J Solid State Electrochem DOI 10.1007/s10008-015-2960-2
ORIGINAL PAPER
Electrochemical supercapacitive properties of polypyrrole thin films: influence of the electropolymerization methods Franciele Wolfart 1 & Deepak P. Dubal 2 & Marcio Vidotti 1 & Rudolf Holze 3 & Pedro Gómez-Romero 2
Received: 2 May 2015 / Revised: 3 July 2015 / Accepted: 7 July 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract A detailed study of the effects of different electropolymerization methods on the supercapacitive properties of polypyrrole (PPy) thin films deposited on carbon cloth is reported. Deposition mechanisms of PPy thin films through cyclic voltammetry (CV), potentiostatic (PS), and galvanostatic (GS) modes have been analyzed. The resulting PPy thin films have been characterized by X-ray photoelectron spectroscopy (XPS), SEM, and TEM. The electrochemical properties of PPy thin films were investigated by cyclic voltammetry and galvanostatic charge/discharge. The results showed that the different electrodeposition modes of synthesis significantly affect the supercapacitive properties of PPy thin films. Among different modes of electrodeposition, PPy synthesized by a potentiostatic mode exhibits maximum specific capacitance of 166 F/g with specific energy of 13 Wh/kg; this is attributed to equivalent proportions of the oxidized and neutral states of PPy. Thus, these results provide a useful orientation for the use of optimized electrodeposition modes for the
Dedicated to José H. Zagal on the occasion of his 65th birthday in appreciation of his contributions to electrocatalysis and to the general development of our electrochemical science in Chile and beyond. * Marcio Vidotti
[email protected] * Pedro Gómez-Romero
[email protected] 1
Grupo de Pesquisa em Macromoléculas e Interfaces, Departamento de Química, Universidade Federal do Paraná, CP 19081, 81531-980 Curitiba, PR, Brazil
2
Institut Català de Nanociència i Nanotecnologia, ICN2 (CSIC-CERCA), Campus UAB, 08193 Bellaterra, Barcelona, Spain
3
Technische Universität Chemnitz, Institut für Chemie, AG Elektrochemie, D-09107 Chemnitz, Germany
growth of PPy thin films to be applied as electrode material in supercapacitors. Keywords Polypyrrole . Electrodeposition . Supercapacitors . Conducting polymers
Introduction Intrinsically conducting polymers have attracted an increasing interest in both academic and industrial communities due to their unique combination of properties leading to many promising applications, such as sensors and biosensors, anticorrosion coatings, supercapacitors and batteries, drug delivery, and artificial muscles [1–7]. Among them, polypyrrole (PPy) is one of the frequently investigated polymers for many of these applications due to its great specific advantages such as relatively easy polymerization, low cost, environmental and thermal stability, high conductivity, and charge storage capability [8]. Recent investigations proved that PPy is a promising electrode material for supercapacitors [9]. A supercapacitor (SC) is an electrical energy storage device in which energy can be stored temporarily either faradaically through surface redox reactions or nonfaradaically through the adsorption of ions at the electrode surface [10]. Contrary to conventional batteries, supercapacitors feature high power densities, fast chargingdischarging, long cycling life, and high reliability [11]. Based on charge storing mechanisms, three different classes of materials, carbon, transition metal oxides, and conducting polymers, can be considered as possible electrodes for supercapacitors [9]. Among the variety of pseudocapacitive materials, PPy is sure-fire suitable candidate for the preparation of supercapacitor electrodes due to its high conductivity and excellent electrochemical properties such as high capacitance and high rate capability [12]. The performance of a
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supercapacitor is largely determined by the electrode materials in terms of their morphology, size, porosity, and electrochemically active surface area. Thus, currently growing interest has concentrated on designing and fabricating electrode materials with controlled microstructures using different synthetic routes. Since PPy was first synthesized in the 1970s, a plethora of articles have been published reporting different synthetic routes and applications. The properties of PPy such as morphology, size, and shape of microstructures and nanostructures, and conductivity strongly depend on the synthetic method. Various methods of synthesis of polypyrrole have been reported in the literature [13–17]. Electropolymerization is a widespread method for the synthesis of PPy including three different modes of polymerization/deposition, namely, cyclic voltammetry (CV), galvanostatic (GS), and potentiostatic (PS) deposition [8]. The PS mode is based on applying a constant potential in order to avoid the occurrence of side reactions at the electrode surface whereas the GS mode is based on a constant current which is independent of the system resistance, thus suitable for deposition of films on large surface areas [18]. In addition, both techniques can be utilized to investigate the nucleation mechanism and growth during polymerization. Apart from this, the potentiodynamic mode (cyclic voltammetry) is commonly used to obtain qualitative information about the redox processes involved in the early stages of the polymerization reaction [19]. Recently, Dubal et al. [11] reported the possibility of using this potentiodynamic mode for the synthesis of different PPy nanostructures for supercapacitor application. Thus, it is clear that using different modes of electrodeposition, the supercapacitive properties of PPy thin films can be manipulated. The focus of the current study is to investigate the influence of different electropolymerization modes on the supercapacitive properties of PPy thin films. First, the electropolymerization of PPy on carbon cloth has been carried out using the different modes introduced above: CV, GS, and PS. The resulting PPy samples were characterized by different physical-chemical techniques in order to study their surface morphology and structure. Moreover, the effect of different modes of electropolymerization on supercapacitive properties of PPy thin films has been investigated by cyclic voltammetry and galvanostatic charge/discharge experiments.
at 0.8 V for 3 min, galvanostatic (GS) mode at 10 mA/cm2 for 5 min, and cyclic voltammetry (CV) mode by scanning between 0 and +0.9 V at 100 mV/s for 12 cycles. A three electrode cell was assembled for the synthesis of PPy which consists of carbon cloth as working electrode, Ag/AgCl as reference electrode, and graphite plate as counter electrode. Surface morphological analyses were carried out by scanning electron microscopy (FEI Quanta 650F Environmental SEM) and transmission electron microscopy (Tecnai G2 F20 S-TWIN HR(S) TEM, FEI). The X-ray photoelectron (XPS)
Experimental The synthesis of polypyrrole has been carried out by electropolymerization. Briefly, the solution for the synthesis of polypyrrole was prepared by mixing 0.1 mol/L of distilled pyrrole and 0.5 mol/L H2SO4 in deionized water (Milli Q, 18 MΩ resistance). The electropolymerization of PPy was carried out by three different modes: potentiostatic (PS) mode
Fig. 1 Synthesis of polypyrrole thin films by different electrodeposition modes: a cyclic voltammetry (CV) mode by scanning between 0 and + 0.9 V for 12 cycles; b galvanostatic (GS) mode at 10 mA/cm2 for 5 min; and c potentiostatic (PS) mode at 0.8 V (vs Ag/AgCl) for 3 min
J Solid State Electrochem Fig. 2 Scheme of the pyrrole polymerization, adapted from [8]
analyses were obtained by X-ray photoelectron spectroscopy ( X P S , S P E C S G e r m a n y, P H O I B O S 1 5 0 ) . T h e supercapacitive performance of PPy thin films was tested in 1 mol/L H2SO4 using cyclic voltammetry (CV) and galvanostatic charge/discharge techniques by assembling three electrode cell comprising a PPy thin film as a working electrode, platinum as a counter electrode, and Ag/AgCl as reference electrode. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 10 kHz to 10 mHz. All electrochemical measurements were carried out with a Bio-Logic VMP3 potentiostat. Fig. 3 XPS spectra of a wide region spectra for all three PPy samples, b N1s for CV/PPy, c N1s for GS/PPy, and d N1s for PS/Ppy, respectively
Results and discussion Figure 1 shows the deposition curves of PPy thin films on carbon cloth through cyclic voltammetry (CV), galvanostatic (GS), and potentiostatic (PS) modes. The nucleation and growth mechanisms of PPy thin films can be explained based on the shape of deposition curves. As seen in Fig. 1a in CV mode, a continuously varied potential is applied, and the resulting current recorded. Upon voltage scanning, different redox reactions of the electroactive species present in solution are made possible at the electrode surface. A sudden increase
J Solid State Electrochem Table 1 Parameters calculated from XPS analysis for polypyrrole samples prepared by different modes of electrodeposition Mode of deposition
Carbon %
Nitrogen %
CV/PPy
89.47
10.53
GS/PPy
95.44
4.56
PS/PPy
84.68
15.32
in current density was typically observed at +0.7 V (vs Ag/ AgCl) (Fig. 1a), which corresponds to the oxidation of pyrrole (Py) monomers into radical cations and the consequent formation of PPy film onto the carbon cloth electrode. The most Fig. 4 Scanning electron micrographs of a, b CV/PPy, c, d GS/PPy, and e, f PS/PPy electrodes at two different magnifications
characteristic feature of this CV mode of PPy deposition is the pause in the deposition between each CV cycle leading to a discontinuous growth and potential sweep of the growing film which is expected to produce uniform PPy microstructures [8, 20]. From the CV mode, we have determined the optimal deposition potential (0.8 V, Ag/AgCl) and current density (10 mA/cm2) for PPy formation which will be further used in PS and GS modes, respectively. Contrary to CV mode, the galvanostatic (GS) mode is associated with the use of a constant current while the change of potential is recorded. Figure 1b shows the deposition curve of PPy thin films at applied constant current density of 10 mA/cm2. In this case, after the formation of the electrochemical double layer, the
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potential starts to decrease which may be attributed to the formation of oligomers onto the electrode with a smaller oxidation potential than the starting monomer. As seen from Fig. 1b, a high initial potential is necessary in order to oxidize monomers into oligomers. Afterwards the constant potential reached is high enough for complete polymerization [8, 20]. Furthermore, Fig. 1c shows the deposition curve of PPy thin films on carbon cloth at a constant potential of 0.8 V (vs Ag/ AgCl) in PS mode. The initial increment in the current density is associated with the formation of double layer at the electrode/solution interface. Then, as it could be expected, the recorded current decreases as the concentration of pyrrole in solution decreases due to the formation of PPy on the electrode (carbon cloth). According to Licona-Sanchez et al., the shape of PPy electrode position curve from PS synthesis (Fig. 1c) can be explained by the superposition of two simultaneous processes that occur at the electrode surface, namely, multiple 3D hemispherical nucleation, and growth, where oxidation of pyrrole takes place on fresh PPy surface [21]. The electropolymerization mechanism of PPy thin films is schematically illustrated in Fig. 2. Briefly, in the first step, the oxidation of pyrrole monomer forms pyrrole radicals at the electrode surface. With highest charge density at the α-position, the coupling between two radicals occurs, resulting in the formation of α,α′-dimers and the loss of two protons driving rearomtaization. These aromatic dimers can further react with new radicals until completion of the polymerization reaction. The aromatic dimer has an unpaired electron delocalized over two pyrrole rings which results in a lower oxidation potential compared with the monomer. This is in good agreement with the observations discussed above for the GS polymerization mode [8]. XPS measurements were carried out in order to investigate the possible oxidation states in the PPy thin films synthesized by different modes of electrodeposition as well as to determine elemental compositions. Figure 3a shows the wide region XPS spectra of CV/PPy, GS/PPy, and PS/PPy. It is seen that all PPy samples show the presence of oxygen (O1s at ~533 eV), nitrogen (N1s at ~400.5 eV), and carbon (C1s at ~284.7 eV) as well as sulfur (S2p at ~162.7 eV), corresponding to typical spectra of sulfate-doped PPy [22, 23]. Carbon and nitrogen are from the polypyrrole backbone whereas sulfur is from the sulfuric acid used in the polymerization process. The presence of O1s may be attributed to the surface oxidation of polypyrrole and/or the presence of sulfate in PPy [1, 22]. It is interesting to note that the different modes of electrodeposition significantly change the proportion of carbon and nitrogen in PPy samples as summarized in Table 1. The PS/PPy sample exhibits considerably higher proportion of nitrogen as compared to GS/PPy and CV/PPy. The deconvolution of N1s peaks in the XPS spectra of all PPy samples gives three components with remarkably different intensities (Fig. 3b–d). The peak at 399.7 eV is attributed to
neutral N in the pyrrole ring (-NH) [22]; it appears in all PPy samples but with different relative intensities. Moreover, the signals at high binding energies associated to the oxidized states of PPy appear in different regions. The peak corresponding to polaron state (-NH·+) is shifted from the peak of neutral N by 0.8, 2.0, and 0.5 eV to higher binding energies for CV/PPy (400.5 eV), GS/PPy (401.6 eV), and PS/PPy (400.2 eV), respectively [22]. In addition, another peak at higher binding energy can be assigned to (=NH·+), as a result of the presence of bipolaron charge carriers [22]. The signal is shifted by 2.0, 3.2, and 1.1 eV to energies for CV/PPy
Fig. 5 TEM images of a CV/PPy, b GS/Ppy, and c PS/PPy samples
J Solid State Electrochem Fig. 6 a CV curves of CV/PPy, GS/Ppy, and PS/Ppy electrodes at 20-mV/s scan rate. b Specific capacitances of PPy samples synthesized by three different modes of electrodeposition at 5 mV/s
(401.7 eV), GS/PPy (402.8 eV), and PS/PPy (400.8 eV), respectively. Interestingly, PS/PPy sample exhibits a smaller shift to higher binding energies than CV/PPy and GS/PPy. The XPS analysis suggests that PPy samples synthesized by different electrochemical modes have different structural fractions of the polymer in the oxidized state. This fact may be related to the presence of defects in the polymer chain, but also to the different conjugation lengths [23, 24]. Figure 4 shows SEM images of PPy thin films on carbon cloth synthesized by different modes of electrodeposition at two different magnifications. All samples have a typical globular microstructure with some minor modifications [25, 21]. Moreover, it is interesting to note that PPy microstructure is uniformly coated on the surface of carbon cloth preserving the 3D fibrous network structure of cloth even after heavy deposition of PPy. Low magnification SEM images reveal that CV/ PPy (Fig. 4a) and GS/PPy (Fig. 4c) samples exhibit a more organized microstructure having typical layer by layer growth while PS/PPy shows relatively uniform 3D growth (Fig. 4e). A careful inspection suggests that CV/PPy and GS/PPy possess more compact and dense surfaces than PS/PPy samples as seen in Fig. 4b, d, f. TEM analyses of PPy samples have also been carried out in order to investigate the shape, size, and distribution of the particles; results are presented in Fig. 5. All samples revealed the aggregated growth of nanoparticles. Compared to SEM analyses, TEM images show a considerable variability of the morphology of PPy samples depending on the modes of electrodeposition used. Interestingly, CV/PPy displays clusters of large distributed particles which may be associated to the discontinuous growth of PPy (Fig. 5a). On the other hand, GS/ PPy and PS/PPy exhibit very tiny particle clusters which can be attributed to the characteristic nucleation and growth mechanism, because in these cases, multiple nucleation of hemispheres results in rough clumps (Fig. 5b, c) [21]. Figure 6a displays the typical cyclic voltammograms (CV) of CV/PPy, GS/PPy, and PS/PPy at constant scan rate of 20 mV/s within −0.1 to 0.8 V (vs Ag/AgCl). The shape of CVs for all PPy samples is not quite rectangular which confirms the existence of pseudocapacitive charge storage. In
addition, the anodic part of the CV curves is nearly symmetric to the cathodic counterpart suggesting excellent reversibility. The specific capacitance values of PPy samples were determined using the following equation: Z Vc 1 Cm ¼ I ðV ÞdV ð1Þ mvðV c−V aÞ V a where Cm is the specific capacitance (F/g), v is the scan rate (mV/s), Vc-Va is the potential range (−0.1 to 0.8 V/Ag/AgCl), I is the response current (mA), and m is the deposited weight of the PPy material on the electrode surface per unit area (g/cm2). The maximum values of the specific capacitance for CV/PPy, GS/PPy, and PS/PPy were found to be 66, 113, and 166 F/g, respectively, at 5-mV/s scan rate. The maximum value of specific capacitance of PPy thin films reported here is comparable to that of bulk PPy granules (152 F/g), PPy nanotubes synthesized using methyl orange template (MO)/ FeCl3 (172–197 F/g) [26], and PPy particles (142 F/g) [27]. The higher value of specific capacitance is obtained for PS/ PPy; it is significantly larger than that of GS/PPy and CV/PPy samples (Fig. 6b). This can be attributed to the ratio of different oxidized states of PPy detected in the XPS analysis. The proportions of the oxidized/neutral states of PPy are different for CV/PPy, GS/PPy, and PS/PPy samples (see Table 2). Thus, it is revealed that the equivalent proportion of the polaron and bipolaron states in PPy leads to better supercapacitive properties. In other words, the fraction of oxidized state of PPy depends on the defects present in the polymer chain, and also on the different conjugate lengths. Both could be associated with different morphologies Table 2 Intensity calculated from XPS analysis for different oxidized states of polypyrrole samples prepared by different modes of electrodeposition Mode of deposition
Ipolaron/netural
Ibipolaron/netural
CV/PPy GS/PPy PS/PPy
0.79 1.28 0.65
1.42 0.93 0.64
J Solid State Electrochem Fig. 7 CVs of PPy samples at different scan rates a CV/PPy, b GS/PPy, c PS/Ppy, d variation of specific capacitance of PPy samples with scan rate
obtained from different electrosynthesis methods. Moreover, the high level of structures in the oxidized state can be Fig. 8 Galvanostatic chargedischarge curves of a CV/PPy, b GS/PPy, c PS/PPy at different current densities in 1 M H2SO4 electrolyte, d specific power versus specific energy of the PPy electrodes
associated to PPy overoxidization caused by oxygen from the electrolyte [24, 23].
J Solid State Electrochem Table 3 Specific energy and specific power values of polypyrrole thin films synthesized by different modes of electrodeposition
Current density (mA/cm2)
Specific energy (Wh/kg)
Specific power (kW/kg)
CV
GS
PS
CV
GS
PS
2
4.54
8.50
13.04
0.26
0.35
0.14
4 8
3.61 3.27
7.25 6.96
10.67 10.52
0.52 1.03
0.69 1.39
0.27 0.53
12
3.15
6.35
9.78
1.55
2.09
0.80
16 20
3.00 2.84
6.14 5.99
9.56 8.18
2.06 2.58
2.78 3.48
1.07 1.34
Figure 7a–c shows CVs of PPy samples synthesized by different modes in 1 mol/L H2SO4 aqueous solution at different scanning rates from 5 to 100 mV/s, respectively. It is observed that the area under each curve increases with increase in scan rate indicating that the current density is directly proportional to the scan rate. Moreover, even at high scan rates of 100 mV/s, all CVs retain their shapes suggesting excellent reversibility of the materials. The variation observed in the behavior with the scan rate is associated with the effective interaction between the ions from the electrolyte and the electrode material [5, 28, 29]. Variation of specific capacitance with scan rate is presented in Fig. 7d. It is observed that the specific capacitance decreases with scan rate which can be attributed to the rapid increase in concentration of ions at the electrode/electrolyte interface at higher scan rate; the diffusion from the bulk of electrolyte solution to interface cannot be established rapidly enough to complete the redox reaction in the electrode material. Galvanostatic charge/discharge (GCD) tests were performed to further evaluate the electrochemical performance of the PPy samples at different current densities. Figure 8a–c shows the GCD curves of CV/PPy, GS/PPy, and PS/PPy at different current densities from 2 to 20 mA/cm2. The CD curves for all PPy samples are not ideally symmetrical also indicating redox reaction in the charge and discharge process (Fig. 8a–c). Moreover, charge/discharge time decreases with increase in current density, showing that the redox reaction is a diffusion-controlled process. Since the redox reactions usually depend on the ingress and egress of ions from the electrolyte, at lower current density, the ions can diffuse into more pores of the electrode. On the other hand, high current densities impose that the redox reaction in the polymer occurs rapidly which reduces the electrode efficiency. The maximum specific energy is obtained for PS/PPy sample with 13 Wh/kg at a specific power of 0.14 kW/kg. The values of specific energy and power for all PPy samples are summarized in Table 3. In agreement with its superior specific capacitance, the PS/PPy samples also have the highest specific energy among the electrodes prepared in this work. These values allow to include these PS/PPy coatings among promising materials for supercapacitor device development [9]. Figure 9a shows the Nyquist plots for CV/PPy, GS/PPy, and PS/PPy at room temperature. The plots consist of a high-
frequency arc region due to the diffusion effect of electrolyte and a low frequency line region. The intercept on the real axis (Z′) represents the equivalent series resistance (ESR), which includes the ionic resistance of the electrolyte, the intrinsic resistance of the active material, and current collector. The frequency at which there is deviation from the semicircle is called as Bknee frequency,^ which reflects the maximum frequency at which capacitive behavior is dominant. The straight line in the low-frequency region angled at −45° to the real Z′ axis can be attributed to the capacitive behavior [30]. The values of equivalent series resistances (ESR) for CV/PPy, GS/PPy, and PS/PPy are found to be 2.31, 2.37, and 2.27 Ω, respectively, which indicates that the ESR value does not
Fig. 9 a Nyquist plots and b Bode phase angle plots for CV/PPy, GS/ PPy, and PS/PPy supercapacitors, inset of a: equivalent circuit
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change obviously with the mode of deposition. Relatively lower ESR of PS/PPy reveals a higher conductivity than CV/PPy and GS/PPy samples. The region with the 45° slope, which is called the Warburg region, is a consequence of the behavior of the porous structure and is often called the distributed resistance/capacitance of PPy. The Bknee^ frequency in the plot is found at 417, 620, and 673 Hz for CV/PPy, GS/PPy, and PS/PPy electrodes, respectively. This suggests that most of the energy stored can be accessible below the frequencies of 417, 620, and 673 Hz for CV/PPy, GS/PPy, and PS/PPy, respectively. The equivalent circuit used for evaluations is shown as the inset of Fig. 9a with the components Rs as the electrolyte resistance in the cell, Rct as the charge transfer resistance, CPE as the constant phase element, and Z w representing the Warburg diffusion. Figure 9b shows the Bode plot of PPy electrodes. The phase angle is very close to −90° for frequencies up to 1 Hz suggesting that the PPy supercapacitor device behaves as an ideal capacitor at low frequencies [31]. On the other hand, the phase angles of the PS/PPy supercapacitor reach −74°, suggesting that the PS/ PPy-based supercapacitor behaves more like an ideal capacitor than other GS/PPy and CV/PPy electrodes. A hypothetical electric circuit consists of components with well-defined electrical properties used to describe the EIS response of the PPybased supercapacitor. The dielectric losses in the supercapacitor due to energy dissipation by an irreversible process lead to hysteresis. The time constant (Γ0 =1/f0) [32] was earlier described as the dielectric relaxation time constant or the supercapacitor factor of merit. The time constants are calculated to be 9.8, 4.5, and 2 ms for CV/PPy, GS/PPy, and PS/PPy, respectively. The difference in the time constant is due to structural changes in the electrodes.
Acknowledgments CAPES Foundation, Ministry of Education of Brazil: Process BEX 3196/14-3. Spanish grant MAT2012-39199-C02-01 is also acknowledged.
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Conclusions I n c o n c l u s i o n , t h e e ff e c t o f d i ff e r e n t m o d e s o f electropolymerization on the supercapacitive properties of PPy thin films has been successfully investigated. The XPS analysis suggests that PPy samples synthesized by different electrochemical modes exhibit different structural contributions in the oxidized states as well as the presence of different proportions of polaron/bipolaron structures in the polymer chain. Surface morphological analysis reveals typical globular microstructures of PPy organized in clustered particles. A significant effect of different modes of electrodeposition on supercapacitive properties of PPy thin films was observed. The PPy thin films deposited on carbon cloth by potentiostatic mode showed better electrochemical properties with specific capacitance of 166 F/g and specific energy of 13 Wh/kg. Thus, this work provides useful orientation for the use of optimal electrodeposition methods for the formation of PPy thin films to be used as electrode material for supercapacitors.
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