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Capacitance and cycling stability of poly(alkoxythiophene) derivative electrodes
Electrochemistry Communications 3 (2001) 16±19
www.elsevier.nl/locate/elecom
Capacitance and cycling stability of poly(alkoxythiophene) derivative electrodes C. Arbizzani
a,*
, M.C. Gallazzi b, M. Mastragostino c, M. Rossi c, F. Soavi
a
a Dip. di Chimica, Universit a di Bologna, ``G. Ciamician'', via Selmi 2, 40126 Bologna, Italy Politecnico di Milano, Dip. di Chimica Industriale e Ingegneria Chimica, P.zza Leonardo da Vinci 32, 20133 Milano, Italy Universit a di Bologna, Unit a Complessa di Istituti di Scienze Chimiche, Radiochimiche e Metallurgiche, via San Donato 15, 40127 Bologna, Italy b
c
Received 18 October 2000; received in revised form 3 November 2000; accepted 3 November 2000
Abstract The functionalization of thiophene rings by electron withdrawing or electron donor groups has been widely studied, and the electrochemical and optical properties of the corresponding polymers have been extensively investigated. Given the good performance of the polyalkoxythiophenes, especially in terms of conductivity and stability of the conducting form, the present study was carried out to evaluate their stability also to repeated voltammetric cycles and their speci®c capacitance for their use as electrode materials in polymer supercapacitors. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Electrode materials; Poly(alkoxythiophene); Poly(4,400 -dipentoxy-40 dicyanovinyl-2,20 :50 ,200 -terthiophene); poly(4,400 -dipentoxy-2,20 :50 ,200 -terthiophene)
1. Introduction The functionalization of molecules is a very useful and elegant tool for controlling their electronic properties [1]. By tuning the HOMO and LUMO energies of monomers, it is possible to control the electrochemical and optical performance of the corresponding polymers. The 3-substitution of the thiophene ring by electron withdrawing or electron donor groups has been widely studied, and the electrochemical and optical properties of the corresponding polymers have been extensively investigated [2±4]. One of the most investigated thiophene-based families is the poly(alkoxythiophenes). Due to the electron donor groups, these materials display the p-doping process at less positive potential than the unsubstituted polymer and, consequently, a good stability of the pdoped state. The dimers 4,40 -alkoxy-2,20 -bithiophene yield polymers that are very interesting for their electronic properties, stability and optical characteristics [5± * Corresponding author. Tel.: +39-051-253164; fax: +39-051-2099456. E-mail address: [email protected] (C. Arbizzani).
7]. The use of symmetrically disubstituted dimers or oligomers instead of monosubstituted monomers produces regiochemically substituted polymers with enhanced electrical and optical properties [8±10]. In order to obtain processable materials, copolymerization of alkyl and alkoxy substituted starting molecules were investigated [11]. The multifunctionalization of the starting molecules with alternating donor±acceptor groups is also another interesting approach to polymer design at the molecular level [12,13]. Given the good performance of the poly(alkoxythiophenes), especially in terms of conductivity and stability of the conducting form, the present study was designed to evaluate their stability also to repeated voltammetric cycles and their speci®c capacitance for use as electrode materials in polymer supercapacitors, which are devices for applications requiring high operating power levels. While electronically conducting polymers yield dierent redox supercapacitor con®gurations, devices with the polymer n-doped form as the negative electrode and the p-doped form as the positive one are the most promising, and polymers able to sustain both p- and n-doping processes are of great interest.
1388-2481/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 ( 0 0 ) 0 0 1 3 9 - 9
C. Arbizzani et al. / Electrochemistry Communications 3 (2001) 16±19
17
Scheme 1.
2. Experimental Poly(4,400 -dipentoxy-40 -dicyanovinyl-2,20 :50 ,200 -terthiophene) (pDPCT) and poly(4,400 -dipentoxy-2,20 :50 ,200 terthiophene) (pDPT) were galvanostatically grown on Pt (0:12 cm2 ) and on tin oxide (TO, 1 cm2 ) electrodes by trimer oxidation. The trimers DPCT and DPT (Scheme 1) were synthesized as reported in Ref. [8,14], respectively. The electrosyntheses were carried out at room temperature in CH3 CN ± 0.2 M tetraethylammonium tetr¯uoroborate (Et4 NBF4 ) ± 5 mM trimer at 0:5 mA cmÿ2 , and to estimate the polymer mass of the electrosynthesized ®lms, the polymer electrodes were galvanostatically discharged (Qd recovered charge). Assuming 100% polymerization eciency, the polymer mass (me ) was obtained by the formula me Qel ÿ Qd MW =2F , where Qel is the electrosynthesis charge and MW is the molecular weight of the monomer unit (three rings). The doping level values (y%) are referred to the monomer unit based on three rings. All chemicals used were reagent-grade products puri®ed before use, and all the electrochemical experiments were performed in a Labmaster 130 MBraun dry-box (oxygen and water content less than 1 ppm) using a 273A PAR potentiostat/galvanostat, a 545 AMEL galvanostat/electrometer and a Solartron 1255 HF frequency response analyzer. A 10 mV ac perturbation was used in the impedance measurements, and data were collected over a frequency range from 10 kHz to 10 mHz, taking 10 points/decade. Spectroelectrochemical measurements were carried out in situ with a Lambda 19 Perkin Elmer spectrometer and a 273A PAR potentiostat/galvanostat. The optical cell was closed in dry-box under argon atmosphere. The electrochemical and the spectroelectrochemical tests were performed in propylene carbonate (PC)±Et4 NBF4 1 and 0.2 M, respectively. All the potential values were measured vs Ag (a quasireference electrode) and expressed vs SCE. The VAg value was checked vs SCE after each electrochemical test.
Fig. 1. Spectra of a pDPCT/TO electrode Qel 0:033 C cmÿ2 by LSs at 50 mV sÿ1 in PC±Et4 NBF4 0.2 M up to dierent ®nal potentials: (a) neutral form; (b) 0.5 V, 2:2 mC cmÿ2 ; (c) 0.7 V, 6:2 mC cmÿ2 ; (d) 0.8 V, 7:7 mC cmÿ2 ; (e) 0.9 V, 9:5 mC cmÿ2 ; (f) back to the neutral form. After each spectrum the ®lm was discharged at 0 V vs SCE.
polymer ®lms as thick as those required for such devices. The characteristics of the doping/undoping process can signi®cantly change with thickness, a fact strongly dependent on polymer morphology, which in turn depends on electrosynthesis conditions [2]. We electrosynthesized pDPCT ®lms with Qel up to 2:5 C cmÿ2 . The cycling voltammetry (CV) of pDPCT in the potential range between ÿ0.015 and 1.285 V shows a reversible oxidation process. The changes in the electronic spectra, recorded at dierent values of the injected charge by linear sweep (LS) and displayed in Fig. 1, are typical of a reversible p-doping process. Repeated CVs in the p-doping potential range evinced the high cycling stability of the p-doped pDPCT. Fig. 2 shows the speci®c capacity (in mAh gÿ1 ) delivered during the reverse scan of the CVs at 20 mV sÿ1 , with a coulombic eciency of the pdoping/undoping process >99%. The ®gure also reports the electrode speci®c capacitance in F gÿ1 : these data were derived from the diagram in which the electrode potential was plotted against the doping charge calculated from the CVs by integration of the current density (during discharge) vs time and by taking into account the mass of the electrode. The slope of the linear part of
3. Results and discussion A real evaluation of the performance of these polymers as electrodes in supercapacitors requires tests on
Fig. 2. Speci®c capacity and capacitance of a pDPCT/Pt electrode Qel 2:5 C cmÿ2 by CVs at 20 mV sÿ1 in PC±Et4 NBF4 1 M between ÿ0.015 and 1.285 V vs SCE.
18
C. Arbizzani et al. / Electrochemistry Communications 3 (2001) 16±19
Fig. 3. Discharge capacity vs voltage of a pDPCT/Pt electrode Qel 2:5 C cmÿ2 of the 220th CV at 20 mV sÿ1 in PC±Et4 NBF4 1 M.
Fig. 5. Speci®c capacity and capacitance of a pDPT/Pt electrode Qel 2:5 C cmÿ2 by CVs at 20 mV sÿ1 in PC±Et4 NBF4 1 M between ÿ0.27 and 1.03 V vs SCE.
the curve represents the reciprocal of the speci®c capacitance for the p-doped polymer. Fig. 3 shows the diagram plotted for the 220th CV. The great cycling stability over one thousand cycles and the high speci®c capacity of the p-doped pDPCT are promising for use in supercapacitors. The scans towards negative potentials, performed to evaluate the n-doping capability of pDPCT, greatly modi®ed the polymer material. To gain insight into these changes, in situ spectroelectrochemical measurements were carried out and Fig. 4 summarizes the changes of the polymer ®lm spectra after negative potential scans. The solid line is the spectrum of the ®lm in the neutral state (after a few cycles in the positive potential domain). The dotted line is the spectrum taken after the injection of negative charge by LS at 50 mV sÿ1 up to ÿ1:4 V, and the dashed line is the spectrum after the ®lm was brought again to the neutral state. The dotted±dashed line is the spectrum of the ®lm that was brought back to the neutral state after insertion of negative charge by LS at 50 mV sÿ1 up to ÿ2.2 V. From this last spectrum it seems that during the scan in the negative potential domain the eect of cyano groups in
the polymer conjugation is lost: the shape of the spectrum and the wavelength of the maximum absorption are similar to those of the polymer yielded from the trimer DPT, without cyano groups [8]. The reliability of an ecient cyclability depends on the mass of the monomer unit. Indeed, heavy thiophenederivative polymers require higher doping levels than polythiophene to meet the criterion of high electrode speci®c capacitance for maximizing the device's speci®c energy, as illustrated in Ref. [15], and this can be a disadvantage because of greater mechanical stress on the polymer during repeated cycles. In the speci®c case, pDPT (see Fig. 5) has to be preferred to pDPCT as positive electrode materials, given that its performance is slightly higher than that of pDPCT and comparable to that of such conventional polymers as poly(3-methylthiophene) [16]. More detailed estimate of the electrochemical properties of pDPT can be carried out by analysis of the impedance spectra with the evaluation of the parameters aecting the electrochemical processes taking place at the electrode. Fig. 6(a) shows impedance spectra at dierent states of charge. The spectra display capacitive behaviors from very low doping levels: charge
Fig. 4. Spectra of a pDPCT/TO electrode Qel 0:033 C cmÿ2 by LSs at 50 mV sÿ1 in PC±Et4 NBF4 0.2 M. Ð undoped; after ÿ5.2 mC cmÿ2 ¯own up to ÿ1.4 V; ± ± ± undoped (after ÿ5.2 mC cmÿ2 ¯own up to ÿ1.4 V); ± ± ± undoped (after ÿ17.8 mC cmÿ2 ¯own up to ÿ2.2 V).
Fig. 6. (a) Impedance spectra and (b) capacitance vs frequency of a pDPT/Pt electrode Qel 2:5 C cmÿ2 at dierent p-doping levels (y%) in PC±Et4 NBF4 1 M.
C. Arbizzani et al. / Electrochemistry Communications 3 (2001) 16±19 Table 1 Re®ned parameters from impedance spectra at dierent p-doping levels (y%) y%
YoL mF snÿ1
n
6 19 35
56.5 79.0 96.3
0.95 0.99 0.99
transfer resistance is very small and the Warburg impedance not discernible as the magni®cation at high frequency shows. Table 1 reports the YoL and n values, which are representative of the limit capacitance (when n 1, YoL limit capacitance). Fig. 6(b) displays the capacitance vs frequency of the pDPT and shows that there is a good agreement between the speci®c capacitance data evaluated from impedance spectra and that from CV data reported in Fig. 5. The pDPT shows good capacitance values also at a very low level of charge and, at the higher doping levels, reaches the limit capacitance values at 0.1 Hz and the 80% of the limit values at 1 Hz, i.e., in a time scale of 10 and 1 s, respectively. It is also notable that the ion diusion process for this ®lm is so fast that the limit resistance is not detectable: consequently, the diusion process does not seriously aect the charge injection even for high ®lm thickness, suggesting a very open structure for pDPT. 4. Conclusions Our data demonstrate that p-doped pDPCT and, preferably, p-doped pDPT can be successfully used as electrode materials in such energy conversion devices as supercapacitors. These p-doped polymers display high
19
cycling stability over one thousand tested cycles and high capacitance values (ca. 190 F gÿ1 ) in a time scale of a few seconds, as supercapacitors require. Acknowledgements The present work was supported by MURST (Electrochemical and Electronic Devices with Polymer Components). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
F. Garnier, Angew. Chem. Int. Ed. Engl. 28 (1989) 513. J. Roncali, Chem. Rev. 92 (1992) 711. J. Roncali, Chem. Rev. 97 (1997) 173. C. Arbizzani, M. Mastragostino, Current Trend in Polymer Science, vol. 2, p. 217, Research Trends, Trivandrum, 1997. K. Faid, R. Cloutier, M. Leclerc, Macromolecules 26 (1993) 2501. M. Dietrich, J. Heinze, Synth. Metals 41±43 (1991) 503. N. Camaioni, G. Casalbore-Miceli, A. Geri, G. Zotti, J. Phys. Chem. D 31 (1998) 1245. G. Zotti, M.C. Gallazzi, G. Zerbi, S.V. Meille, Synth. Metals 73 (1995) 217. E. Villa, E. Agosti, C. Castiglioni, M.C. Gallazzi, G. Zerbi, J. Chem. Phys. 105 (1996) 9461. M.C. Gallazzi, L. Castellani, R.A. Marin, G. Zerbi, J. Polym. Sci. A 31 (1993) 3339. R. Hanna, M. Leclerc, Chem. Mater. 8 (1996) 1512. F. Demanze, A. Yassar, F. Garnier, Adv. Mater. 7 (1995) 907. L.E. Bolivar-Marinez, M.C. dos Santos, D.S. Galv~ ao, J. Phys. Chem 100 (1996) 11029. M.C. Gallazzi, F. Toscano, D. Paganuzzi, C. Bertarelli, Macromol. Chem. Phys. (submitted). M. Mastragostino, C. Arbizzani, R. Paraventi, A. Zanelli, J. Electrochem. Soc. 147 (2000) 407. M. Mastragostino, C. Arbizzani, F. Soavi, J. Power Sources (in press).
www.elsevier.nl/locate/elecom
Capacitance and cycling stability of poly(alkoxythiophene) derivative electrodes C. Arbizzani
a,*
, M.C. Gallazzi b, M. Mastragostino c, M. Rossi c, F. Soavi
a
a Dip. di Chimica, Universit a di Bologna, ``G. Ciamician'', via Selmi 2, 40126 Bologna, Italy Politecnico di Milano, Dip. di Chimica Industriale e Ingegneria Chimica, P.zza Leonardo da Vinci 32, 20133 Milano, Italy Universit a di Bologna, Unit a Complessa di Istituti di Scienze Chimiche, Radiochimiche e Metallurgiche, via San Donato 15, 40127 Bologna, Italy b
c
Received 18 October 2000; received in revised form 3 November 2000; accepted 3 November 2000
Abstract The functionalization of thiophene rings by electron withdrawing or electron donor groups has been widely studied, and the electrochemical and optical properties of the corresponding polymers have been extensively investigated. Given the good performance of the polyalkoxythiophenes, especially in terms of conductivity and stability of the conducting form, the present study was carried out to evaluate their stability also to repeated voltammetric cycles and their speci®c capacitance for their use as electrode materials in polymer supercapacitors. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Electrode materials; Poly(alkoxythiophene); Poly(4,400 -dipentoxy-40 dicyanovinyl-2,20 :50 ,200 -terthiophene); poly(4,400 -dipentoxy-2,20 :50 ,200 -terthiophene)
1. Introduction The functionalization of molecules is a very useful and elegant tool for controlling their electronic properties [1]. By tuning the HOMO and LUMO energies of monomers, it is possible to control the electrochemical and optical performance of the corresponding polymers. The 3-substitution of the thiophene ring by electron withdrawing or electron donor groups has been widely studied, and the electrochemical and optical properties of the corresponding polymers have been extensively investigated [2±4]. One of the most investigated thiophene-based families is the poly(alkoxythiophenes). Due to the electron donor groups, these materials display the p-doping process at less positive potential than the unsubstituted polymer and, consequently, a good stability of the pdoped state. The dimers 4,40 -alkoxy-2,20 -bithiophene yield polymers that are very interesting for their electronic properties, stability and optical characteristics [5± * Corresponding author. Tel.: +39-051-253164; fax: +39-051-2099456. E-mail address: [email protected] (C. Arbizzani).
7]. The use of symmetrically disubstituted dimers or oligomers instead of monosubstituted monomers produces regiochemically substituted polymers with enhanced electrical and optical properties [8±10]. In order to obtain processable materials, copolymerization of alkyl and alkoxy substituted starting molecules were investigated [11]. The multifunctionalization of the starting molecules with alternating donor±acceptor groups is also another interesting approach to polymer design at the molecular level [12,13]. Given the good performance of the poly(alkoxythiophenes), especially in terms of conductivity and stability of the conducting form, the present study was designed to evaluate their stability also to repeated voltammetric cycles and their speci®c capacitance for use as electrode materials in polymer supercapacitors, which are devices for applications requiring high operating power levels. While electronically conducting polymers yield dierent redox supercapacitor con®gurations, devices with the polymer n-doped form as the negative electrode and the p-doped form as the positive one are the most promising, and polymers able to sustain both p- and n-doping processes are of great interest.
1388-2481/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 ( 0 0 ) 0 0 1 3 9 - 9
C. Arbizzani et al. / Electrochemistry Communications 3 (2001) 16±19
17
Scheme 1.
2. Experimental Poly(4,400 -dipentoxy-40 -dicyanovinyl-2,20 :50 ,200 -terthiophene) (pDPCT) and poly(4,400 -dipentoxy-2,20 :50 ,200 terthiophene) (pDPT) were galvanostatically grown on Pt (0:12 cm2 ) and on tin oxide (TO, 1 cm2 ) electrodes by trimer oxidation. The trimers DPCT and DPT (Scheme 1) were synthesized as reported in Ref. [8,14], respectively. The electrosyntheses were carried out at room temperature in CH3 CN ± 0.2 M tetraethylammonium tetr¯uoroborate (Et4 NBF4 ) ± 5 mM trimer at 0:5 mA cmÿ2 , and to estimate the polymer mass of the electrosynthesized ®lms, the polymer electrodes were galvanostatically discharged (Qd recovered charge). Assuming 100% polymerization eciency, the polymer mass (me ) was obtained by the formula me Qel ÿ Qd MW =2F , where Qel is the electrosynthesis charge and MW is the molecular weight of the monomer unit (three rings). The doping level values (y%) are referred to the monomer unit based on three rings. All chemicals used were reagent-grade products puri®ed before use, and all the electrochemical experiments were performed in a Labmaster 130 MBraun dry-box (oxygen and water content less than 1 ppm) using a 273A PAR potentiostat/galvanostat, a 545 AMEL galvanostat/electrometer and a Solartron 1255 HF frequency response analyzer. A 10 mV ac perturbation was used in the impedance measurements, and data were collected over a frequency range from 10 kHz to 10 mHz, taking 10 points/decade. Spectroelectrochemical measurements were carried out in situ with a Lambda 19 Perkin Elmer spectrometer and a 273A PAR potentiostat/galvanostat. The optical cell was closed in dry-box under argon atmosphere. The electrochemical and the spectroelectrochemical tests were performed in propylene carbonate (PC)±Et4 NBF4 1 and 0.2 M, respectively. All the potential values were measured vs Ag (a quasireference electrode) and expressed vs SCE. The VAg value was checked vs SCE after each electrochemical test.
Fig. 1. Spectra of a pDPCT/TO electrode Qel 0:033 C cmÿ2 by LSs at 50 mV sÿ1 in PC±Et4 NBF4 0.2 M up to dierent ®nal potentials: (a) neutral form; (b) 0.5 V, 2:2 mC cmÿ2 ; (c) 0.7 V, 6:2 mC cmÿ2 ; (d) 0.8 V, 7:7 mC cmÿ2 ; (e) 0.9 V, 9:5 mC cmÿ2 ; (f) back to the neutral form. After each spectrum the ®lm was discharged at 0 V vs SCE.
polymer ®lms as thick as those required for such devices. The characteristics of the doping/undoping process can signi®cantly change with thickness, a fact strongly dependent on polymer morphology, which in turn depends on electrosynthesis conditions [2]. We electrosynthesized pDPCT ®lms with Qel up to 2:5 C cmÿ2 . The cycling voltammetry (CV) of pDPCT in the potential range between ÿ0.015 and 1.285 V shows a reversible oxidation process. The changes in the electronic spectra, recorded at dierent values of the injected charge by linear sweep (LS) and displayed in Fig. 1, are typical of a reversible p-doping process. Repeated CVs in the p-doping potential range evinced the high cycling stability of the p-doped pDPCT. Fig. 2 shows the speci®c capacity (in mAh gÿ1 ) delivered during the reverse scan of the CVs at 20 mV sÿ1 , with a coulombic eciency of the pdoping/undoping process >99%. The ®gure also reports the electrode speci®c capacitance in F gÿ1 : these data were derived from the diagram in which the electrode potential was plotted against the doping charge calculated from the CVs by integration of the current density (during discharge) vs time and by taking into account the mass of the electrode. The slope of the linear part of
3. Results and discussion A real evaluation of the performance of these polymers as electrodes in supercapacitors requires tests on
Fig. 2. Speci®c capacity and capacitance of a pDPCT/Pt electrode Qel 2:5 C cmÿ2 by CVs at 20 mV sÿ1 in PC±Et4 NBF4 1 M between ÿ0.015 and 1.285 V vs SCE.
18
C. Arbizzani et al. / Electrochemistry Communications 3 (2001) 16±19
Fig. 3. Discharge capacity vs voltage of a pDPCT/Pt electrode Qel 2:5 C cmÿ2 of the 220th CV at 20 mV sÿ1 in PC±Et4 NBF4 1 M.
Fig. 5. Speci®c capacity and capacitance of a pDPT/Pt electrode Qel 2:5 C cmÿ2 by CVs at 20 mV sÿ1 in PC±Et4 NBF4 1 M between ÿ0.27 and 1.03 V vs SCE.
the curve represents the reciprocal of the speci®c capacitance for the p-doped polymer. Fig. 3 shows the diagram plotted for the 220th CV. The great cycling stability over one thousand cycles and the high speci®c capacity of the p-doped pDPCT are promising for use in supercapacitors. The scans towards negative potentials, performed to evaluate the n-doping capability of pDPCT, greatly modi®ed the polymer material. To gain insight into these changes, in situ spectroelectrochemical measurements were carried out and Fig. 4 summarizes the changes of the polymer ®lm spectra after negative potential scans. The solid line is the spectrum of the ®lm in the neutral state (after a few cycles in the positive potential domain). The dotted line is the spectrum taken after the injection of negative charge by LS at 50 mV sÿ1 up to ÿ1:4 V, and the dashed line is the spectrum after the ®lm was brought again to the neutral state. The dotted±dashed line is the spectrum of the ®lm that was brought back to the neutral state after insertion of negative charge by LS at 50 mV sÿ1 up to ÿ2.2 V. From this last spectrum it seems that during the scan in the negative potential domain the eect of cyano groups in
the polymer conjugation is lost: the shape of the spectrum and the wavelength of the maximum absorption are similar to those of the polymer yielded from the trimer DPT, without cyano groups [8]. The reliability of an ecient cyclability depends on the mass of the monomer unit. Indeed, heavy thiophenederivative polymers require higher doping levels than polythiophene to meet the criterion of high electrode speci®c capacitance for maximizing the device's speci®c energy, as illustrated in Ref. [15], and this can be a disadvantage because of greater mechanical stress on the polymer during repeated cycles. In the speci®c case, pDPT (see Fig. 5) has to be preferred to pDPCT as positive electrode materials, given that its performance is slightly higher than that of pDPCT and comparable to that of such conventional polymers as poly(3-methylthiophene) [16]. More detailed estimate of the electrochemical properties of pDPT can be carried out by analysis of the impedance spectra with the evaluation of the parameters aecting the electrochemical processes taking place at the electrode. Fig. 6(a) shows impedance spectra at dierent states of charge. The spectra display capacitive behaviors from very low doping levels: charge
Fig. 4. Spectra of a pDPCT/TO electrode Qel 0:033 C cmÿ2 by LSs at 50 mV sÿ1 in PC±Et4 NBF4 0.2 M. Ð undoped; after ÿ5.2 mC cmÿ2 ¯own up to ÿ1.4 V; ± ± ± undoped (after ÿ5.2 mC cmÿ2 ¯own up to ÿ1.4 V); ± ± ± undoped (after ÿ17.8 mC cmÿ2 ¯own up to ÿ2.2 V).
Fig. 6. (a) Impedance spectra and (b) capacitance vs frequency of a pDPT/Pt electrode Qel 2:5 C cmÿ2 at dierent p-doping levels (y%) in PC±Et4 NBF4 1 M.
C. Arbizzani et al. / Electrochemistry Communications 3 (2001) 16±19 Table 1 Re®ned parameters from impedance spectra at dierent p-doping levels (y%) y%
YoL mF snÿ1
n
6 19 35
56.5 79.0 96.3
0.95 0.99 0.99
transfer resistance is very small and the Warburg impedance not discernible as the magni®cation at high frequency shows. Table 1 reports the YoL and n values, which are representative of the limit capacitance (when n 1, YoL limit capacitance). Fig. 6(b) displays the capacitance vs frequency of the pDPT and shows that there is a good agreement between the speci®c capacitance data evaluated from impedance spectra and that from CV data reported in Fig. 5. The pDPT shows good capacitance values also at a very low level of charge and, at the higher doping levels, reaches the limit capacitance values at 0.1 Hz and the 80% of the limit values at 1 Hz, i.e., in a time scale of 10 and 1 s, respectively. It is also notable that the ion diusion process for this ®lm is so fast that the limit resistance is not detectable: consequently, the diusion process does not seriously aect the charge injection even for high ®lm thickness, suggesting a very open structure for pDPT. 4. Conclusions Our data demonstrate that p-doped pDPCT and, preferably, p-doped pDPT can be successfully used as electrode materials in such energy conversion devices as supercapacitors. These p-doped polymers display high
19
cycling stability over one thousand tested cycles and high capacitance values (ca. 190 F gÿ1 ) in a time scale of a few seconds, as supercapacitors require. Acknowledgements The present work was supported by MURST (Electrochemical and Electronic Devices with Polymer Components). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
F. Garnier, Angew. Chem. Int. Ed. Engl. 28 (1989) 513. J. Roncali, Chem. Rev. 92 (1992) 711. J. Roncali, Chem. Rev. 97 (1997) 173. C. Arbizzani, M. Mastragostino, Current Trend in Polymer Science, vol. 2, p. 217, Research Trends, Trivandrum, 1997. K. Faid, R. Cloutier, M. Leclerc, Macromolecules 26 (1993) 2501. M. Dietrich, J. Heinze, Synth. Metals 41±43 (1991) 503. N. Camaioni, G. Casalbore-Miceli, A. Geri, G. Zotti, J. Phys. Chem. D 31 (1998) 1245. G. Zotti, M.C. Gallazzi, G. Zerbi, S.V. Meille, Synth. Metals 73 (1995) 217. E. Villa, E. Agosti, C. Castiglioni, M.C. Gallazzi, G. Zerbi, J. Chem. Phys. 105 (1996) 9461. M.C. Gallazzi, L. Castellani, R.A. Marin, G. Zerbi, J. Polym. Sci. A 31 (1993) 3339. R. Hanna, M. Leclerc, Chem. Mater. 8 (1996) 1512. F. Demanze, A. Yassar, F. Garnier, Adv. Mater. 7 (1995) 907. L.E. Bolivar-Marinez, M.C. dos Santos, D.S. Galv~ ao, J. Phys. Chem 100 (1996) 11029. M.C. Gallazzi, F. Toscano, D. Paganuzzi, C. Bertarelli, Macromol. Chem. Phys. (submitted). M. Mastragostino, C. Arbizzani, R. Paraventi, A. Zanelli, J. Electrochem. Soc. 147 (2000) 407. M. Mastragostino, C. Arbizzani, F. Soavi, J. Power Sources (in press).