- Email: [email protected]
Conducting polymers as electrode materials in supercapacitors
Solid State Ionics 148 (2002) 493 – 498 www.elsevier.com/locate/ssi
Conducting polymers as electrode materials in supercapacitors Marina Mastragostino a,*, Catia Arbizzani b, Francesca Soavi a a Radiochimiche e Metallurgiche, Unita’ Complessa di Istituti, Scienze Chimiche, Radiochimiche e Metallurgiche, University of Bologna, via San Donato 15, I-40127 Bologna, Italy b Department of Chemistry ‘‘G. Ciamician’’, University of Bologna, via Selmi 2, I-40126 Bologna, Italy
Abstract This paper summarizes the performance data of conventional and especially designed thiophene-based conducting polymers for use as positive and negative electrodes in n/p type supercapacitors. Performance data of polymer composite electrodes are also compared with those of high surface area carbon-based composite electrodes. On the basis of capacity, capacitance and electrode charging resistance data, we selected the best electrode materials, and assembled and tested galvanostatic charge – discharge cycles n/p type pMeT-based supercapacitors and hybrid supercapacitors with pMeT as positive electrode active material and activated carbon as negative. The results of this investigation demonstrate that a conventional polymer such as pMeT can be successfully used in the supercapacitor technology when a hybrid configuration is realized; its use is, indeed, a great advantage because the hybrid supercapacitor outperforms the double-layer carbon supercapacitors presently on the market in terms of specific energy and power. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical supercapacitor; Hybrid supercapacitor; p-Doping; n-Doping; n/p type supercapacitor; Poly3-methylthiophene
1. Introduction Supercapacitors and batteries are energy storage and conversion systems which satisfy, in a complementary way, the requirements of high specific power and energy, respectively. In the last years, the significative advances achieved in the materials for supercapacitors brought about an extension of their application to the field of rechargeable batteries and, at present, the best performing devices on the market are the double-layer supercapacitors with activated carbons of high surface area (1500 –2400 m2 g 1) as electrode active materials. Amelioration in these types of devices is expected by lowering the equiv*
Corresponding author. Tel.: +39-051-209-9798; fax: +39-051209-9365. E-mail address: [email protected] (M. Mastragostino).
alent series resistance (ESR) and this can be attained by optimizing the pore size distribution of the carbon materials. Electronically conducting polymers (ECPs) are promising materials for the realization of high performance supercapacitors, as they are characterized by high specific capacitances (the charge processes pertain to the whole polymer mass and not only to the surface, as in the case of double-layer activated carbons) and by high conductivities in the charged states; furthermore, their charge–discharge processes are generally fast. These features suggest the possibility to develop devices with low ESR and high specific energy and power. The most promising polymer supercapacitor configuration in terms of charge storage capacity and cell voltage and able to outperform the double-layer carbon supercapacitors is the n/ p type, i.e. the configuration in which an n-doped
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 0 9 3 - 0
494
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
as in Ref. [1]. The composite electrodes were prepared by mixing active materials (pMeT or activated carbon AC1 and AC3), conducting additive (graphite, SFG44, Timcal, or acetylene black, AB, Hoechst), binder (carboxy methyl cellulose, Cmc, Aldrich) and PTFE (Du Pont), and the supercapacitors were assembled with propylene carbonate (PC, Fluka) —1 M Et4NBF4 as electrolyte, as in Ref. [4]. The cyclic voltammetries (CVs) of the electrodes were performed at 20 mV s 1 with a 273A PAR potentiostat/galvanostat. Impedance measurements were carried out in three-electrode mode using a Solartron SI 1255 frequency response analyzer coupled with a 273A PAR potentiostat/galvanostat; an AC amplitude of 5 mV was used and data were collected in the frequency range of 100 kHz – 10 mHz taking 10 points per decade. The cyclability performance of the supercapacitors was tested by repeated charge – discharge galvanostatic cycles at different current densities and cut-off potentials with a 273A PAR potentiostat/galvanostat and a Perkin Elmer VMP. An Ag quasi-reference electrode, whose potential was checked vs. SCE for each set of experiments, was used for the experiments in three-electrode mode and to monitor the electrode potentials during the charge– discharge cycles. All the electrochemical tests were performed in a MBraun Labmaster 150 dry box filled with argon, as reported in Ref. [4].
polymer and a p-doped one are used as negative and positive electrodes, respectively. However, this is not an easy-to-realize configuration and the difficulties are mainly related to the n-doping process. The charge– discharge processes of polymer-based supercapacitors are faradic and those of the n/p devices take place in a cell potential range settled by the n- and p-doping – undoping process potentials of the polymer electrodes. We studied the thiophene-based polymer materials, such as poly(dithieno[3,4-b:3V,4V-d]thiophene) (pDTT1) and poly(3-p-fluorophenylthiophene) (pFPT), especially designed to optimize the working potential range of each electrode, by taking into account the need to realize high cell voltages compatible with the breakdown potentials of the electrolyte on the current collectors, as well as a conventional polymer such as poly(3-methylthiophene) (pMeT) for the development of the n/p type supercapacitors [1,2]. We also developed a new type of polymer supercapacitor, a hybrid device based on pMeT as positive electrode and activated carbon as negative [3]. In this paper, capacity and capacitance data of the above reported polymers in the p- and n-doped forms, as well as the data of positively and negatively charged activated carbons, are reported and discussed in view of their use as positive and negative electrodes in supercapacitors. Data from the charge– discharge galvanostatic cycles of the n/p type and C/ECP hybrid supercapacitors are also reported and the pros and cons of the use of polymer electrodes are discussed.
3. Results and discussion Table 1 summarizes the specific capacity and capacitance data, obtained by CVs at 20 mV s 1, and the linear potential excursion of each electrode during the discharge of the p- and n-doped forms of pFPT, pDTT1 and pMeT, directly electrosynthesized on the current collectors (polymer mass loading was in the range from 4 to 9 mg cm 2). We selected the
2. Experimental Poly(3-p-fluorophenylthiophene) and poly(dithieno[3,4-b:3V,4V-d]thiophene) were galvanostatically grown on carbon paper electrodes and poly(3-methylthiophene) on Pt electrodes by monomer oxidation, Table 1 Electrode specific capacitance and capacity from CVs at 20 mV s ECP
pFPT pDTT1 pMeT
1
of polymers electrosynthesized on current collectors
p-Doping
n-Doping
CV range (V vs. SCE)
Capacitance (F g 1)
Capacity (mAh g 1)
1.0/ 0.2 1.0/ 0.2 1.15/ 0.2
95 110 220
19 19 62
CV range (V vs. SCE) 1.7/ 1.0 1.5/ 0.2 2.0/ 1.0
Capacitance (F g 1)
Capacity (mAh g 1)
80 75 165
9 17 26
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
p- and n-doping potential range of the CVs for each polymer so as to assure the maximum coulombic efficiency of the charge – discharge processes. The specific capacitance of these polymer electrodes was estimated from the slope of the linear part of the diagrams derived from CVs in which the electrode potential was plotted against the p- and n-doping specific charges calculated by integration of the current density during discharge. The choice of the current collectors was determined by the polymer adherence and by the breakdown potentials of the electrolyte, particularly in the n-doping domain. The pMeT was electrosynthesized on platinum because of the negative potentials required for its n-doping, whereas the electrosynthesis of pFPT and pDTT1 was performed on carbon paper. The pMeT shows the highest specific capacity and capacitance of both the p- and n-doped forms, with the values of the p-doped form significantly higher than those of the n-doped. The capacitance data of pFPT of the p- and n-doped forms are similar, although lower than 100 F g 1, but the capacity value of the n-doped form is half of that of the p-doped, and significantly lower than that of the p- and n-doped pDTT1; a low capacity value like that of the n-doped pFPT makes the polymer unsuitable for an n/p type supercapacitor. The pDTT1 shows a very symmetric behavior with respect to the p-doping and n-doping capacity, and, to a lesser extent, the capacitance of the two doped forms. This is an advantage for the positive and negative electrode mass balancing in an n/p type supercapacitor, even if only the capacitance of the p-doped form reaches 100 F g 1. While with pDTT1, an n/p type device with electrodes of almost the same polymer mass loading can be realized, with pMeT it is necessary to couple electrodes of significantly different mass. This pMeT ‘‘trouble’’ is strongly rewarded by the higher capacity and capacitance values of its p- and n-doped forms, and by the lower cost, when compared to pDTT1. The choice of a conventional polymer such as pMeT, which can be easily produced starting from low-cost commercial monomer, allowed the realization of large area composite electrodes which are of great interest for the industrial scale up; we especially focused our attention on the development of the n/p type supercapacitors with these pMeT composite electrodes and demonstrated their cycling stability over several thousands cycles [2].
495
The preparation of composite electrodes laminated on proper current collectors involves the addition of carbon and binder, since the pMeT is a powder with scarce binding properties. The presence of carbon assures a good electric contact between the polymer particles and current collectors, and the optimization of the polymer to carbon and binder ratio in the composite electrodes are important issues. Composite polymer electrodes with 30%, 55% and 80% w/w of pMeT were prepared and tested in both the p- and n-doped states by impedance spectroscopy in three-electrode mode and the spectra of the composite electrodes with 80% of pMeT are shown in Fig. 1a and d. While in the p-doped state, the composite electrodes with 80% of pMeT displayed a low electrode charge resistance (2 V cm2) and a high capacitance (220 F g 1 of pMeT, i.e. the same value obtained from CVs of pMeT electrosynthesized on current collectors and reported in Table 1 [1]), in the n-doped state, they displayed a very high electrode charge resistance which makes their use unsuitable as negative electrodes in n/p type supercapacitors. Therefore, the maximum percentage of pMeT allowed in the composites to be used as negative electrodes is 55%. Fig. 1b, c, e, f shows the spectra of double-layer carbon composite electrodes, based on two high surface area activated carbons commercially available (AC1 and AC3 type), tested both as positive and negative electrodes. These spectra demonstrate that the capacitance of the activated carbon composite materials not only depends on the carbon surface area (higher than 2000 m2 g 1 for both AC1 and AC3) but also on their pore size distribution, which in turn significantly affects the resistance to charge them. The composite electrodes based on the AC3 carbon are interesting for their low resistances (9 and 2.5 V cm2 for the positive and the negative electrodes, respectively) but their capacitances are lower (74 and 84 F g 1 for the positive and the negative electrodes, respectively) than those of the composite based on AC1 carbon (125 and 158 F g 1 for the positive and the negative electrodes, respectively). A comparison of the spectra of the activated carbon and the pMeT composite electrodes clearly demonstrates the great advantage in the use of a pMeT-based positive electrode. In fact, from the point of view of the double-layer supercapacitors, the replacement of the positive carbon electrode with a pMeT-based is an advantage to attain the lowering of the device ESR.
496
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
maintaining the total mass loading of the two electrodes constant, at about 18 mg cm 2 [3,4]. The proper balancing of the positive and negative electrode masses was an important issue: while the n/p type pMeT-based supercapacitor was assembled by considering the specific capacitances of the two electrodes (taking place the faradic charge processes of pMeT in a potential range of ca. 0.5 V for each electrode), the hybrid supercapacitor was assembled by taking into account both the specific capacitances and the different potential ranges for the charge processes of the two electrode materials. In fact, a potential excursion of at least 2 V for the electrostatic charge process of the activated carbon electrode is required to reach cell voltages higher than 2.5 V for the charged hybrid supercapacitor. Therefore, the n/p type supercapacitor was assembled with the active mass of the negative electrode double than that of the positive (7.6 and 3.1 mg cm 2, respectively), whereas the hybrid supercapacitor was assembled with an active mass loading of the negative electrode lower than that of the positive (6.5 and 9.4 mg cm 2). Fig. 2 compares the voltage profiles during the galvanostatic discharge at 5 mA cm 2 of the n/p type and hybrid supercapacitors vs. the capacity expressed in mAh g 1 of the total active electrode materials. Note
Fig. 1. Impedance spectra of composite electrodes positively (a, b, c) and negatively (d, e, f) charged and with ca. 9 mg cm 2 of active mass (except electrode (d) with 7.5 mg cm 2). The electrode compositions were (a) pMeT 80% – AB 15% – binder 5%; (b, e) AC1 90% – SFG44 5% – binder 5%; (c, f) AC3 80% – SFG44 15% – binder 5%; (d) pMeT 80% – SFG44 15% – binder 5%.
The same advantage is from the point of view of the n/ p polymer supercapacitor in which the negative electrode, the most resistive, is replaced by a double-layer activated carbon. Consequently, an appealing supercapacitor configuration is a hybrid configuration with the negative electrode based on 90% AC1 doublelayer carbon and the positive based on 80% pMeT electrodes. However, we have also continued the investigation of the n/p type supercapacitor with the negative electrode based on 55% pMeT and the positive based on 80% pMeT electrodes. These prospects have been confirmed by tests on supercapacitors which were assembled by roughly
Fig. 2. Voltage profiles of (——) the n/p type supercapacitor and of ( – – ) the hybrid supercapacitor vs. the delivered capacity in mAh per gram of total active materials during a galvanostatic discharge at 5 mA cm 2. The electrode compositions were pMeT 80% – AB 15% – binder 5% for the pMeT positive electrodes, pMeT 55% – SFG44 5% – binder 5% for the pMeT negative electrode and AC1 90% – SFG44 5% – binder 5% for the activated carbon electrode.
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
that the profiles are referred to the discharge in different potential ranges (the cut-off potential of the hybrid supercapacitor was carefully selected). The figure clearly evidences the different ESR of the two devices, which are in agreement with the data attained from the impedance spectroscopy. From the profiles in Fig. 2, it is possible to calculate the capacitances of the two devices. The capacitance per gram of the total active materials is almost the same for the n/p type and the hybrid supercapacitors (37 and 39 F g 1, respectively), while that per gram of the total composite materials is quite different, i.e. 22 F g 1 for the n/p type supercapacitor to be compared with 33 F g 1 for the hybrid device. This is for the lower active material content in the pMeT negative electrode, and as discussed above, increasing the percentage of the electroactive polymer seems to be a nonfeasible goal. Because of the shorter potential excursion of the discharge process, also the specific capacity, related to the total active materials, of the n/p type device is lower (9 mAh g 1) than that of the hybrid (19 mAh g 1), and even worse, if the total composite mass is taken into account. Fig. 3 shows the Ragone plot for the hybrid supercapacitor attained with the data from the galva-
497
nostatic charge– discharge cycles at different current densities. Specific energy and power densities were calculated by taking into account the total composite masses, by the formulas E = imVdt/m and Pav =E/Dtd, where i is the current density, V is the cell voltage during discharge down to 1.0 V, m is the total mass of the positive and negative composite electrodes per cm2 (mass of current collectors is not included) and Dtd is the discharge time. In the Ragone plot, the data of the double-layer carbon supercapacitor based on the AC1 carbon (cut-off potentials 2.8– 1.0 V) are also included for comparison [3]. From Fig. 3, it is clear that the hybrid device, as expected, outperforms the double-layer supercapacitor, particularly at high current densities, since the resistance related to the charge process of the polymer positive electrode is lower than that of the carbon positive electrode, as discussed above.
4. Conclusions Our study on conducting polymers for the supercapacitor applications demonstrates that a conventional polymer such as pMeT can be successfully used in the supercapacitor technology when a hybrid configuration is realized. While the pMeT application in the n/p type supercapacitors cannot be envisioned, mainly because a high polymer content in the composite, such as 80%, is not viable for the negative electrode, the replacement of the positive activated carbon electrode of a double-layer carbon supercapacitor with a pMeT electrode is a great advantage because the hybrid supercapacitor outperforms the double-layer carbon supercapacitors presently on the market, both in terms of specific energy and power.
Acknowledgements
Fig. 3. Ragone plot for the hybrid supercapacitor (E) (cut-off: 1.0 – 3.0 V) and the double-layer carbon supercapacitor (*) (cut-off: 1.0 – 2.8 V). Labels indicate the current density in mA cm 2. The electrode compositions were pMeT 80% – AB 15% – binder 5% for the pMeT electrode and AC1 90% – SFG44 5% – binder 5% for the activated carbon electrode.
Project funded by Progetto Finalizzato ‘‘Materiali Speciali per Tecnologie Avanzate II’’ (contract no. 99.01.824.PF34), by MURST 60% and by European Commission within JOULE III (contract no. JOE3CT97-0047, SCOPE project). The authors wish to thank the partners in the SCOPE project CNAM (France), ENEA’s Department of Energy (Italy), Arcotronics Italia (Italy) and CEAC-Exide (France),
498
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
for the contribution in the development of hybrid supercapacitors.
References [1] M. Mastragostino, C. Arbizzani, R. Paraventi, A. Zanelli, J. Electrochem. Soc. 147 (2000) 407.
[2] M. Mastragostino, R. Paraventi, A. Zanelli, J. Electrochem. Soc. 147 (2000) 3167. [3] A. Di Fabio, A. Giorgi, M. Mastragostino, F. Soavi, J. Electrochem. Soc. 148 (2001) A845. [4] C. Arbizzani, M. Mastragostino, F. Soavi, J. Power Sources, 100 (2001) 164.
Conducting polymers as electrode materials in supercapacitors Marina Mastragostino a,*, Catia Arbizzani b, Francesca Soavi a a Radiochimiche e Metallurgiche, Unita’ Complessa di Istituti, Scienze Chimiche, Radiochimiche e Metallurgiche, University of Bologna, via San Donato 15, I-40127 Bologna, Italy b Department of Chemistry ‘‘G. Ciamician’’, University of Bologna, via Selmi 2, I-40126 Bologna, Italy
Abstract This paper summarizes the performance data of conventional and especially designed thiophene-based conducting polymers for use as positive and negative electrodes in n/p type supercapacitors. Performance data of polymer composite electrodes are also compared with those of high surface area carbon-based composite electrodes. On the basis of capacity, capacitance and electrode charging resistance data, we selected the best electrode materials, and assembled and tested galvanostatic charge – discharge cycles n/p type pMeT-based supercapacitors and hybrid supercapacitors with pMeT as positive electrode active material and activated carbon as negative. The results of this investigation demonstrate that a conventional polymer such as pMeT can be successfully used in the supercapacitor technology when a hybrid configuration is realized; its use is, indeed, a great advantage because the hybrid supercapacitor outperforms the double-layer carbon supercapacitors presently on the market in terms of specific energy and power. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical supercapacitor; Hybrid supercapacitor; p-Doping; n-Doping; n/p type supercapacitor; Poly3-methylthiophene
1. Introduction Supercapacitors and batteries are energy storage and conversion systems which satisfy, in a complementary way, the requirements of high specific power and energy, respectively. In the last years, the significative advances achieved in the materials for supercapacitors brought about an extension of their application to the field of rechargeable batteries and, at present, the best performing devices on the market are the double-layer supercapacitors with activated carbons of high surface area (1500 –2400 m2 g 1) as electrode active materials. Amelioration in these types of devices is expected by lowering the equiv*
Corresponding author. Tel.: +39-051-209-9798; fax: +39-051209-9365. E-mail address: [email protected] (M. Mastragostino).
alent series resistance (ESR) and this can be attained by optimizing the pore size distribution of the carbon materials. Electronically conducting polymers (ECPs) are promising materials for the realization of high performance supercapacitors, as they are characterized by high specific capacitances (the charge processes pertain to the whole polymer mass and not only to the surface, as in the case of double-layer activated carbons) and by high conductivities in the charged states; furthermore, their charge–discharge processes are generally fast. These features suggest the possibility to develop devices with low ESR and high specific energy and power. The most promising polymer supercapacitor configuration in terms of charge storage capacity and cell voltage and able to outperform the double-layer carbon supercapacitors is the n/ p type, i.e. the configuration in which an n-doped
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 0 9 3 - 0
494
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
as in Ref. [1]. The composite electrodes were prepared by mixing active materials (pMeT or activated carbon AC1 and AC3), conducting additive (graphite, SFG44, Timcal, or acetylene black, AB, Hoechst), binder (carboxy methyl cellulose, Cmc, Aldrich) and PTFE (Du Pont), and the supercapacitors were assembled with propylene carbonate (PC, Fluka) —1 M Et4NBF4 as electrolyte, as in Ref. [4]. The cyclic voltammetries (CVs) of the electrodes were performed at 20 mV s 1 with a 273A PAR potentiostat/galvanostat. Impedance measurements were carried out in three-electrode mode using a Solartron SI 1255 frequency response analyzer coupled with a 273A PAR potentiostat/galvanostat; an AC amplitude of 5 mV was used and data were collected in the frequency range of 100 kHz – 10 mHz taking 10 points per decade. The cyclability performance of the supercapacitors was tested by repeated charge – discharge galvanostatic cycles at different current densities and cut-off potentials with a 273A PAR potentiostat/galvanostat and a Perkin Elmer VMP. An Ag quasi-reference electrode, whose potential was checked vs. SCE for each set of experiments, was used for the experiments in three-electrode mode and to monitor the electrode potentials during the charge– discharge cycles. All the electrochemical tests were performed in a MBraun Labmaster 150 dry box filled with argon, as reported in Ref. [4].
polymer and a p-doped one are used as negative and positive electrodes, respectively. However, this is not an easy-to-realize configuration and the difficulties are mainly related to the n-doping process. The charge– discharge processes of polymer-based supercapacitors are faradic and those of the n/p devices take place in a cell potential range settled by the n- and p-doping – undoping process potentials of the polymer electrodes. We studied the thiophene-based polymer materials, such as poly(dithieno[3,4-b:3V,4V-d]thiophene) (pDTT1) and poly(3-p-fluorophenylthiophene) (pFPT), especially designed to optimize the working potential range of each electrode, by taking into account the need to realize high cell voltages compatible with the breakdown potentials of the electrolyte on the current collectors, as well as a conventional polymer such as poly(3-methylthiophene) (pMeT) for the development of the n/p type supercapacitors [1,2]. We also developed a new type of polymer supercapacitor, a hybrid device based on pMeT as positive electrode and activated carbon as negative [3]. In this paper, capacity and capacitance data of the above reported polymers in the p- and n-doped forms, as well as the data of positively and negatively charged activated carbons, are reported and discussed in view of their use as positive and negative electrodes in supercapacitors. Data from the charge– discharge galvanostatic cycles of the n/p type and C/ECP hybrid supercapacitors are also reported and the pros and cons of the use of polymer electrodes are discussed.
3. Results and discussion Table 1 summarizes the specific capacity and capacitance data, obtained by CVs at 20 mV s 1, and the linear potential excursion of each electrode during the discharge of the p- and n-doped forms of pFPT, pDTT1 and pMeT, directly electrosynthesized on the current collectors (polymer mass loading was in the range from 4 to 9 mg cm 2). We selected the
2. Experimental Poly(3-p-fluorophenylthiophene) and poly(dithieno[3,4-b:3V,4V-d]thiophene) were galvanostatically grown on carbon paper electrodes and poly(3-methylthiophene) on Pt electrodes by monomer oxidation, Table 1 Electrode specific capacitance and capacity from CVs at 20 mV s ECP
pFPT pDTT1 pMeT
1
of polymers electrosynthesized on current collectors
p-Doping
n-Doping
CV range (V vs. SCE)
Capacitance (F g 1)
Capacity (mAh g 1)
1.0/ 0.2 1.0/ 0.2 1.15/ 0.2
95 110 220
19 19 62
CV range (V vs. SCE) 1.7/ 1.0 1.5/ 0.2 2.0/ 1.0
Capacitance (F g 1)
Capacity (mAh g 1)
80 75 165
9 17 26
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
p- and n-doping potential range of the CVs for each polymer so as to assure the maximum coulombic efficiency of the charge – discharge processes. The specific capacitance of these polymer electrodes was estimated from the slope of the linear part of the diagrams derived from CVs in which the electrode potential was plotted against the p- and n-doping specific charges calculated by integration of the current density during discharge. The choice of the current collectors was determined by the polymer adherence and by the breakdown potentials of the electrolyte, particularly in the n-doping domain. The pMeT was electrosynthesized on platinum because of the negative potentials required for its n-doping, whereas the electrosynthesis of pFPT and pDTT1 was performed on carbon paper. The pMeT shows the highest specific capacity and capacitance of both the p- and n-doped forms, with the values of the p-doped form significantly higher than those of the n-doped. The capacitance data of pFPT of the p- and n-doped forms are similar, although lower than 100 F g 1, but the capacity value of the n-doped form is half of that of the p-doped, and significantly lower than that of the p- and n-doped pDTT1; a low capacity value like that of the n-doped pFPT makes the polymer unsuitable for an n/p type supercapacitor. The pDTT1 shows a very symmetric behavior with respect to the p-doping and n-doping capacity, and, to a lesser extent, the capacitance of the two doped forms. This is an advantage for the positive and negative electrode mass balancing in an n/p type supercapacitor, even if only the capacitance of the p-doped form reaches 100 F g 1. While with pDTT1, an n/p type device with electrodes of almost the same polymer mass loading can be realized, with pMeT it is necessary to couple electrodes of significantly different mass. This pMeT ‘‘trouble’’ is strongly rewarded by the higher capacity and capacitance values of its p- and n-doped forms, and by the lower cost, when compared to pDTT1. The choice of a conventional polymer such as pMeT, which can be easily produced starting from low-cost commercial monomer, allowed the realization of large area composite electrodes which are of great interest for the industrial scale up; we especially focused our attention on the development of the n/p type supercapacitors with these pMeT composite electrodes and demonstrated their cycling stability over several thousands cycles [2].
495
The preparation of composite electrodes laminated on proper current collectors involves the addition of carbon and binder, since the pMeT is a powder with scarce binding properties. The presence of carbon assures a good electric contact between the polymer particles and current collectors, and the optimization of the polymer to carbon and binder ratio in the composite electrodes are important issues. Composite polymer electrodes with 30%, 55% and 80% w/w of pMeT were prepared and tested in both the p- and n-doped states by impedance spectroscopy in three-electrode mode and the spectra of the composite electrodes with 80% of pMeT are shown in Fig. 1a and d. While in the p-doped state, the composite electrodes with 80% of pMeT displayed a low electrode charge resistance (2 V cm2) and a high capacitance (220 F g 1 of pMeT, i.e. the same value obtained from CVs of pMeT electrosynthesized on current collectors and reported in Table 1 [1]), in the n-doped state, they displayed a very high electrode charge resistance which makes their use unsuitable as negative electrodes in n/p type supercapacitors. Therefore, the maximum percentage of pMeT allowed in the composites to be used as negative electrodes is 55%. Fig. 1b, c, e, f shows the spectra of double-layer carbon composite electrodes, based on two high surface area activated carbons commercially available (AC1 and AC3 type), tested both as positive and negative electrodes. These spectra demonstrate that the capacitance of the activated carbon composite materials not only depends on the carbon surface area (higher than 2000 m2 g 1 for both AC1 and AC3) but also on their pore size distribution, which in turn significantly affects the resistance to charge them. The composite electrodes based on the AC3 carbon are interesting for their low resistances (9 and 2.5 V cm2 for the positive and the negative electrodes, respectively) but their capacitances are lower (74 and 84 F g 1 for the positive and the negative electrodes, respectively) than those of the composite based on AC1 carbon (125 and 158 F g 1 for the positive and the negative electrodes, respectively). A comparison of the spectra of the activated carbon and the pMeT composite electrodes clearly demonstrates the great advantage in the use of a pMeT-based positive electrode. In fact, from the point of view of the double-layer supercapacitors, the replacement of the positive carbon electrode with a pMeT-based is an advantage to attain the lowering of the device ESR.
496
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
maintaining the total mass loading of the two electrodes constant, at about 18 mg cm 2 [3,4]. The proper balancing of the positive and negative electrode masses was an important issue: while the n/p type pMeT-based supercapacitor was assembled by considering the specific capacitances of the two electrodes (taking place the faradic charge processes of pMeT in a potential range of ca. 0.5 V for each electrode), the hybrid supercapacitor was assembled by taking into account both the specific capacitances and the different potential ranges for the charge processes of the two electrode materials. In fact, a potential excursion of at least 2 V for the electrostatic charge process of the activated carbon electrode is required to reach cell voltages higher than 2.5 V for the charged hybrid supercapacitor. Therefore, the n/p type supercapacitor was assembled with the active mass of the negative electrode double than that of the positive (7.6 and 3.1 mg cm 2, respectively), whereas the hybrid supercapacitor was assembled with an active mass loading of the negative electrode lower than that of the positive (6.5 and 9.4 mg cm 2). Fig. 2 compares the voltage profiles during the galvanostatic discharge at 5 mA cm 2 of the n/p type and hybrid supercapacitors vs. the capacity expressed in mAh g 1 of the total active electrode materials. Note
Fig. 1. Impedance spectra of composite electrodes positively (a, b, c) and negatively (d, e, f) charged and with ca. 9 mg cm 2 of active mass (except electrode (d) with 7.5 mg cm 2). The electrode compositions were (a) pMeT 80% – AB 15% – binder 5%; (b, e) AC1 90% – SFG44 5% – binder 5%; (c, f) AC3 80% – SFG44 15% – binder 5%; (d) pMeT 80% – SFG44 15% – binder 5%.
The same advantage is from the point of view of the n/ p polymer supercapacitor in which the negative electrode, the most resistive, is replaced by a double-layer activated carbon. Consequently, an appealing supercapacitor configuration is a hybrid configuration with the negative electrode based on 90% AC1 doublelayer carbon and the positive based on 80% pMeT electrodes. However, we have also continued the investigation of the n/p type supercapacitor with the negative electrode based on 55% pMeT and the positive based on 80% pMeT electrodes. These prospects have been confirmed by tests on supercapacitors which were assembled by roughly
Fig. 2. Voltage profiles of (——) the n/p type supercapacitor and of ( – – ) the hybrid supercapacitor vs. the delivered capacity in mAh per gram of total active materials during a galvanostatic discharge at 5 mA cm 2. The electrode compositions were pMeT 80% – AB 15% – binder 5% for the pMeT positive electrodes, pMeT 55% – SFG44 5% – binder 5% for the pMeT negative electrode and AC1 90% – SFG44 5% – binder 5% for the activated carbon electrode.
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
that the profiles are referred to the discharge in different potential ranges (the cut-off potential of the hybrid supercapacitor was carefully selected). The figure clearly evidences the different ESR of the two devices, which are in agreement with the data attained from the impedance spectroscopy. From the profiles in Fig. 2, it is possible to calculate the capacitances of the two devices. The capacitance per gram of the total active materials is almost the same for the n/p type and the hybrid supercapacitors (37 and 39 F g 1, respectively), while that per gram of the total composite materials is quite different, i.e. 22 F g 1 for the n/p type supercapacitor to be compared with 33 F g 1 for the hybrid device. This is for the lower active material content in the pMeT negative electrode, and as discussed above, increasing the percentage of the electroactive polymer seems to be a nonfeasible goal. Because of the shorter potential excursion of the discharge process, also the specific capacity, related to the total active materials, of the n/p type device is lower (9 mAh g 1) than that of the hybrid (19 mAh g 1), and even worse, if the total composite mass is taken into account. Fig. 3 shows the Ragone plot for the hybrid supercapacitor attained with the data from the galva-
497
nostatic charge– discharge cycles at different current densities. Specific energy and power densities were calculated by taking into account the total composite masses, by the formulas E = imVdt/m and Pav =E/Dtd, where i is the current density, V is the cell voltage during discharge down to 1.0 V, m is the total mass of the positive and negative composite electrodes per cm2 (mass of current collectors is not included) and Dtd is the discharge time. In the Ragone plot, the data of the double-layer carbon supercapacitor based on the AC1 carbon (cut-off potentials 2.8– 1.0 V) are also included for comparison [3]. From Fig. 3, it is clear that the hybrid device, as expected, outperforms the double-layer supercapacitor, particularly at high current densities, since the resistance related to the charge process of the polymer positive electrode is lower than that of the carbon positive electrode, as discussed above.
4. Conclusions Our study on conducting polymers for the supercapacitor applications demonstrates that a conventional polymer such as pMeT can be successfully used in the supercapacitor technology when a hybrid configuration is realized. While the pMeT application in the n/p type supercapacitors cannot be envisioned, mainly because a high polymer content in the composite, such as 80%, is not viable for the negative electrode, the replacement of the positive activated carbon electrode of a double-layer carbon supercapacitor with a pMeT electrode is a great advantage because the hybrid supercapacitor outperforms the double-layer carbon supercapacitors presently on the market, both in terms of specific energy and power.
Acknowledgements
Fig. 3. Ragone plot for the hybrid supercapacitor (E) (cut-off: 1.0 – 3.0 V) and the double-layer carbon supercapacitor (*) (cut-off: 1.0 – 2.8 V). Labels indicate the current density in mA cm 2. The electrode compositions were pMeT 80% – AB 15% – binder 5% for the pMeT electrode and AC1 90% – SFG44 5% – binder 5% for the activated carbon electrode.
Project funded by Progetto Finalizzato ‘‘Materiali Speciali per Tecnologie Avanzate II’’ (contract no. 99.01.824.PF34), by MURST 60% and by European Commission within JOULE III (contract no. JOE3CT97-0047, SCOPE project). The authors wish to thank the partners in the SCOPE project CNAM (France), ENEA’s Department of Energy (Italy), Arcotronics Italia (Italy) and CEAC-Exide (France),
498
M. Mastragostino et al. / Solid State Ionics 148 (2002) 493–498
for the contribution in the development of hybrid supercapacitors.
References [1] M. Mastragostino, C. Arbizzani, R. Paraventi, A. Zanelli, J. Electrochem. Soc. 147 (2000) 407.
[2] M. Mastragostino, R. Paraventi, A. Zanelli, J. Electrochem. Soc. 147 (2000) 3167. [3] A. Di Fabio, A. Giorgi, M. Mastragostino, F. Soavi, J. Electrochem. Soc. 148 (2001) A845. [4] C. Arbizzani, M. Mastragostino, F. Soavi, J. Power Sources, 100 (2001) 164.