.1 M NaOH solution in the presence of various concentrations of methanol at a potential sweep rate of 10 mV s−1 . At Ni–MWCNT–POAP/GC electrode, oxidation of methanol appeared as a typical electrocatalytic response in alkaline media by Ni(OH)2 /NiOOH [28]. The anodic electrochemical reactions can be summarized as follows [18]:
Fig. 9. Cyclic voltammograms of the Ni–MWNT–POAP/GC electrode in 0.1 M NaOH solution in the presence of 0–8 mM of methanol in the solution. Potential sweep rate was 10 mV s−1 .
NiO–OH + OH− → NiO2 + H2 O + e−
(5)
Whereas the reduction reaction observed can be simplified as: NiO2 + 2H2 O + 2e− → Ni(OH)2 + 2OH−
(6)
Ni2+ + 2OH− → Ni(OH)2
(3)
Eqs. (4)–(6) suggest that the product obtained was a mixture of NiO2 and NiO–OH. Methanol is oxidized on the modified surface according to following reaction:
Ni(OH)2 + OH− → NiO–OH + H2 O + e−
(4)
Methanol + NiIII O–OH → NiII (OH)2 + product
(7)
Fig. 10. Nyquist diagrams of Ni–MWNT–POAP/GC electrode in the presence 0.1 (1), 0.2 (2), 0.4 (3), 0.8 mM (4), methanol in 0.1 M NaOH solution and corresponding fitted curves. DC potential is 650 mV. (5) and (6) are Nyquist diagram in 700 mV/Ag, AgCl.
A. Ehsani et al. / Synthetic Metals 161 (2011) 1760–1765
The anodic current in the positive sweep was proportional to the bulk concentration of methanol and any increase in the concentration of methanol caused an almost proportional linear enhancement of the anodic current. So, catalytic electrooxidation of methanol on Ni–MWCNT–POAP/GC seems to be certain. Fig. 10 presents the Nyquist diagrams of POAP/MWCNT–NiO electrodes recorded at 650 mV/Ag, AgCl dc-offset both in the presence of methanol in 0.1 M NaOH solution. In the methanol’s concentration range of 0.1–0.8 M a steady decrease of the diameter of the semi-circle is witnessed. At further higher concentrations the Nyquist plot extended to the fourth quadrate in the low frequencies and an inductive loop appeared. The equivalent circuits compatible with the results are presented for both the low (inset a) and high (inset b) limits of methanol concentrations. In these circuits, Rs , CPE and Rct represent solution resistance, a constant phase element corresponding to the double layer capacitance and the charge transfer resistance associated with the oxidation of methanol. RL and L signify the characteristics of the inductive element representing the adsorption of the intermediates at higher methanol concentrations. However at potential higher than 650 mV/Ag–AgCl, the diagrams represented negative differential resistance and rolled over the real axis at low frequencies and went to the second quadrant. The diagram crossed the negative real axis at some finite frequencies indicating a dynamic instability of the stationary state. This negative impedance is characteristic of systems capable of exhibiting galvanostatic potential oscillation. 4. Conclusions We have demonstrated a simple and general strategy, namely in situ electropolymerization by using the ionic surfactant as electrolyte, for dispersing disordered CNTs within conducting polymer/carbon nanotubes composite films. Surfactant SDS has played a key role in the synthesis of CP/CNT composite films. We have introduced the MWCNT/POAP composite electrode to improve the specific capacitance and power characteristic of electrochemical capacitance. The MWCNT has an obvious improvement effect, which makes the composites have more active sites for faradic reaction and larger specific capacitance than pure POAP. Also, it results in enhanced electric conductivity, lower the resistance, and facilitate the charge-transfer of the composites. In comparison with a Ni–POAP/GC, a Ni–MWCNT–POAP/GC electrode shows a better catalytic performance for the electrocatalytic oxidation of methanol.
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Acknowledgement Grateful acknowledgement is made to Iranian Nano Council for support of this work. References [1] C.M. Niu, E.K. Sichel, R. Hoch, D. Moy, Appl. Phys. Lett. 70 (1997) 1480– 1482. [2] L.S. Schadler, S.C. Giannaris, P.M. Ajayan, Appl. Phys. Lett. 73 (1998) 3842– 3844. [3] H.D. Wagner, O. Lourie, Y. Feldman, R. Tenne, Appl. Phys. Lett. 72 (1998) 188–190. [4] D. Qian, E.C. Dickey, R. Andrews, T. Rantell, Appl. Phys. Lett. 76 (2000) 2868–2870. [5] M. Hughes, G.Z. Chen, M.S.P. Shaffer, D.J. Fray, Chem. Mater. 14 (2002) 1610–1613. [6] X.H. Li, B. Wu, J. Huang, J. Zhang, Z.F. Liu, H.L. Li, Carbon 41 (2003) 1670–1673. [7] W.A. de Heer, J.M. Bonard, K. Fauth, A. Cha telain, L. Forro, D. Ugarte, Adv. Mater. 9 (1997) 87–89. [8] C.N.R. Rao, R. Sen, B.C. Satishkumar, A. Govindaraj, Chem. Commun. (1998) 1525–1526. [9] Z. Liu, Z. Shen, T. Zhu, S. Hou, L. Ying, Z. Shi, et al., Langmuir 16 (2000) 3569–3573. [10] P.M. Ajayan, Adv. Mater. 7 (1995) 489–491. [11] K. Yamamoto, S. Akita, Y. Nakayama, J. Appl. Phys. 35 (1996) 917–918. [12] R. Haggenmueller, H.H. Gommans, A.G. Rinzler, J.E. Fischer, K.I. Winey, Chem. Phys. Lett. 330 (2000) 219–225. [13] Y. Dror, W. Salalha, R.L. Khalfin, Y. Cohen, A.L. Yarin, E. Zussman, Langmuir 19 (2003) 7012–7020. [14] M. Gao, S. Huang, L. Dai, G. Wallace, R. Gao, Z. Wang, Angew. Chem. Int. Ed. 39 (2000) 3664–3667. [15] H. Garmestani, M.S. Al-Haik, K. Dahmen, R. Tannenbaum, D. Li, S.S. Sablin, Adv. Mater. 15 (2003) 1918–1921. [16] T. Kimura, H. Ago, M. Tobita, S. Ohshima, M. Kyotanyi, M. Yumura, Adv. Mater. 14 (2002) 1380–1383. [17] M.G. Mahjani, A. Ehsani, M. Jafarian, Synth. Met. 160 (2010) 1252–1259. [18] A. Ehsani, M.G. Mahjani, M. Jafarian, A. Naeemy, Prog. Org. Coat. 69 (2010) 510–516. [19] M.F. Islam, E. Rojas, D.M. Bergey, A.T. Johnson, A.G. Yodh, Nano Lett. 3 (2003) 269–273. [20] X.T. Zhang, J. Zhang, R.M. Wang, Z.F. Liu, Carbon 42 (2004) 1455–1461. [21] N. Bahrami Panah, M.G. Mahjani, M. Jafarian, Prog. Org. Coat. 64 (2009) 33–38. [22] Y. Kang, T.A. Taton, J. Am. Chem. Soc. 125 (2003) 5650–5651. [23] V.C. Moore, M.S. Strano, E.H. Haroz, R.H. Hauge, R.E. Smalley, Nano Lett. 3 (2003) 1379–1382. [24] M.S. Kumar, T.H. Kim, S.H. Lee, S.M. Song, J.W. Yang, K.S. Nahm, E.K. Suh, Chem. Phys. Lett. 383 (2004) 235–239. [25] J. Kaizer, R. Csonka, G. Speier, J. Mol. Catal. A: Chem. 180 (2002) 91–96. [26] S.A.M. Refaey, G. Schwitzgebel, O. Schneider, Synth. Met. 98 (1999) 183– 192. [27] M. Baibarac, I. Baltog, S. Lefrant, J.Y. Mevellec, O. Chauvet, Chem. Mater. 15 (2003) 4149–4156. [28] M. Jafarian, R.B. Moghaddam, M.G. Mahjani, F. Gobal, J. Appl. Electrochem. 36 (2006) 913–918.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
An electrochemical study of the synthesis and properties of multi-walled carbon nanotube/poly ortho aminophenol composites A. Ehsani, M.G. Mahjani ∗ , M. Jafarian Department of Chemistry, Faculty of Science, K. N. Toosi University of Technology, P.O. Box 15875-4416, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 13 February 2011 Received in revised form 12 June 2011 Accepted 13 June 2011 Available online 12 July 2011 Keywords: POAP Composite CNTs Impedance
a b s t r a c t Composites of multi-walled carbon nanotubes (MWCNTs) and poly ortho aminophenol (POAP) with good uniformity for use as electrodes in electrochemical capacitors were prepared by electropolymerization by using the ionic surfactant as electrolyte, for dispersing CNTs within conducting polymer/carbon nanotube composite films. The capacitance properties were investigated with cyclic voltammetry (CV), discharge tests and ac impedance spectroscopy. The composite electrode shows much higher specific capacitance, better power characteristics and is more promising for application in the capacitor than a pure POAP electrode. The effect and role of MWCNT in the composite electrode are discussed in detail. In comparison with a Ni–POAP/glassy carbon (GC), a Ni–MWCNT–POAP/GC electrode shows a better catalytic performance for the electrocatalytic oxidation of methanol. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) have a novel structure, a narrow distribution size, highly accessible surface area, low resistivity, and high stability [1]. It has been shown experimentally that the introduction of CNTs into a polymer matrix improves the electric conductivity as well as the mechanical properties of the original polymer matrix [2–4]. Composites of carbon nanotube and polypyrrole were also prepared to enhance its electrochemical capacitance performance [5]. The high surface area and conductivity of CNTs may improve the redox properties of conducting polymers. Recently, it has been found that single-walled carbon nanotube and polyaniline (SWCNT/PANI) composites with good uniformity can be formed by the polymerization of aniline containing well-dissolved SWCNT [6]. The CNT in the composites adhere strongly to the PANI matrix by the formation of a charge-transfer complex rather than the weak van der Waals interactions between them, which improves dispersion of SWCNTs into the PANI matrix, and results in enhanced electric conductivity. CNTs have been aligned through different techniques such as template-directed fabrication, template-free chemical vapor deposition, self-assembly, mechanical stretching, electrophoresis, etc. [7–11]. However, when CNTs introduced into a polymeric matrix, disperse randomly, they lose their orientations. Because of the unique one-dimensional structure of nanotubes, a high anisotropy
∗ Corresponding author. Tel.: +98 21 22853551; fax: +98 21 22853650. E-mail address: [email protected] (M.G. Mahjani). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.020
is expected for nanotube–polymer composites. Hence, fabrication of high-performance nanotube-based composites should take into consideration the alignment of CNTs in a certain direction. Melt [12] or electrical spinning [13] of a composite through a narrow hole leads to nanotube alignment, but the method provides only fibers. The use of perpendicularly aligned CNTs for making composite materials with polymers should offer additional advantages to many applications with great ease [14]. Magnetic field assisted alignment of carbon nanotubes in a polymeric matrix is another very powerful strategy [15,16]. Aminophenols are interesting members of the class of substituted anilines. The hydroxyl group in the phenyl ring can be oxidized to quinine and quinine can be reduced again. POAP gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerized in acidic solution [17,18]. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7. Herein, we describe a simple strategy for aligning of disordered CNT within the conducting polymer matrix by in situ electropolymerization using an ionic surfactant as the supporting electrolyte. The CNTs were first dispersed in an aqueous solution containing an ionic surfactant similar to the procedure reported by Islam et al. [19]. Then electroactive monomer (OAP) was added into the above mixture and finally electrochemical reaction was preceded at the surface of the glassy carbon (GC) electrode. In the present work POAP/MWCNT composite was synthesized by normal pulse voltammetry (NPV) methods and electrochemical properties of film were investigated by using electrochemical
A. Ehsani et al. / Synthetic Metals 161 (2011) 1760–1765
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Fig. 1. The surface morphology (SEM) of (a) POAP films, (b, c) POAP film in presence of MWCNT in two magnification.
techniques, viz. CV and electrochemical impedance spectroscopy (EIS).
2. Experimental The chemicals used in this work were of Merck origin and used without further purification. Typical MWCNT were uniform with outer diameters in the range of 5–20 nm and lengths up to several hundreds of nanometers were used. Their hollow cores and multi-layer walls could be clearly seen by transmission electron microscopy (TEM) observation [20]. All electrochemical measurements were carried out in a conventional three electrodes cell, powered by a potentiosat/galvanostat (EG&G273A) and a frequency response analyzer (EG&G, 1025). The system was run by a PC through M270 and M398 software using GPIB interface. The frequency ranges of 100 kHz to 100 mHz and modulation amplitude of 5 mV were employed for impedance studies. POAP films electrodeposited on a GC disk of 0.0314 cm2 areas were employed as working electrode. Saturated calomel electrode (SCE) and a platinum wire were used as reference and counter electrodes respectively. POAP/MWCNT composites were prepared by NPV with 5 mV potential increment and plus width 0.005 s in a stirring solution containing 0.01 M monomer, 0.5 M HClO4 , 0.1 M LiCO4 , 5.0 × 10−3 M sodium dodecyl sulfate (SDS) and 8% of MWCNT in suspension. SDS was used as an additive in order to suspend MWCNT particles and improve the stability and electroactivity of the resulting films. The effect of SDS on the electropolymerization of conducting polymer in aqueous solutions was investigated [18]. The coated electrode with POAP/MWCNT composite was thoroughly washed with distilled water and dried. In order to incorporate Ni(II) ions into the POAP composite film, the freshly electropolymerized POAP/GC was placed at open circuit in a well stirred aqueous solution of 0.1 M nickel sulfate. The thickness of the deposited film on graphite was estimated from the charge consumed in reducing the polymer (Q) and the molar concentration of electro-active sites in the film, which is obtained by faraday law [21]. For a given Q the molar concentration of electro-active sites in the film and then the mass of deposited polymer was estimated, considering a value of 2 for electron transfer and 109 for OAP molecular weight. Assuming a value of 1.33 g cm−3 for POAP films density at 25 ◦ C and the geometric area of GC as 0.0314 cm2 , the polymer film thickness can easily be calculated. The microstructure of the obtained films was investigated by a VEGA/TESCAN scanning electron microscope. The transmission electron microscopy (TEM) was performed using a CEM 902A ZEISS transmission electron microscope, with an accelerating voltage of 80 kV. The impedance data was analyzed and fitted by Zplot/Zview software (Scribner Associated Inc.).
3. Results and discussion Fig. 1a and b shows the scanning electron microscopy (SEM) micrographs of the surface of the pure POAP and POAP/MWCNT composite films on the GC electrode respectively and Fig. 2 shows transmission electron microscopy (TEM) of POAP/MWCNT that obtained by NPV with potential scale of −0.2 to +0.9 V in an aqueous solution of 0.01 M monomer, 0.5 M HClO4 , 0.1 M LiCO4 , 5.0 × 10−3 M SDS and 8% of MWCNT in different magnification. These images confirm that the POAP has been wrapped on the surface of MWCNTs or MWCNTs have been dispersed in the polymer matrix. It was clearly observed that a uniform film was deposited at the surface of the GC electrode. There were so many one-dimensional nanostructures that stood almost vertically on the deposited films. Such nanostructures were of roughly uniform size with diameters in the range of several hundreds of nanometers. However, these one-dimensional nanostructures could not be seen in the absence of CNTs and by keeping other experimental parameters constant, which indicated that the growth of these nanostructures at the surface of the GC electrode has something to do with CNTs. It is expected that this difference in surface morphology will play an important role in subsequent processes, e.g., impedance spectra. From these investigations, it is evident that micelleencapsulated carbon nanotube composite nanostructures would form by using SDS to disperse carbon nanotubes in aqueous solution [17,22,23], and that OAP could enter the interiors of micelle-encapsulated carbon nanotubes and locate at the interfaces between surfactants and CNT [19,20]. These micelleencapsulated CNT composite nanostructures were with random orientation without an electric field existing. By exerting positive voltage to GC electrode, due to orientation of CNT in the presence of an electric field [24], these micelle-encapsulated CNT composite nanostructures at the vicinities of the GC electrode would orient towards the surface of the GC electrode. Meanwhile, due to the interaction between positive GC electrode and SDS negative ions, the micelle-encapsulated carbon nanotube composite nanostructures would move towards and as far as the surface of the GC electrode. Once micelle encapsulated CNT composite nanostructures ran into the GC electrode, if potentially high enough, OAP located at the interfaces between surfactants and carbon nanotubes would polymerize at the surfaces of both the GC electrode and CNT. Fig. 3 displays the FT-IR spectra of ortho-aminophenol (a), POAP (b), and POAP/MWCNTs (c). FT-IR spectrum for ortho-aminophenol shows a typical profile with two peaks at 3378 cm−1 and 3311 cm−1 due to the asymmetrical and symmetrical N–H stretching vibrations, two peaks at 1510 cm−1 and 1473 cm−1 are the characteristic bands of the C C stretching vibration mode for benzenoid rings,
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Fig. 2. TEM images of POAP/MWCNT in different magnification.
and the bands at 1401 cm−1 and 1220 cm−1 can be attributed to the C–O–H deformation vibration and the C–O stretching vibration respectively. Spectrums b and c present POAP and POAP/MWCNT patterns which synthesized at different conditions. POAP/MWCNT has been synthesized in the presence of surfactant and MWCNT. The spectrum of POAP shows two peaks at 3380 cm−1 and 1602 cm−1 due to the characteristic bands of the N–H stretching vibrations and the axial stretching of the C O groups in the POAP structure. The peaks in the region of 1400–1600 cm−1 are attributed to the stretching of C–H and C C groups. The peak at 1384 cm−1 is clearly seen that could be assigned to the C–N stretching vibration of a secondary aromatic amine. The band at 1121 cm−1 is ascribed to the stretching of the C–O–C linkages and further support the o-aminophenol changed into POAP [25]. The spectrum of POAP/MWCNT (c) is similar in the main peaks to that of the POAP pattern however with different intensity, and it is well known that the peaks of pristine MWCNTs is very weak. These results confirm that the MWCNTs have been dispersed in the polymer matrix. To elucidate the effect of MWCNTs on the property of POAP films, electrochemical performance of composite films was evaluated by carrying out CV measurements in 0.5 M HClO4 , as shown in Fig. 4. The cyclic voltammograms of the MWCNT/POAP films
(Fig. 4(2)) showed a couple of strong and broad oxidation and reduction waves. Their wave currents were stronger than that of pure POAP films electrodes (Fig. 4(1)). The voltammetric behavior of both films is similar and the CV curves show capacitive-like responses approximately rectangular in shape. The voltammograms reveal the electrodes are stable in HClO4 solution within the sweeping potential range. Close comparisons of CV curves between MWCNT/POAP electrode and pure POAP electrode show that a MWCNT/POAP electrode not only displays a higher background current in the potential sweep but also there exist faradic currents, which are believed to arise from the contribution of the loaded MWCNT. Owing to the higher current in the voltammograms of the MWCNT/POAP electrode than a pure POAP electrode, a larger capacitance for capacitors equipped with MWCNT/POAP electrode can be anticipated. It is interesting to see that the supercapacitor made POAP/MWCNT show a wide-open rectangular shaped voltammogram with a charge density of 450 F g−1 . The specific capacitance, SC, can be obtained from the following equation for CV measurements: SC =
2i sm
(1)
where i denotes the average cathodic current, s is the applied potential sweep rate, and m is the mass of each active material on the electrode. To illustrate the performance of the resulting electrodes in supercapacitors, constant current discharge was performed to measure the capacitance in a 0.5 M HClO4 solution. Typical galvanostatic discharge curves at a current of 0.1 mA between 0 and 0.8 V are shown in Fig. 5. It can be clearly seen that the MWCNT/POAP electrode cell presents higher capacitance than the pure POAP and this
Fig. 3. FT-IR spectrum of (a) ortho-aminophenol, (b) POAP, and (c) POAP/MWCNTs.
Fig. 4. Comparative cyclic voltammograms for POAP and POAP/MWCNT electrodes with a potential sweep rate of 100 mV s−1 between −0.2 and 0.85 V in acidic solution.
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Fig. 5. Galvanostatic discharge curves of the POAP film at 0.1 mA, (1) in presence of MWCNT, (2) in absence of MWCNT.
has been considered as the contribution of faradic pseudo capacitance of MWCNT/POAP supercapacitor. In the case of electrochemical system, EIS can reveal information regarding processes occurring in the polymer matrix when it is doped. This may include kinetic values of the doping process and parameters of the diffusion of ions into the polymers [26]. EIS was analyzed for POAP films in two different synthesis conditions in acidic solution of HClO4 and LiClO4 . Fig. 6 shows the Nyquist diagrams of electrodes in open circuit potential (OCP). The plot in Fig. 6 depicts a single semi-circle in the high frequency region and a straight line in the low-frequency region for all spectra. The highfrequency arc is the overall contact impedance generated from the electrical connection between MWCNT/POAP composites and the backing plate as well as the charge transfer at the contact interface between the electrode and the electrolyte solution. In spite of the similar shape of the impedance spectra, there is an obvious difference between the diameters of the four semi-circles. That is, the diameters of the semicircles decline greatly with the increasing doping content of MWCNTs in POAP. In other words, the bulk-film transport of electrons and the charge transfer resistance (Rct ) of MWCNT/POAP composite films are much lower than that of the pure POAP films. This means that the MWCNTs inside the POAP matrix may lead to a faster electron transport in the bulk-film and charge transfer in the parallel POAP film/solution interface and MWCNTs/solution interface, compared to that in the originally single POAP film/solution interface. In conducting polymer/carbon
Fig. 6. Nyquist diagrams of POAP films in OCP in acidic solution in two magnification, (1) POAP, (2) POAP/MWCNT film.
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Fig. 7. C2 vs E dependences for the POAP film with (1) presence MWCNT, (2) absence MWCNT.
nanotubes (CP/CNTs) composites, it is suggested that either the polymer functionalizes the CNTs [27], or the CPs are doped with CNTs, i.e., a charge transfer occurs between the two constituents. This fact may suggest that the MWCNT has an obvious improvement effect, which makes the composites have more active sites for faradic reactions and a larger specific capacitance than pure POAP. Also, this result in enhanced electric conductivity, lowers the resistance, and facilitates the charge-transfer of the composites. According to the our previous works [17,18], the behavior of space charge capacitance vs E (potential) dependence and the observed capacitance values were typical of a thin film semiconductor electrode and suggest the formation of space charge or depletion region within the polymer film. This conclusion is consistent with numerous result of other author who demonstrated the formation of a space charge region (SCR) on conducting polymer modified electrodes. At the negative potentials, the space charge region extends over whole film. As the potential drops across the SCR decrease, its thickness becomes smaller than the film thickness, and a quasi-neutral region is formed and the electrode capacitance increases. The dependence of the electrode capacitance on the potential in the simplest case is given by the Mott–Schottky equation [17]: C −2 =
2 εε0 eNA2
E − Efb −
KT e
(2)
where C is the space charge capacitance, ε is the dielectric constant of the polymer, ε0 is the permittivity of free space, e is the elementary charge, k is Boltzmann’s constant, T is the absolute temperature, N is the carrier density that set up the space charge, (E − Efb ) is the absolute value of the potential drop across the SCR. The flat band potential Efb is the potential at which the thickness of the SCR is zero. From Eq. (2) it follow that C−2 vs E line are known as Mott–Schottky plots. Fig. 7 presents the C−2 vs E dependences for the POAP film in the presence and absence of MWCNTs, that obtained from Nyquist plots of films in different offset potential. One can see that each curve has a linear portion that can be described by the Mott–Schottky law to a first approximation. Extrapolating these linear portions, we can estimate the values 0.21 and 0.23 V for the flat band potential of film in presence and absence of CNTs respectively. The values of the carrier density in two cases can be also obtained 8.98 × 1019 and 2.28 × 1019 /cm3 for the film in the presence and absence of CNTs respectively from the slop of the plots. The higher density of carriers in the presence of CNTs than pure POAP film results in doping of CNTs into the polymer film. Furthermore, the negative slopes of Mott–Schottky plots of them show that we can categorize them as p-type semiconductors that space charge is established by counter anions. In this case, polarons or bipolarons act as holes in ordinary semiconductors.
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Fig. 8. Cyclic voltammograms of Ni–MWNT–POAP/GC and Ni–POAP/GC electrode in 0.1 M NaOH solution in the presence of 3 mM methanol at a potential sweep rate of 10 mV s−1 .
Fig. 8 shows cyclic voltammograms of Ni–MWCNT–POAP/GC electrode in 0.1 M NaOH solution in the presence of 0.1 M methanol at a potential sweep rate of 10 mV s−1 . The larger methanol response at the Ni–MWCNT–POAP/GC in respect to Ni–POAP/GC electrode is proposed to be the Ni–MWCNT–POAP/GC enhances the catalytic properties of nickel oxide through fine dispersion of the catalyst particles into the conductive polymer matrix to result in a drastic increase in surface area. Fig. 9 shows cyclic voltammograms of Ni–MWCNT–POAP/GC electrode in 0.1 M NaOH solution in the presence of various concentrations of methanol at a potential sweep rate of 10 mV s−1 . At Ni–MWCNT–POAP/GC electrode, oxidation of methanol appeared as a typical electrocatalytic response in alkaline media by Ni(OH)2 /NiOOH [28]. The anodic electrochemical reactions can be summarized as follows [18]:
Fig. 9. Cyclic voltammograms of the Ni–MWNT–POAP/GC electrode in 0.1 M NaOH solution in the presence of 0–8 mM of methanol in the solution. Potential sweep rate was 10 mV s−1 .
NiO–OH + OH− → NiO2 + H2 O + e−
(5)
Whereas the reduction reaction observed can be simplified as: NiO2 + 2H2 O + 2e− → Ni(OH)2 + 2OH−
(6)
Ni2+ + 2OH− → Ni(OH)2
(3)
Eqs. (4)–(6) suggest that the product obtained was a mixture of NiO2 and NiO–OH. Methanol is oxidized on the modified surface according to following reaction:
Ni(OH)2 + OH− → NiO–OH + H2 O + e−
(4)
Methanol + NiIII O–OH → NiII (OH)2 + product
(7)
Fig. 10. Nyquist diagrams of Ni–MWNT–POAP/GC electrode in the presence 0.1 (1), 0.2 (2), 0.4 (3), 0.8 mM (4), methanol in 0.1 M NaOH solution and corresponding fitted curves. DC potential is 650 mV. (5) and (6) are Nyquist diagram in 700 mV/Ag, AgCl.
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The anodic current in the positive sweep was proportional to the bulk concentration of methanol and any increase in the concentration of methanol caused an almost proportional linear enhancement of the anodic current. So, catalytic electrooxidation of methanol on Ni–MWCNT–POAP/GC seems to be certain. Fig. 10 presents the Nyquist diagrams of POAP/MWCNT–NiO electrodes recorded at 650 mV/Ag, AgCl dc-offset both in the presence of methanol in 0.1 M NaOH solution. In the methanol’s concentration range of 0.1–0.8 M a steady decrease of the diameter of the semi-circle is witnessed. At further higher concentrations the Nyquist plot extended to the fourth quadrate in the low frequencies and an inductive loop appeared. The equivalent circuits compatible with the results are presented for both the low (inset a) and high (inset b) limits of methanol concentrations. In these circuits, Rs , CPE and Rct represent solution resistance, a constant phase element corresponding to the double layer capacitance and the charge transfer resistance associated with the oxidation of methanol. RL and L signify the characteristics of the inductive element representing the adsorption of the intermediates at higher methanol concentrations. However at potential higher than 650 mV/Ag–AgCl, the diagrams represented negative differential resistance and rolled over the real axis at low frequencies and went to the second quadrant. The diagram crossed the negative real axis at some finite frequencies indicating a dynamic instability of the stationary state. This negative impedance is characteristic of systems capable of exhibiting galvanostatic potential oscillation. 4. Conclusions We have demonstrated a simple and general strategy, namely in situ electropolymerization by using the ionic surfactant as electrolyte, for dispersing disordered CNTs within conducting polymer/carbon nanotubes composite films. Surfactant SDS has played a key role in the synthesis of CP/CNT composite films. We have introduced the MWCNT/POAP composite electrode to improve the specific capacitance and power characteristic of electrochemical capacitance. The MWCNT has an obvious improvement effect, which makes the composites have more active sites for faradic reaction and larger specific capacitance than pure POAP. Also, it results in enhanced electric conductivity, lower the resistance, and facilitate the charge-transfer of the composites. In comparison with a Ni–POAP/GC, a Ni–MWCNT–POAP/GC electrode shows a better catalytic performance for the electrocatalytic oxidation of methanol.
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