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Electrosynthesis of poly ortho aminophenol films and nanoparticles: A comparative study
Synthetic Metals 162 (2012) 199–204
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrosynthesis of poly ortho aminophenol films and nanoparticles: A comparative study 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 19 July 2011 Received in revised form 21 November 2011 Accepted 28 November 2011 Available online 19 December 2011 Keywords: Nanoparticle Electrosynthesis Fractal Supercapacitor
a b s t r a c t This paper aims to investigate the relationship between surface morphology of poly ortho aminophenol (POAP) films and synthesis conditions. POAP with different particle sizes was deposited on a glassy carbon (GC) electrode using direct current (DC) and pulse techniques. POAP nanoparticles with good electrochemical stability and high doping degree were obtained by applying ultra short on time current pulse for polymerization. The surface morphology of POAP films was revealed by using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge studies showed that the polymer obtained by pulses have higher electrochemical performance in acidic solution. Moreover, nanoparticles of POAP have a highly porous structure, high fractal dimension and large specific surface area, which can be employed as an excellent electrode material in supercapacitors. It was also found that the morphology of POAP varies with the synthesis conditions of the pulse. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymer nanostructure has been a subject of growing interest in recent years for their promising application in microelectronics, sensors, solar cells, etc. [1–4]. As for sensing application, conducting polymer nanostructures including nano-rod, nanoparticle and nano-tube have a larger surface area than their conventional bulk counterparts. Therefore, they have the capability of offering amplified sensitivity and real time response as a result of enhanced interaction among conducting polymer and analyte [5–9]. In recent years, the pulse polymerization method to prepare conducting polymer nanoparticle has attracted more attention [10]. Pulsed polymerization is a novel process, which is known for micro and nano-structuring of the metals and semiconductors [10–12]. Exercising the pulse variables of polymerization, a drastic improvement in conducting polymer capacitance, operating potential and stability for long cycle life is realized. During the time when the current pulse is ON, charges allow polymer chain to nucleate over the substrate surface only for a very short period followed by the off time pulses that do not start fresh nucleation but terminate the growing chains. Relatively longer off time helps the already grown chains to oxidize completely and orient over the surface with the fullest conjugation before the next pulse
∗ Corresponding author. Tel.: +98 21 2285355; 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.2011.11.032
charges and another polymer chain nucleates over it. During the off pulse growth on the initial sites of the electrode is frustrated and hence the growth on the fresh sites of the electrode is more probable leading to a large number of equivalent nucleation and growth sites. The supercapacitor device made by pulse films exhibited very high specific capacitance in comparison to the direct current (DC) polymerized film in aqueous electrolyte. In recent years some kinds of conducting polymer nanostructure, such as polypyrrole and polyaniline, have been successfully prepared using this method [10]. Aminophenols are remarkable members of the class of substituted anilines. The hydroxyl group in the phenyl ring can be oxidized to quinine that can be reduced again. POAP gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerized in acidic solution [13]. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7. Peak current increase and peak potential are displaced to more negative potentials as the pH decreases. The variety of finding for conductivity of the POAP film reported in our previous works [14–16] show that the electrochemical response of POAP is strongly influenced by the experimental procedure used to produce the polymer film. The fractal geometry of conducting polymers demonstrates the growing pattern of the deposited films which depend on the experimental condition. The degree of surface disorder may also be revealed by the fractal approach [17]. Electron micrographs of conducting polymers can also provide useful information on the
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Fig. 1. The surface morphology of (a); pulsed polymerized POAP by applying a very short on current pulse for 5 ms, (b); DC polymerized POAP film and (c); TEM image of pulsed polymerized POAP.
structural behavior that depends on the technique used for preparation. However, it is difficult to extract quantitative information from the electron micrograph [18]. In the present work, the POAP particles with different sizes were synthesized by DC and pulse polymerization methods. The effect of synthesis condition was investigated by using fractal dimension and electrochemical techniques, viz. cyclic voltammetry and electrochemical impedance spectroscopy. 2. Experimental The chemicals used in this work were of Merck origin and used without further purification. Electrochemical synthesis of polymer was carried out in a conventional three-electrode cell powered by a -Autolab potentiostat/galvanostat, type III (The Netherlands). Electrochemical measurements were carried out in a conventional three electrodes cell, powered by a potentiostat/galvanostat (EG&G273A) and a frequency response analyzer (EG&G, 1025). The system was run by a PC through M270 and M398 software via a GPIB interface. The frequency range of 100 kHz to 100 mHz and modulation amplitude of 5 mV were employed for impedance studies. POAP films electrodeposited on a GC disk electrode of 2 mm diameter were employed as working electrode. Saturated calomel electrode (SCE) and a platinum wire were used as reference and counter electrodes, respectively. Polymerization was carried out at constant current density (1 mA/cm2 ) and pulses for a very short period of time. Pulse on time was varied from 5 ms to 200 s and pulse off time was chosen as constant 100 ms. No current was applied during off period. Polymerization charge was constant in all the growth experiments by keeping the total growth time as 200 s with applied current density of 1 mA/cm2 . Total numbers of on pulses were set to complete a growth time equivalent of 200 s. Polymerized electrodes were characterized by cyclic voltammetry, impedance spectroscopy and charge discharge cycles in 0.5 M HClO4 . 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.
effect on the polymer structure and the chain length is obviously short in comparison to DC electrochemical or chemical methods of polymerization. Moreover the short polymer chains are the ideal structures for supercapacitor electrode due to their improved hydrophilic character [19–23]. The improvement in the electrochemical performance of the pulsed polymerized POAP over DC is clearly seen in the cyclic voltammetry curves. Fig. 2 displays the FT-IR spectra of pulse electropolymerized (a) and DC electropolymerized POAP (b). The spectrum of POAP shows two peaks at 3380 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 that the o-aminophenol changed into POAP. The spectrums of POAP are similar in the main peaks although with different intensity. These results can support different morphology for POAP. Fig. 3 shows a comparative study of the cyclic voltammograms for DC and pulsed POAP electrodes with a potential sweep rate of 100 mV s−1 between −0.2 and 0.85 V in acidic solution. As seen, the voltammograms show two well-defined peaks, the corresponding peak currents of which increased upon decreasing size of the polymer particle that obtained in pulse method. The voltammetric behavior of both films is similar and the CV curves show capacitivelike responses that are approximately rectangular in shape. It is remarkable that the growth charge for both polymer films are the same whereas the voltammetric charge in the blank solution for the pulsed polymerized is high. Essentially the pulsed polymerized POAP electrode show higher charge storage than the DC
3. Results and discussion The scanning and transmission electron microscopy images of pulsed polymerized POAP are shown in Fig. 1a and c, respectively. The polymer was prepared by applying a very short on- current pulse for 5 ms that represents the nano particle kind structure with an average size of 10–30 nm. The polymer exhibited highly porous morphology with distinct structures of uniform size and larger surface area in comparison with DC polymerized film (1b). The film morphology in both cases was porous whereas the pulse polymerized film showed more ordered growth of POAP. The application of ultra short current pulse for polymerization has definite
Fig. 2. FT-IR spectrum of pulsed (a) and DC (b) polymerized POAP.
A. Ehsani et al. / Synthetic Metals 162 (2012) 199–204
Fig. 3. Comparative cyclic voltammograms for DC and pulsed POAP electrodes with a potential sweep rate of 100 mV s−1 between −0.2 and 0.85 V in acidic solution.
polymerized as expected to store high electrochemical energy and higher pseudo-capacitance. The specific capacitance, SC, can be obtained from the following equation for CV measurements [24]: 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. Electrode weighting, before and after the deposition process, yields the amount of the active material onto the plates. It is interesting to see that the supercapacitor made by pulse polymerized POAP shows a wide open rectangular shaped voltammogram with a charge density of 380 F/g .Whereas the DC POAP film shows nearly rectangular voltammetric shape with maximum specific capacitance of 130 F/g. The increase in the current due to increased scan rates show the linear variation in capacitive currents of pulsed and DC, POAP electrodes as a function of cyclic voltammetric scan rates. Interestingly, the shape of CV curve even at high scan rate (200 mV s−1 ) is rectangular and the capacitive current of POAP film is unexpectedly high. The higher slope of the linear trend of CV currents from pulsed POAP film is resulted by the ordered and improved POAP structures. It is known that the polymer film deposited in DC mode is supposed to have long interconnected chains that affect both the hydrophilic character of the material and the electronic conduction through irregular and undefined polymer structure. Improved performance of POAP is attributed to the presence of a lesser density of defects such as the carbonyl group in polymer structure during polymerization by pulsed technique. The formation of carbonyl in the polymer matrix has been ascribed to the attack of water molecules due to the high reactivity of the polymer radical. The nucleophilic attack by water produces hydroxyl bonding that can be further oxidized to carbonyl bonds [25]. This process can be minimized by ultra short on time pulse polymerization which consequently reduces the density of defects [26]. It has been shown that the good electrochemical reversibility and stability of POAP may be linked to the intrinsic structure of conducting polymer prepared under different conditions. The results show that disorder effects due to interchain links rise with increasing pulse on time (polymerization rate). This observation is acceptable because at a larger pulse on time the polymerization rate is higher which results in a higher reactivity of the radicals, probably resulting in a higher density of interchain links, and the side chains distributed within the polymer result in a decrease in the specific capacitance due to increased polymer resistance [26,27].
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The results of pulse on time effect on the capacitive character of the POAP film show that the film polymerized for <50 ms pulse on time has high capacitance and a higher energy density. Increasing the pulse on time >50 ms, capacitance decreases due to increased defect density of the polymer film at higher polymerization rates. The shorter POAP chain with reduced defect density has an important effect on the polymer stability in longer cycle life. The stability of pulsed POAP electrode was examined by means of a cycle life test performed for many of cycles. A POAP supercapacitor in 0.5 M HClO4 was subjected to 200 charge/discharge cycles at a current density of 1 mA/cm2 . Nearly 10% decay in specific capacity was observed in first 50 cycles that is generally seen in supercapacitors devices. The long charge/discharge cycling test shows the high stability of pulsed POAP electrode whereas the DC polymerized electrode could not sustain the first 50 charge discharge cycles at 1 mA/cm2 . Fig. 4 illustrates the performance of the POAP supercapacitor under constant current charge and discharge. In order to compare these supercapacitors at the same condition, all of the tested supercapacitors were charged to 0.3 V, since it is the maximum potential of doping and dedoping for POAP. As expected, in the two cases, POAP film has a typical linear decay of voltage with time increasing. It shows that the capacitances of the pulse supercapacitors materials are much higher than that of DC supercapacitor. The surface morphology of POAP film was studied by using the fractal concept. It has been reported [28,29] that for a diffusion controlled redox transition, which occurred via diffusion of an electroreactant species to a target surface, followed by heterogeneous electron transfer, there is a power dependence between the peak current (Ipc ) in cyclic voltammograms and the corresponding potential sweep rate (): Ipc = F ˛
(2)
where ˛ is the fractal parameter and F is a proportionality factor. Thus, the fractal parameter can be obtained easily by plotting the peak current against the sweep rate in log–log scale. On the other hand, the fractal parameter is related to the fractal dimension (Df ) of the electrode surface as [30]: ˛=
Df − 1 2
(3)
Eq. (3) is applicable to electrochemical methods, as it has been successfully used for calculating the fractal dimension of electrode surface. Based on this information, Figs. 5 and 6 represent cyclic voltammograms of POAP films that were recorded in different potential sweep rates in the range of 100–500 mV s−1 . Inset in Figs. 5 and 6 represents the relationship between the anodic peak current and potential sweep rate in log–log scale, obtained for the POAP films with different synthesis condition. The slope of the lines gives the value for each film. Substituting the value in Eq. (3), gives the fractal dimensions of 2.72 and 2.88 for POAP films synthesis by DC and pulse, respectively. Fig. 7 represents cyclic voltammograms for POAP film in solution of 0.2 M NaNO3 containing 0.002 M [Fe(CN)6 ]3− recorded in potential sweep rate 100 mV s−1 . Here the electrochemically reversible redox couple ferri/ferrocyanide was used as a suitable probe to examine the active surface area of the films. For analytical purpose, it was assumed that the diffusion of oxidized, [Fe(CN)6 ]3− , and reduced species, [Fe(CN)6 ]4− , in the electrolyte is semi-infinite and one dimensional. The solution initially contains only oxidized species, the bulk concentration of these species being constantly away from the electrode, while the corresponding concentration of reduced species is zero. The supporting electrolyte, NaNO3 , is assumed to present at a sufficient concentration so that the contribution of the redox couple to migration can be neglected. Since the
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Fig. 4. Galvanostatic charge–discharge curves of the POAP film at 1 mA, for pulse and DC polymerization.
POAP synthesized in acidic solution is electroactive when the pH of the medium is less than 4 and is electroinactive when the pH is neutral or basic, under the experimental conditions used, ipeak was background corrected so that the only reversible reaction occurring with the redox couple is [Fe(CN)6 ]3− + e− [Fe(CN)6 ]4− The high peak current of this redox in the POAP film synthesized by pulse method can be related to high surface area of the film and size of the polymer particle. Electrochemical impedance spectroscopy (EIS) was analyzed for POAP films in two different synthesis conditions in acidic solution. Fig. 8 shows the Nyquist diagrams of electrodes in OCP. A single semi-circle in the high frequency region and a straight line in the low frequency region can be observed for spectra. As seen, the imaginary part of both plots at low frequency region is almost perpendicular to the real part of resistance proving a mainly pure capacitive behavior of the system. Inset in Fig. 8, shows the
Fig. 5. Cyclic voltammograms of POAP films was obtained in pulse polymerization were recorded in different potential sweep rates in the range of 100–500 mV s−1 . Inset represents the relationship between the anodic peak current and potential sweep rate in log–log scale, obtained for the POAP film.
extended high frequency regions for both plots. It could be found that, at the same conditions, the pulse based supercapacitor possesses a lower charge transfer resistance and a better conductivity, which is an important factor in the fast redox systems, especially for supercapacitors. In other words, the bulk-film transport of electrons and the charge transfer resistance (Rct ) of pulse POAP films are lower than that of the DC synthesized POAP films. This means that the film with nanoparticle may lead to a faster electron transport in the film and charge transfer in the parallel POAP/solution interface. This fact may suggest that pulse POAP film has an obvious improvement effect which makes the films have more active sites for faradic reaction and a higher specific capacitance than DC, POAP films. Consequently film resistance is lowered which facilitates chargetransfer in the films. The experimental results were fitted using an appropriate equivalent circuit [31] depicted as an inset in Fig. 8 and the electrical parameters found from fitting were represented in Table 1. As it is shown, the circuit consists of a parallel combination
Fig. 6. Cyclic voltammograms of POAP films was obtained in DC polymerization were recorded in different potential sweep rates in the range of 100–500 mV s−1 . Inset represents the relationship between the anodic peak current and potential sweep rate in log–log scale, obtained for the POAP film.
A. Ehsani et al. / Synthetic Metals 162 (2012) 199–204
203
Fig. 9. C−2 vs. E dependences for the POAP film with (1) pulse and (2) DC.
Fig. 7. Cyclic voltammograms for POAP film in solution of 0.2 M NaNO3 containing 0.002 M [Fe(CN)6 ]3− , were recorded in potential sweep rate 100 mV s−1 .
electrodes. At the negative potentials, the space charge region extends over the whole film. As the potential drops across the SCR decrease, their 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 [16]: C −2 =
Fig. 8. Nyquist diagrams of POAP films (DC and pulse polymerization) obtained in OCP in acidic solution. Insets show the extended high frequency regions for both plots and the equivalent circuit used to fit the experimental data, respectively.
of a charge transfer resistance with a double layer capacitance in series with a Faradic capacitance, in addition to the solution resistance at high frequency extreme. The shape of the Nyquist plots can be considered as an indication of the porous structure behavior, in good agreement with the SEM characterization. According to our previous works [14,16] 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 results of other authors who demonstrated the formation of a space charge region (SCR) on conducting polymer modified Table 1 Electrical parameters for the POAP film obtained at OCP.
Pulse DC
2 εε0 eNA2
E − Efb −
KT e
(4)
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. (4) it follows that C−2 vs. E line are known as Mott–Schottky plots. Fig. 9 present the C−2 vs. E dependences for the POAP film with pulse and DC 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 0.22 and 0.19 V for the flat band potential of film in pulse and DC, respectively. The values of the carrier density in two cases can be also obtained 7.65 × 1019 and 1.86 × 1019 /cm3 for the film in pulse and DC respectively from the slop of the plots. 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 [14].
4. Conclusion POAP films with different morphology prepared by the DC and the pulse methods, and the corresponding capacitance performance of POAP with different morphology were investigated. The results show that, the nanoparticles, which were obtained by pulse method, have better capacitance performance. This is mainly due to large surface area, higher fractal dimension and better electronic and ionic conductivity of POAP nanoparticles, which lead to greater double-layer capacitance and faradic pseudo capacitance.
Acknowledgement
Rs /
Rct /
T0 × 105 /F
n1
T0 × 105 /F
n2
18.33 40
64.82 277
57 49
0.59 0.53
28 12
0.88 0.84
The authors would like to express their deep gratitude to the Iranian Nano Council for supporting this work.
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Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrosynthesis of poly ortho aminophenol films and nanoparticles: A comparative study 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 19 July 2011 Received in revised form 21 November 2011 Accepted 28 November 2011 Available online 19 December 2011 Keywords: Nanoparticle Electrosynthesis Fractal Supercapacitor
a b s t r a c t This paper aims to investigate the relationship between surface morphology of poly ortho aminophenol (POAP) films and synthesis conditions. POAP with different particle sizes was deposited on a glassy carbon (GC) electrode using direct current (DC) and pulse techniques. POAP nanoparticles with good electrochemical stability and high doping degree were obtained by applying ultra short on time current pulse for polymerization. The surface morphology of POAP films was revealed by using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge studies showed that the polymer obtained by pulses have higher electrochemical performance in acidic solution. Moreover, nanoparticles of POAP have a highly porous structure, high fractal dimension and large specific surface area, which can be employed as an excellent electrode material in supercapacitors. It was also found that the morphology of POAP varies with the synthesis conditions of the pulse. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymer nanostructure has been a subject of growing interest in recent years for their promising application in microelectronics, sensors, solar cells, etc. [1–4]. As for sensing application, conducting polymer nanostructures including nano-rod, nanoparticle and nano-tube have a larger surface area than their conventional bulk counterparts. Therefore, they have the capability of offering amplified sensitivity and real time response as a result of enhanced interaction among conducting polymer and analyte [5–9]. In recent years, the pulse polymerization method to prepare conducting polymer nanoparticle has attracted more attention [10]. Pulsed polymerization is a novel process, which is known for micro and nano-structuring of the metals and semiconductors [10–12]. Exercising the pulse variables of polymerization, a drastic improvement in conducting polymer capacitance, operating potential and stability for long cycle life is realized. During the time when the current pulse is ON, charges allow polymer chain to nucleate over the substrate surface only for a very short period followed by the off time pulses that do not start fresh nucleation but terminate the growing chains. Relatively longer off time helps the already grown chains to oxidize completely and orient over the surface with the fullest conjugation before the next pulse
∗ Corresponding author. Tel.: +98 21 2285355; 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.2011.11.032
charges and another polymer chain nucleates over it. During the off pulse growth on the initial sites of the electrode is frustrated and hence the growth on the fresh sites of the electrode is more probable leading to a large number of equivalent nucleation and growth sites. The supercapacitor device made by pulse films exhibited very high specific capacitance in comparison to the direct current (DC) polymerized film in aqueous electrolyte. In recent years some kinds of conducting polymer nanostructure, such as polypyrrole and polyaniline, have been successfully prepared using this method [10]. Aminophenols are remarkable members of the class of substituted anilines. The hydroxyl group in the phenyl ring can be oxidized to quinine that can be reduced again. POAP gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerized in acidic solution [13]. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7. Peak current increase and peak potential are displaced to more negative potentials as the pH decreases. The variety of finding for conductivity of the POAP film reported in our previous works [14–16] show that the electrochemical response of POAP is strongly influenced by the experimental procedure used to produce the polymer film. The fractal geometry of conducting polymers demonstrates the growing pattern of the deposited films which depend on the experimental condition. The degree of surface disorder may also be revealed by the fractal approach [17]. Electron micrographs of conducting polymers can also provide useful information on the
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A. Ehsani et al. / Synthetic Metals 162 (2012) 199–204
Fig. 1. The surface morphology of (a); pulsed polymerized POAP by applying a very short on current pulse for 5 ms, (b); DC polymerized POAP film and (c); TEM image of pulsed polymerized POAP.
structural behavior that depends on the technique used for preparation. However, it is difficult to extract quantitative information from the electron micrograph [18]. In the present work, the POAP particles with different sizes were synthesized by DC and pulse polymerization methods. The effect of synthesis condition was investigated by using fractal dimension and electrochemical techniques, viz. cyclic voltammetry and electrochemical impedance spectroscopy. 2. Experimental The chemicals used in this work were of Merck origin and used without further purification. Electrochemical synthesis of polymer was carried out in a conventional three-electrode cell powered by a -Autolab potentiostat/galvanostat, type III (The Netherlands). Electrochemical measurements were carried out in a conventional three electrodes cell, powered by a potentiostat/galvanostat (EG&G273A) and a frequency response analyzer (EG&G, 1025). The system was run by a PC through M270 and M398 software via a GPIB interface. The frequency range of 100 kHz to 100 mHz and modulation amplitude of 5 mV were employed for impedance studies. POAP films electrodeposited on a GC disk electrode of 2 mm diameter were employed as working electrode. Saturated calomel electrode (SCE) and a platinum wire were used as reference and counter electrodes, respectively. Polymerization was carried out at constant current density (1 mA/cm2 ) and pulses for a very short period of time. Pulse on time was varied from 5 ms to 200 s and pulse off time was chosen as constant 100 ms. No current was applied during off period. Polymerization charge was constant in all the growth experiments by keeping the total growth time as 200 s with applied current density of 1 mA/cm2 . Total numbers of on pulses were set to complete a growth time equivalent of 200 s. Polymerized electrodes were characterized by cyclic voltammetry, impedance spectroscopy and charge discharge cycles in 0.5 M HClO4 . 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.
effect on the polymer structure and the chain length is obviously short in comparison to DC electrochemical or chemical methods of polymerization. Moreover the short polymer chains are the ideal structures for supercapacitor electrode due to their improved hydrophilic character [19–23]. The improvement in the electrochemical performance of the pulsed polymerized POAP over DC is clearly seen in the cyclic voltammetry curves. Fig. 2 displays the FT-IR spectra of pulse electropolymerized (a) and DC electropolymerized POAP (b). The spectrum of POAP shows two peaks at 3380 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 that the o-aminophenol changed into POAP. The spectrums of POAP are similar in the main peaks although with different intensity. These results can support different morphology for POAP. Fig. 3 shows a comparative study of the cyclic voltammograms for DC and pulsed POAP electrodes with a potential sweep rate of 100 mV s−1 between −0.2 and 0.85 V in acidic solution. As seen, the voltammograms show two well-defined peaks, the corresponding peak currents of which increased upon decreasing size of the polymer particle that obtained in pulse method. The voltammetric behavior of both films is similar and the CV curves show capacitivelike responses that are approximately rectangular in shape. It is remarkable that the growth charge for both polymer films are the same whereas the voltammetric charge in the blank solution for the pulsed polymerized is high. Essentially the pulsed polymerized POAP electrode show higher charge storage than the DC
3. Results and discussion The scanning and transmission electron microscopy images of pulsed polymerized POAP are shown in Fig. 1a and c, respectively. The polymer was prepared by applying a very short on- current pulse for 5 ms that represents the nano particle kind structure with an average size of 10–30 nm. The polymer exhibited highly porous morphology with distinct structures of uniform size and larger surface area in comparison with DC polymerized film (1b). The film morphology in both cases was porous whereas the pulse polymerized film showed more ordered growth of POAP. The application of ultra short current pulse for polymerization has definite
Fig. 2. FT-IR spectrum of pulsed (a) and DC (b) polymerized POAP.
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Fig. 3. Comparative cyclic voltammograms for DC and pulsed POAP electrodes with a potential sweep rate of 100 mV s−1 between −0.2 and 0.85 V in acidic solution.
polymerized as expected to store high electrochemical energy and higher pseudo-capacitance. The specific capacitance, SC, can be obtained from the following equation for CV measurements [24]: 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. Electrode weighting, before and after the deposition process, yields the amount of the active material onto the plates. It is interesting to see that the supercapacitor made by pulse polymerized POAP shows a wide open rectangular shaped voltammogram with a charge density of 380 F/g .Whereas the DC POAP film shows nearly rectangular voltammetric shape with maximum specific capacitance of 130 F/g. The increase in the current due to increased scan rates show the linear variation in capacitive currents of pulsed and DC, POAP electrodes as a function of cyclic voltammetric scan rates. Interestingly, the shape of CV curve even at high scan rate (200 mV s−1 ) is rectangular and the capacitive current of POAP film is unexpectedly high. The higher slope of the linear trend of CV currents from pulsed POAP film is resulted by the ordered and improved POAP structures. It is known that the polymer film deposited in DC mode is supposed to have long interconnected chains that affect both the hydrophilic character of the material and the electronic conduction through irregular and undefined polymer structure. Improved performance of POAP is attributed to the presence of a lesser density of defects such as the carbonyl group in polymer structure during polymerization by pulsed technique. The formation of carbonyl in the polymer matrix has been ascribed to the attack of water molecules due to the high reactivity of the polymer radical. The nucleophilic attack by water produces hydroxyl bonding that can be further oxidized to carbonyl bonds [25]. This process can be minimized by ultra short on time pulse polymerization which consequently reduces the density of defects [26]. It has been shown that the good electrochemical reversibility and stability of POAP may be linked to the intrinsic structure of conducting polymer prepared under different conditions. The results show that disorder effects due to interchain links rise with increasing pulse on time (polymerization rate). This observation is acceptable because at a larger pulse on time the polymerization rate is higher which results in a higher reactivity of the radicals, probably resulting in a higher density of interchain links, and the side chains distributed within the polymer result in a decrease in the specific capacitance due to increased polymer resistance [26,27].
201
The results of pulse on time effect on the capacitive character of the POAP film show that the film polymerized for <50 ms pulse on time has high capacitance and a higher energy density. Increasing the pulse on time >50 ms, capacitance decreases due to increased defect density of the polymer film at higher polymerization rates. The shorter POAP chain with reduced defect density has an important effect on the polymer stability in longer cycle life. The stability of pulsed POAP electrode was examined by means of a cycle life test performed for many of cycles. A POAP supercapacitor in 0.5 M HClO4 was subjected to 200 charge/discharge cycles at a current density of 1 mA/cm2 . Nearly 10% decay in specific capacity was observed in first 50 cycles that is generally seen in supercapacitors devices. The long charge/discharge cycling test shows the high stability of pulsed POAP electrode whereas the DC polymerized electrode could not sustain the first 50 charge discharge cycles at 1 mA/cm2 . Fig. 4 illustrates the performance of the POAP supercapacitor under constant current charge and discharge. In order to compare these supercapacitors at the same condition, all of the tested supercapacitors were charged to 0.3 V, since it is the maximum potential of doping and dedoping for POAP. As expected, in the two cases, POAP film has a typical linear decay of voltage with time increasing. It shows that the capacitances of the pulse supercapacitors materials are much higher than that of DC supercapacitor. The surface morphology of POAP film was studied by using the fractal concept. It has been reported [28,29] that for a diffusion controlled redox transition, which occurred via diffusion of an electroreactant species to a target surface, followed by heterogeneous electron transfer, there is a power dependence between the peak current (Ipc ) in cyclic voltammograms and the corresponding potential sweep rate (): Ipc = F ˛
(2)
where ˛ is the fractal parameter and F is a proportionality factor. Thus, the fractal parameter can be obtained easily by plotting the peak current against the sweep rate in log–log scale. On the other hand, the fractal parameter is related to the fractal dimension (Df ) of the electrode surface as [30]: ˛=
Df − 1 2
(3)
Eq. (3) is applicable to electrochemical methods, as it has been successfully used for calculating the fractal dimension of electrode surface. Based on this information, Figs. 5 and 6 represent cyclic voltammograms of POAP films that were recorded in different potential sweep rates in the range of 100–500 mV s−1 . Inset in Figs. 5 and 6 represents the relationship between the anodic peak current and potential sweep rate in log–log scale, obtained for the POAP films with different synthesis condition. The slope of the lines gives the value for each film. Substituting the value in Eq. (3), gives the fractal dimensions of 2.72 and 2.88 for POAP films synthesis by DC and pulse, respectively. Fig. 7 represents cyclic voltammograms for POAP film in solution of 0.2 M NaNO3 containing 0.002 M [Fe(CN)6 ]3− recorded in potential sweep rate 100 mV s−1 . Here the electrochemically reversible redox couple ferri/ferrocyanide was used as a suitable probe to examine the active surface area of the films. For analytical purpose, it was assumed that the diffusion of oxidized, [Fe(CN)6 ]3− , and reduced species, [Fe(CN)6 ]4− , in the electrolyte is semi-infinite and one dimensional. The solution initially contains only oxidized species, the bulk concentration of these species being constantly away from the electrode, while the corresponding concentration of reduced species is zero. The supporting electrolyte, NaNO3 , is assumed to present at a sufficient concentration so that the contribution of the redox couple to migration can be neglected. Since the
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Fig. 4. Galvanostatic charge–discharge curves of the POAP film at 1 mA, for pulse and DC polymerization.
POAP synthesized in acidic solution is electroactive when the pH of the medium is less than 4 and is electroinactive when the pH is neutral or basic, under the experimental conditions used, ipeak was background corrected so that the only reversible reaction occurring with the redox couple is [Fe(CN)6 ]3− + e− [Fe(CN)6 ]4− The high peak current of this redox in the POAP film synthesized by pulse method can be related to high surface area of the film and size of the polymer particle. Electrochemical impedance spectroscopy (EIS) was analyzed for POAP films in two different synthesis conditions in acidic solution. Fig. 8 shows the Nyquist diagrams of electrodes in OCP. A single semi-circle in the high frequency region and a straight line in the low frequency region can be observed for spectra. As seen, the imaginary part of both plots at low frequency region is almost perpendicular to the real part of resistance proving a mainly pure capacitive behavior of the system. Inset in Fig. 8, shows the
Fig. 5. Cyclic voltammograms of POAP films was obtained in pulse polymerization were recorded in different potential sweep rates in the range of 100–500 mV s−1 . Inset represents the relationship between the anodic peak current and potential sweep rate in log–log scale, obtained for the POAP film.
extended high frequency regions for both plots. It could be found that, at the same conditions, the pulse based supercapacitor possesses a lower charge transfer resistance and a better conductivity, which is an important factor in the fast redox systems, especially for supercapacitors. In other words, the bulk-film transport of electrons and the charge transfer resistance (Rct ) of pulse POAP films are lower than that of the DC synthesized POAP films. This means that the film with nanoparticle may lead to a faster electron transport in the film and charge transfer in the parallel POAP/solution interface. This fact may suggest that pulse POAP film has an obvious improvement effect which makes the films have more active sites for faradic reaction and a higher specific capacitance than DC, POAP films. Consequently film resistance is lowered which facilitates chargetransfer in the films. The experimental results were fitted using an appropriate equivalent circuit [31] depicted as an inset in Fig. 8 and the electrical parameters found from fitting were represented in Table 1. As it is shown, the circuit consists of a parallel combination
Fig. 6. Cyclic voltammograms of POAP films was obtained in DC polymerization were recorded in different potential sweep rates in the range of 100–500 mV s−1 . Inset represents the relationship between the anodic peak current and potential sweep rate in log–log scale, obtained for the POAP film.
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203
Fig. 9. C−2 vs. E dependences for the POAP film with (1) pulse and (2) DC.
Fig. 7. Cyclic voltammograms for POAP film in solution of 0.2 M NaNO3 containing 0.002 M [Fe(CN)6 ]3− , were recorded in potential sweep rate 100 mV s−1 .
electrodes. At the negative potentials, the space charge region extends over the whole film. As the potential drops across the SCR decrease, their 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 [16]: C −2 =
Fig. 8. Nyquist diagrams of POAP films (DC and pulse polymerization) obtained in OCP in acidic solution. Insets show the extended high frequency regions for both plots and the equivalent circuit used to fit the experimental data, respectively.
of a charge transfer resistance with a double layer capacitance in series with a Faradic capacitance, in addition to the solution resistance at high frequency extreme. The shape of the Nyquist plots can be considered as an indication of the porous structure behavior, in good agreement with the SEM characterization. According to our previous works [14,16] 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 results of other authors who demonstrated the formation of a space charge region (SCR) on conducting polymer modified Table 1 Electrical parameters for the POAP film obtained at OCP.
Pulse DC
2 εε0 eNA2
E − Efb −
KT e
(4)
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. (4) it follows that C−2 vs. E line are known as Mott–Schottky plots. Fig. 9 present the C−2 vs. E dependences for the POAP film with pulse and DC 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 0.22 and 0.19 V for the flat band potential of film in pulse and DC, respectively. The values of the carrier density in two cases can be also obtained 7.65 × 1019 and 1.86 × 1019 /cm3 for the film in pulse and DC respectively from the slop of the plots. 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 [14].
4. Conclusion POAP films with different morphology prepared by the DC and the pulse methods, and the corresponding capacitance performance of POAP with different morphology were investigated. The results show that, the nanoparticles, which were obtained by pulse method, have better capacitance performance. This is mainly due to large surface area, higher fractal dimension and better electronic and ionic conductivity of POAP nanoparticles, which lead to greater double-layer capacitance and faradic pseudo capacitance.
Acknowledgement
Rs /
Rct /
T0 × 105 /F
n1
T0 × 105 /F
n2
18.33 40
64.82 277
57 49
0.59 0.53
28 12
0.88 0.84
The authors would like to express their deep gratitude to the Iranian Nano Council for supporting this work.
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