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Electropolymerization of poly(3-methylthiophene) in pyrrolidinium-based ionic liquids for hybrid supercapacitors
Electrochimica Acta 53 (2008) 7967–7971
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electropolymerization of poly(3-methylthiophene) in pyrrolidinium-based ionic liquids for hybrid supercapacitors Maurizio Biso a , Marina Mastragostino a,∗,1 , Maria Montanino b , Stefano Passerini b,1 , Francesca Soavi a,1 a b
University of Bologna, Department of Metal Science, Electrochemistry and Chemical Techniques, Via San Donato 15, 40127 Bologna, Italy Agency for the New Technologies, Energy and the Environment (ENEA), TER Department, Centro Ricerche Casaccia, Via Anguillarese 301, Rome 00123, Italy
a r t i c l e
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Article history: Received 13 February 2008 Received in revised form 5 June 2008 Accepted 7 June 2008 Available online 17 June 2008 Keywords: Poly(3-methylthiophene) (pMeT) Pyrrolidinium-based ionic liquid Electropolymerization Hybrid supercapacitor Specific capacitance
a b s t r a c t The ionic liquids (ILs) N-butyl-N-methyl-pyrrolidinium trifluoromethanesulfonate (PYR14 Tf) and N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide (PYR13 FSI) are investigated as electropolymerization media for poly(3-methylthiophene) (pMeT) in view of their use in carbon/IL/pMeT hybrid supercapacitors. Data on the viscosity, solvent polarity, conductivity and electrochemical stability of PYR14 Tf and PYR13 FSI as well as the effect of their properties on the electropolymerization and electrochemical performance of pMeT, which features >200 Fg−1 at 60 ◦ C when prepared and tested in such ILs, are reported and discussed; the results of the electrochemical characterization in N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide of the so-obtained pMeT are also given, for comparison. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The interest in non-aqueous, high-energy supercapacitors freed of safety concerns and with attention to the operating temperature is growing worldwide [1]. One of the strategies which can be pursued to reach the high specific energy requirement is the use of pseudocapacitive electrode materials of charge storage capability higher than that of double-layer carbon electrodes. Electronically conducting polymers, which can be doped/undoped by fast faradic processes that give rise to specific pseudocapacitance even higher than 200 F g−1 , are under study mainly as positive electrode materials. Poly(3-methylthiophene) (pMeT) is one of the most attracting polymers because it is based on 3-substituted thiophene ring which, avoiding ␣, mislinking during polymerization, brings to a polymer of good electronic properties and, featuring low molecular weight, positively affects the specific properties of the material [2]. While pMeT can be successfully used as positive electrode for its high p-doping/undoping cycling stability, it can barely be used as negative electrode for its low conductivity, charge-storage capability and stability in the n-doped state. On the other hand, high performance supercapacitors can be devel-
∗ Corresponding author. Tel.: +39 51 2099798; fax: +39 51 2099365. E-mail address: [email protected] (M. Mastragostino). 1 ISE member. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.06.008
oped by coupling the pMeT positive electrode with a carbonaceous negative one that upon cycling is negatively charged/discharged by electrostatic process [3,4]. Such hybrid configuration allows to attain high cell voltages, even higher than 3.0 V, because the negative electrode can experience a wide potential excursion limited only by the electrochemical stability window (ESW) of the electrolyte. This is where ionic liquid (IL) electrolytes, and particularly those based on the N-alkyl-N-methyl-pyrrolidinium cation, are called upon. The wide ESW, high thermal stability, good conductivity and low vapor pressure of such ILs make them particularly attracting for high-voltage supercapacitors operating above room temperature (RT), i.e. in operative conditions where conventional organic electrolytes such as those based on acetonitrile lack in safety [5]. Indeed, the use of the hydrophobic IL N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14 TFSI) in carbon/IL/pMeT hybrid supercapacitor allowed to reach at 60 ◦ C the maximum cell voltage of 3.6 V over 16,000 charge/discharge cycles [6]. However, using IL in supercapacitors requires that the electrode materials are properly developed in order to deliver in IL at least the high specific capacitance exhibited in conventional electrolytes, and, as it concerns pMeT, the polymer electrode should be electrosynthesized in the same IL used to assemble the supercapacitor [7,8]. ILs are effective growth media for conducting polymers and while they are considered green, recyclable solvents for chemical and electrochemical synthesis procedures, their use results in significantly altered film morphologies with
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respect to those obtained by polymerization in conventional electrolytes [9–14]. Particularly, we reported that polymerization in PYR14 TFSI yield to pMeT with thinner interconnected polymer chains than those of the material synthesized in acetonitrile-tetra alkylammonium salt and such morphology provides an easier electrolyte access to the polymer p-doping sites, so that capacitances even exceeding 195 F g−1 can be reached in PYR14 TFSI at 60 ◦ C [7,8]. Despite the good results achieved by using PYR14 TFSI, in order to develop IL-based supercapacitors of high specific performance operating in a wide temperature range, new ILs of improved conductivity and lower formula weight than the former without penalty in ESW should be used. Indeed, while the PYR14 TFSI’s ESW and conductivity above RT are of interest for supercapacitor application, the latter significantly lowers below RT. Furthermore, given that upon the charge/discharge of hybrid supercapacitors the IL ions are taken from/released to the electrolyte to balance the electrodes’ charges, enough IL has to be used in the cells so as not to limit the charge storage in the electrodes. Therefore, the high formula weight of PYR14 TFSI (422.41 g mol−1 , mainly contributed by the heavy TFSI− anion) may negatively affect the gravimetric performance of the complete supercapacitor module. In this work pyrrolidinium-based ILs with lighter anions than TFSI− , namely the N-butyl-N-methyl-pyrrolidinium trifluoromethanesulfonate (PYR14 Tf) and N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide (PYR13 FSI), are investigated in view of their use in pMeT-based hybrid supercapacitors with attention to their conductivity and ESW. The results of the study on the pMeT electropolymerization and electrochemical characterization in such PYR14 Tf and PYR13 FSI electrolytes are also reported and discussed in relation to the properties of the investigated ILs, data on the electrochemical characterization in PYR14 TFSI of the soobtained pMeT are included, too. 2. Experimental 2.1. Ionic liquid synthesis and characterization The PYR13 FSI was synthesized in ambient air using Nmethylpyrrolidine (Aldrich), bromopropane (Aldrich) and lithium bis(fluorosulfonyl)imide (LiFSI from Dai-Ichi Kogyo Seiyaku DKS Co., Ltd.) as described elsewhere [15], and then transferred to a dry-room with a relative humidity lower than 0.2% at 20 ◦ C. The PYR14 Tf (99%) and PYR14 TFSI (purum 98%) were commercial products from Solvionic and Solvent Innovation, respectively. Before their characterization the three ILs were dried under dynamic vacuum for 10 h at 50 ◦ C followed by additional 10 h at 110 ◦ C (Büchi B-585 glass oven, Varian SH-100 oil-free vacuum pump). Thermogravimetric measurements (TGA) were carried out with a Mettler Toledo TGA/SDTA A851 System in the temperature range from 25 to 600 ◦ C under N2 flow with a scanning rate of 20 ◦ C min−1 ; an alumina pan with lid was used. The solvent polarity of the ILs was evaluated in ambient atmosphere on the basis of the 2,6-diphenyl-4-(2,4,6triphenylpyridinium-1-yl)phenolate (Reichardt dye, Sigma– Aldrich) solvatochromism [16] measured by a PerkinElmer Lambda 19 spectrometer; the data are reported in terms of normalized N molar transition energy EN T with ET = 0 for TMS and 1 for water. The viscosity, conductivity, and ESW of the ILs were determined on samples stored and tested in the dry-room which exhibited water contents below 1 ppm as evaluated by Karl-Fisher titration (METTLER TOLEDO DL32 coulometer). The viscosity measurements were carried out from RT up to 80 ◦ C (0.1 ◦ C/min heating scan
rate) using a HAAKE RheoStress 600 rheometer. The conductivity of the ILs was determined by a conductivity meter AMEL 160 in the temperature ranging from −40 to 100 ◦ C by using a climatic test chamber (Binder GmbH MK53) controlled by a home-made software. The ILs were housed in sealed, glass conductivity cells (AMEL 192/K1) equipped with two porous platinum electrodes (cell constant of 1.0 ± 0.1 cm). The ESW experiments were performed by linear sweep voltammetry (LSV) using a three-electrode, glass micro-cell, filled with a small amount of IL (0.4 ml). The working electrode was a glass-sealed platinum (E-DAQ), the counter was a platinum foil (about 0.5 cm2 ), and the reference was a silver wire immersed in a 0.01 M solution of AgSO3 CF3 (Aldrich) in PYR14 TFSI, separated from the cell compartment with a fine glass frit (E-DAQ). This reference electrode, often used today [17], has been proved stable for at least three weeks. The potential of this electrode, frequently controlled against a 5 mM solution of Ferrocene in PYR14 TFSI, was found to be +0.39 V vs. Fc/Fc+ (at 20 ◦ C). High purity argon (Air Liquide) was flushed over the electrolyte for at least 30 min before the start of the experiment and continued during the experiment. Back-diffusion of air was prevented flowing the outgoing gas through silicon oil. 2.2. pMeT electropolymerization and characterization pMeT was electropolymerized at RT in single compartment cells loaded with 1 M solutions of methylthiophene (MeT, Aldrich, distilled before use) in PYR13 FSI and PYR14 Tf (dried over night at 100 ◦ C under dynamic vacuum by Büchi Glass Oven B-580) and housed in a dry box (MBraun Labmaster 130 or TEKATOM with Ar atmosphere). The polymers grown in PYR13 FSI and in PYR14 Tf are labelled pMeT-a and pMeT-b, respectively. The polymerization was performed by cyclic voltammetry (CV) at 50 mV/s in various potential windows and by chronopotentiometry (GLV) with current density of 10 mA cm−2 followed by undoping at −2.5 mA cm−2 in order to evaluate the electrochemical stoichiometry of polymerization and, thus, the amount of polymerized pMeT in the undoped state. The working electrode was a glassy carbon (GC, 0.07 cm2 area), the counter a carbon paper electrode with area and charge storage capability significantly higher than those of the working, and the reference was a silver wire whose potential measured vs. the reversible redox couple ferrocene/ferrocinium (Fc/Fc+ ) was EAg = (EFc/Fc+ + 0.200 ± 0.010) V; hereinafter the electrode potentials are given vs. Fc/Fc+ . The electropolymerizations were carried in IL–1 M MeT solutions because at lower monomer concentrations, such as 0.1 M, pMeT films with bad adhesion to the GC current collector were obtained and during the voltammetric polymer growth at 50 mV s−1 formation of soluble oligomers was observed. The specific capacitance of the pMeT electrodes which featured 3–5 mg cm−2 of polymer was evaluated from the voltammetric discharges at 20 mV s−1 and 60 ◦ C (controlled by a Thermoblock FALC oven) by the slope of the electrode potential vs. integral over time of the current. The electropolymerization of pMeT and the following electrochemical measurements were performed with a PerkinElmer VMP multichannel potentiostat/galvanostat. 3. Results and discussion The study on the properties of PYR13 FSI and PYR14 Tf ILs was carried out at first in view of their use in pMeT-based hybrid supercapacitors. Table 1 compares the formula weight, the temperature at which thermal decomposition of the IL takes place with a 5% weight loss (T5% w.l. ), the melting temperature (Tm ), and the normal-
M. Biso et al. / Electrochimica Acta 53 (2008) 7967–7971
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Table 1 Formula weight, temperature at which thermal decomposition of the IL takes place with a 5% weight loss (T5% w.l. ), melting temperature (Tm ), normalized solvent polarity (EN T ) at RT of the PYR13 FSI, PYR14 Tf, and PYR14 TFSI ILs Formula weight (g mol−1 )
IL PYR13 FSI PYR14 Tf PYR14 TFSI a b c
308.36 291.33 422.41
T5% w.l. (K) a
490 670 683
Tm (K) a
256 276b 270c
EN T at RT 0.56 0.63 0.57
from ref. [19]. from ref. [5]. from ref. [20].
ized solvent polarity (EN T ) at RT of such ILs with those of PYR14 TFSI. The Tf− and FSI− -based ILs feature a 30% lower formula weight than PYR14 TFSI, mainly for their lighter anions than TFSI− , and while the PYR14 Tf has almost the same thermal stability than PYR14 TFSI and can be used up to 300 ◦ C, the PYR13 FSI starts decomposing above 220 ◦ C, both far above the temperatures of interest for carbon/IL/pMeT hybrid supercapacitor applications whose operation regime is also affected by the polymer decomposition at about 180 ◦ C [18]. Among the three ILs, that feature EN T values roughly corresponding to those of short-chain primary and secondary alcohols [16], the PYR14 Tf is that with the highest solvent polarity and this reflects on its highest melting temperature and viscosity. The viscosity of PYR14 Tf, PYR14 TFSI, and PYR13 FSI at 20 ◦ C is 158, 95 and 45 mPa s, respectively, but it approaches 20–10 mPa s at 80 ◦ C for all the ILs investigated. The viscosity and the melting points of the ILs affect their conductivity which is of 2.0, 2.6, and 7.4 mS cm−1 at room temperature for PYR14 Tf, PYR14 TFSI, and PYR13 FSI, respectively. The conductivity dramatically drops below 0.1 mS cm−1 at ca. 10 ◦ C for PYR14 Tf and −5 ◦ C for PYR13 FSI and PYR14 TFSI, i.e. approaching the Tm , thus marking the lower temperature limit for the use of these ILs in supercapacitors. The linear sweep voltammetries in the investigated ILs reported in Fig. 1 indicate that their ESW at RT is wide up to 5 V and, given that the pMeT p-doping/undoping typically takes place at ca. 1 V vs. Fc/Fc+ and that a negative carbon electrode could be polarized down to −3 V vs. Fc/Fc+ without overcoming the cathodic limit of the ILs, the use of PYR13 FSI and PYR14 Tf in carbon/IL/pMeT hybrid supercapacitors could allow to reach cell voltages even approaching 4.0 V and high specific energy provided that high specific capacitance is attained. With the aim of developing polymer electrodes of high performance in PYR13 FSI and PYR14 Tf, we performed a study on pMeT electropolymerization in these ILs. Voltammetric polymer growths were carried out at first and Fig. 2 reports the cyclic voltammetries
Fig. 1. LSVs at 5 mVs−1 and RT of platinum electrodes in (a) PYR13 FSI, (b) PYR14 Tf, and (c) PYR14 TFSI ionic liquids.
Fig. 2. Voltammetric polymerization on GC electrode at 50 mV s−1 and RT of (a) pMeT-a in PYR13 FSI–1 M MeT and (b, c) pMeT-b in PYR14 Tf–1 M MeT; the dashed lines are the CVs of the bare GC in neat (a) PYR13 FSI and (b,c) PYR14 Tf ILs.
(CVs) at 50 mV s−1 and RT that provided pMeT-a (Fig. 2a) and pMeTb (Fig. 2b and c). On the basis of the charges involved during the anodic and cathodic voltammetric scans, 60 cycles were necessary to yield in PYR13 FSI–1 M MeT 3 mg cm−2 of undoped pMeT, value calculated excluding any contribution to the loading of IL eventually trapped in the polymer; during the last cycle the polymer reached a doping level of 25%, thus indicating an electrochemical stoichiometry typical for electrosynthesis of pMeT in conventional electrolyte solutions, which is of ca. 2.3 F/mol of polymerized monomer unit. Fig. 2b shows that in the case of the polymerization in PYR14 Tf–1 M MeT the CV currents were lower than those recorded in PYR13 FSI when almost the same potential range with the anodic limit of 1.3 V vs. Fc/Fc+ was used. This cannot be ascribed only to the conductivity of the PYR14 Tf which is lower than PYR13 FSI’s, rather it has to be related to its higher polarity which reflects on the stability of the radical cation that propagates the electropolymerization. The radical cation MeT•+ produced at the electrode surface can be stabilized by the IL anions which are present at a high concentration of ca. 4 mol L−1 in the polymerization bath, and this may be more marked when the most polar IL, i.e. PYR14 Tf, is used. Such phenomenon may favor the diffusion of MeT•+ far from the electrode surface, so that radical coupling may take place not only on the electrode surface but also in solution, bringing to an electrochemical polymerization yield lower than that attainable in PYR13 FSI, as also confirmed by the slow current increase with cycles of the CVs reported in Fig. 2b. The Fig. 2c shows that the shift of the anodic scan limit up to 1.6 V vs. Fc/Fc+ is effective for high pMeT polymerization yield and rate. Indeed, by this shift the concentration of the radical cation at the electrode surface increases and 20 cycles are enough to provide ca 3.4 mg cm−2 of undoped pMeT, as evaluated
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on the basis of the charges involved upon the anodic and cathodic scans. In order to investigate the effect of the different ILs on the properties of pMeT polymerized at the same rate, galvanostatic synthesis conditions GLV were adopted and the Fig. 3 reports the working electrode potential profiles upon the polymer growth of pMeT-a in PYR13 FSI–1 M MeT (Fig. 3a) and pMeT-b in PYR14 Tf–1 M MeT (Fig. 3b) at 10 mA cm−2 and the subsequent undoping at −2.5 mA cm−2 . In both cases the doping level was 28%, confirming the results of the previous voltammetric study, and thus 4.7 mg cm−2 of undoped pMeT was obtained. Notably, such GLV polymerizations were performed without stirring because we have evidences that it is deleterious for the polymerization yield, thus further confirming the importance of a high radical cation concentration at the electrode surface. The pMeT-a and pMeT-b electrodes were tested by CVs at 20 mV s−1 and 60 ◦ C in the neat PYR13 FSI and PYR14 Tf and the results are reported in Fig. 4. Fig. 4a shows that in the case of pMeT-a tested in PYR13 FSI the p-doping onset is at more positive potentials and the doping/undoping peaks are less separated with respect to the case of pMeT-b tested in PYR14 Tf. From the plots reported in Fig. 4b, specific capacitance values of 255 F g−1 and 210 F g−1 are evaluated for pMeT-a and pMeT-b, respectively. These are quite interesting results that demonstrate that PYR13 FSI and PYR14 Tf provide polymer electrodes performing in the same ILs like and even better than pMeT grown and tested in PYR14 TFSI [7,8], with the advantage of being electrolytes of lower weight and, in the case of PYR13 FSI, of higher conductivity in wider temperature range than PYR14 TFSI, thus more promising for hybrid supercapacitor applications. The different performance of pMeT-a and pMeT-b electrodes can be explained with the different nature of the ILs used for the tests and a different “quality” of the polymer. The two electrodes have the same charge storage capability and can deliver up to 60 mAh g−1 corresponding to a 20% doping level. However, Fig. 4b shows that such charge is delivered in electrode potential ranges narrower in PYR13 FSI than in PYR14 Tf, thus indicating that the pseudocapacitive response is faster in the former IL than
Fig. 4. (a) CVs at 20 mV s−1 and 60 ◦ C reported in terms of specific current (I) and (b) corresponding plots of the electrode potential vs. integral over time of the discharge specific current (Q) of pMeT-a tested in PYR13 FSI (solid lines, 4.7 mg cm−2 of undoped pMeT electropolymerized in PYR13 FSI) and of pMeT-b tested in PYR14 Tf (dashed lines, 4.8 mg cm−2 of undoped pMeT electropolymerized in PYR14 Tf).
in the latter. This can be explained with the higher conductivity of PYR13 FSI (13.2 mS cm−1 at 60 ◦ C) than PYR14 Tf (5.5 mScm−1 at 60 ◦ C), and, thus, with the higher mobility of the counterions involved in the p-doping/undoping processes. In order to further investigate the effect of the IL on the pMeT performance, we also cycled pMeT-a and pMeT-b in the same PYR14 TFSI IL at 60 ◦ C and we found that the specific capacitance of each electrode decreased by a 10% with respect to the values exhibited in PYR13 FSI and PYR14 Tf, respectively, with the capacitance of the former being always higher than the latter’s. This cannot be related to the conductivity of PYR14 TFSI that at 60 ◦ C is of 6.0 mS cm−1 and almost the same as PYR14 Tf’s. Rather, it can be argued that PYR13 FSI and PYR14 Tf, being incorporated in the polymer matrix upon the electropolymerization, may act as templates and affect the “quality” of the pMeT-a and pMeT-b electrodes. Particularly, the use of these ILs as growing media may provide tailored polymer morphologies which favor the transport of the FSI− and Tf− counterions required for the doping process with respect that of the larger TFSI− anion. 4. Conclusions
Fig. 3. Voltage profiles of the working electrode upon galvanostatic electropolymerization on GC at RT of (a) pMeT-a in PYR13 FSI–1 M MeT and (b) pMeT-b in PYR14 Tf–1 M MeT at 10 mA cm−2 followed by undoping at −2.5 mA cm−2 ; from the polymerization and undoping charges the undoped polymer loading is 4.7 mg cm−2 .
The PYR13 FSI and PYR14 Tf ILs display conductivities, ESWs and formula weights which are of interest for application in safe and high specific energy supercapacitors, particularly in those operating above RT. Furthermore, they are very effective media for the electropolymerization of pMeT electrodes which can deliver in the same ILs more than 200 F g−1 at 60 ◦ C. The pseudocapacitive response of the pMeT electrodes depends on the ILs used for their
M. Biso et al. / Electrochimica Acta 53 (2008) 7967–7971
electropolymerization and for the electrochemical tests. Indeed, irrespective of the IL used for the capacitance test, pMeT performs better when it is synthesized in PYR13 FSI than in PYR14 Tf and this suggests that ILs may have a templating effect that tailors polymer morphology for a favorable transport of the counterions upon the p-doping process. On the other hand, also the different conductivity of the ILs, affecting the p-doping/undoping rate, contributes to the higher pseudocapacitive response of the polymer in PYR13 FSI than in PYR14 Tf. Hence, this study indicates that PYR13 FSI and PYR14 Tf ILs and pMeT electrodes electropolymerized in such electrolytes could be used to develop carbon/IL/pMeT hybrid supercapacitors operating with high cell voltages up to 4.0 V even above RT and work is in progress in our laboratory to prove the feasibility of this strategy with attention to cycling stability. Acknowledgements Work partially funded by the European Commission in the Sixth Framework Programme, Sub-programme Sustainable Surface Transport, under contract no. TST4-CT-2005-518307 (Project ILHYPOS “Ionic Liquid-based Hybrid Supercapacitor”). Dario Cericola is acknowledged for his contribute on the determination of solvent polarity of ILs.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electropolymerization of poly(3-methylthiophene) in pyrrolidinium-based ionic liquids for hybrid supercapacitors Maurizio Biso a , Marina Mastragostino a,∗,1 , Maria Montanino b , Stefano Passerini b,1 , Francesca Soavi a,1 a b
University of Bologna, Department of Metal Science, Electrochemistry and Chemical Techniques, Via San Donato 15, 40127 Bologna, Italy Agency for the New Technologies, Energy and the Environment (ENEA), TER Department, Centro Ricerche Casaccia, Via Anguillarese 301, Rome 00123, Italy
a r t i c l e
i n f o
Article history: Received 13 February 2008 Received in revised form 5 June 2008 Accepted 7 June 2008 Available online 17 June 2008 Keywords: Poly(3-methylthiophene) (pMeT) Pyrrolidinium-based ionic liquid Electropolymerization Hybrid supercapacitor Specific capacitance
a b s t r a c t The ionic liquids (ILs) N-butyl-N-methyl-pyrrolidinium trifluoromethanesulfonate (PYR14 Tf) and N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide (PYR13 FSI) are investigated as electropolymerization media for poly(3-methylthiophene) (pMeT) in view of their use in carbon/IL/pMeT hybrid supercapacitors. Data on the viscosity, solvent polarity, conductivity and electrochemical stability of PYR14 Tf and PYR13 FSI as well as the effect of their properties on the electropolymerization and electrochemical performance of pMeT, which features >200 Fg−1 at 60 ◦ C when prepared and tested in such ILs, are reported and discussed; the results of the electrochemical characterization in N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide of the so-obtained pMeT are also given, for comparison. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The interest in non-aqueous, high-energy supercapacitors freed of safety concerns and with attention to the operating temperature is growing worldwide [1]. One of the strategies which can be pursued to reach the high specific energy requirement is the use of pseudocapacitive electrode materials of charge storage capability higher than that of double-layer carbon electrodes. Electronically conducting polymers, which can be doped/undoped by fast faradic processes that give rise to specific pseudocapacitance even higher than 200 F g−1 , are under study mainly as positive electrode materials. Poly(3-methylthiophene) (pMeT) is one of the most attracting polymers because it is based on 3-substituted thiophene ring which, avoiding ␣, mislinking during polymerization, brings to a polymer of good electronic properties and, featuring low molecular weight, positively affects the specific properties of the material [2]. While pMeT can be successfully used as positive electrode for its high p-doping/undoping cycling stability, it can barely be used as negative electrode for its low conductivity, charge-storage capability and stability in the n-doped state. On the other hand, high performance supercapacitors can be devel-
∗ Corresponding author. Tel.: +39 51 2099798; fax: +39 51 2099365. E-mail address: [email protected] (M. Mastragostino). 1 ISE member. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.06.008
oped by coupling the pMeT positive electrode with a carbonaceous negative one that upon cycling is negatively charged/discharged by electrostatic process [3,4]. Such hybrid configuration allows to attain high cell voltages, even higher than 3.0 V, because the negative electrode can experience a wide potential excursion limited only by the electrochemical stability window (ESW) of the electrolyte. This is where ionic liquid (IL) electrolytes, and particularly those based on the N-alkyl-N-methyl-pyrrolidinium cation, are called upon. The wide ESW, high thermal stability, good conductivity and low vapor pressure of such ILs make them particularly attracting for high-voltage supercapacitors operating above room temperature (RT), i.e. in operative conditions where conventional organic electrolytes such as those based on acetonitrile lack in safety [5]. Indeed, the use of the hydrophobic IL N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14 TFSI) in carbon/IL/pMeT hybrid supercapacitor allowed to reach at 60 ◦ C the maximum cell voltage of 3.6 V over 16,000 charge/discharge cycles [6]. However, using IL in supercapacitors requires that the electrode materials are properly developed in order to deliver in IL at least the high specific capacitance exhibited in conventional electrolytes, and, as it concerns pMeT, the polymer electrode should be electrosynthesized in the same IL used to assemble the supercapacitor [7,8]. ILs are effective growth media for conducting polymers and while they are considered green, recyclable solvents for chemical and electrochemical synthesis procedures, their use results in significantly altered film morphologies with
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respect to those obtained by polymerization in conventional electrolytes [9–14]. Particularly, we reported that polymerization in PYR14 TFSI yield to pMeT with thinner interconnected polymer chains than those of the material synthesized in acetonitrile-tetra alkylammonium salt and such morphology provides an easier electrolyte access to the polymer p-doping sites, so that capacitances even exceeding 195 F g−1 can be reached in PYR14 TFSI at 60 ◦ C [7,8]. Despite the good results achieved by using PYR14 TFSI, in order to develop IL-based supercapacitors of high specific performance operating in a wide temperature range, new ILs of improved conductivity and lower formula weight than the former without penalty in ESW should be used. Indeed, while the PYR14 TFSI’s ESW and conductivity above RT are of interest for supercapacitor application, the latter significantly lowers below RT. Furthermore, given that upon the charge/discharge of hybrid supercapacitors the IL ions are taken from/released to the electrolyte to balance the electrodes’ charges, enough IL has to be used in the cells so as not to limit the charge storage in the electrodes. Therefore, the high formula weight of PYR14 TFSI (422.41 g mol−1 , mainly contributed by the heavy TFSI− anion) may negatively affect the gravimetric performance of the complete supercapacitor module. In this work pyrrolidinium-based ILs with lighter anions than TFSI− , namely the N-butyl-N-methyl-pyrrolidinium trifluoromethanesulfonate (PYR14 Tf) and N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide (PYR13 FSI), are investigated in view of their use in pMeT-based hybrid supercapacitors with attention to their conductivity and ESW. The results of the study on the pMeT electropolymerization and electrochemical characterization in such PYR14 Tf and PYR13 FSI electrolytes are also reported and discussed in relation to the properties of the investigated ILs, data on the electrochemical characterization in PYR14 TFSI of the soobtained pMeT are included, too. 2. Experimental 2.1. Ionic liquid synthesis and characterization The PYR13 FSI was synthesized in ambient air using Nmethylpyrrolidine (Aldrich), bromopropane (Aldrich) and lithium bis(fluorosulfonyl)imide (LiFSI from Dai-Ichi Kogyo Seiyaku DKS Co., Ltd.) as described elsewhere [15], and then transferred to a dry-room with a relative humidity lower than 0.2% at 20 ◦ C. The PYR14 Tf (99%) and PYR14 TFSI (purum 98%) were commercial products from Solvionic and Solvent Innovation, respectively. Before their characterization the three ILs were dried under dynamic vacuum for 10 h at 50 ◦ C followed by additional 10 h at 110 ◦ C (Büchi B-585 glass oven, Varian SH-100 oil-free vacuum pump). Thermogravimetric measurements (TGA) were carried out with a Mettler Toledo TGA/SDTA A851 System in the temperature range from 25 to 600 ◦ C under N2 flow with a scanning rate of 20 ◦ C min−1 ; an alumina pan with lid was used. The solvent polarity of the ILs was evaluated in ambient atmosphere on the basis of the 2,6-diphenyl-4-(2,4,6triphenylpyridinium-1-yl)phenolate (Reichardt dye, Sigma– Aldrich) solvatochromism [16] measured by a PerkinElmer Lambda 19 spectrometer; the data are reported in terms of normalized N molar transition energy EN T with ET = 0 for TMS and 1 for water. The viscosity, conductivity, and ESW of the ILs were determined on samples stored and tested in the dry-room which exhibited water contents below 1 ppm as evaluated by Karl-Fisher titration (METTLER TOLEDO DL32 coulometer). The viscosity measurements were carried out from RT up to 80 ◦ C (0.1 ◦ C/min heating scan
rate) using a HAAKE RheoStress 600 rheometer. The conductivity of the ILs was determined by a conductivity meter AMEL 160 in the temperature ranging from −40 to 100 ◦ C by using a climatic test chamber (Binder GmbH MK53) controlled by a home-made software. The ILs were housed in sealed, glass conductivity cells (AMEL 192/K1) equipped with two porous platinum electrodes (cell constant of 1.0 ± 0.1 cm). The ESW experiments were performed by linear sweep voltammetry (LSV) using a three-electrode, glass micro-cell, filled with a small amount of IL (0.4 ml). The working electrode was a glass-sealed platinum (E-DAQ), the counter was a platinum foil (about 0.5 cm2 ), and the reference was a silver wire immersed in a 0.01 M solution of AgSO3 CF3 (Aldrich) in PYR14 TFSI, separated from the cell compartment with a fine glass frit (E-DAQ). This reference electrode, often used today [17], has been proved stable for at least three weeks. The potential of this electrode, frequently controlled against a 5 mM solution of Ferrocene in PYR14 TFSI, was found to be +0.39 V vs. Fc/Fc+ (at 20 ◦ C). High purity argon (Air Liquide) was flushed over the electrolyte for at least 30 min before the start of the experiment and continued during the experiment. Back-diffusion of air was prevented flowing the outgoing gas through silicon oil. 2.2. pMeT electropolymerization and characterization pMeT was electropolymerized at RT in single compartment cells loaded with 1 M solutions of methylthiophene (MeT, Aldrich, distilled before use) in PYR13 FSI and PYR14 Tf (dried over night at 100 ◦ C under dynamic vacuum by Büchi Glass Oven B-580) and housed in a dry box (MBraun Labmaster 130 or TEKATOM with Ar atmosphere). The polymers grown in PYR13 FSI and in PYR14 Tf are labelled pMeT-a and pMeT-b, respectively. The polymerization was performed by cyclic voltammetry (CV) at 50 mV/s in various potential windows and by chronopotentiometry (GLV) with current density of 10 mA cm−2 followed by undoping at −2.5 mA cm−2 in order to evaluate the electrochemical stoichiometry of polymerization and, thus, the amount of polymerized pMeT in the undoped state. The working electrode was a glassy carbon (GC, 0.07 cm2 area), the counter a carbon paper electrode with area and charge storage capability significantly higher than those of the working, and the reference was a silver wire whose potential measured vs. the reversible redox couple ferrocene/ferrocinium (Fc/Fc+ ) was EAg = (EFc/Fc+ + 0.200 ± 0.010) V; hereinafter the electrode potentials are given vs. Fc/Fc+ . The electropolymerizations were carried in IL–1 M MeT solutions because at lower monomer concentrations, such as 0.1 M, pMeT films with bad adhesion to the GC current collector were obtained and during the voltammetric polymer growth at 50 mV s−1 formation of soluble oligomers was observed. The specific capacitance of the pMeT electrodes which featured 3–5 mg cm−2 of polymer was evaluated from the voltammetric discharges at 20 mV s−1 and 60 ◦ C (controlled by a Thermoblock FALC oven) by the slope of the electrode potential vs. integral over time of the current. The electropolymerization of pMeT and the following electrochemical measurements were performed with a PerkinElmer VMP multichannel potentiostat/galvanostat. 3. Results and discussion The study on the properties of PYR13 FSI and PYR14 Tf ILs was carried out at first in view of their use in pMeT-based hybrid supercapacitors. Table 1 compares the formula weight, the temperature at which thermal decomposition of the IL takes place with a 5% weight loss (T5% w.l. ), the melting temperature (Tm ), and the normal-
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Table 1 Formula weight, temperature at which thermal decomposition of the IL takes place with a 5% weight loss (T5% w.l. ), melting temperature (Tm ), normalized solvent polarity (EN T ) at RT of the PYR13 FSI, PYR14 Tf, and PYR14 TFSI ILs Formula weight (g mol−1 )
IL PYR13 FSI PYR14 Tf PYR14 TFSI a b c
308.36 291.33 422.41
T5% w.l. (K) a
490 670 683
Tm (K) a
256 276b 270c
EN T at RT 0.56 0.63 0.57
from ref. [19]. from ref. [5]. from ref. [20].
ized solvent polarity (EN T ) at RT of such ILs with those of PYR14 TFSI. The Tf− and FSI− -based ILs feature a 30% lower formula weight than PYR14 TFSI, mainly for their lighter anions than TFSI− , and while the PYR14 Tf has almost the same thermal stability than PYR14 TFSI and can be used up to 300 ◦ C, the PYR13 FSI starts decomposing above 220 ◦ C, both far above the temperatures of interest for carbon/IL/pMeT hybrid supercapacitor applications whose operation regime is also affected by the polymer decomposition at about 180 ◦ C [18]. Among the three ILs, that feature EN T values roughly corresponding to those of short-chain primary and secondary alcohols [16], the PYR14 Tf is that with the highest solvent polarity and this reflects on its highest melting temperature and viscosity. The viscosity of PYR14 Tf, PYR14 TFSI, and PYR13 FSI at 20 ◦ C is 158, 95 and 45 mPa s, respectively, but it approaches 20–10 mPa s at 80 ◦ C for all the ILs investigated. The viscosity and the melting points of the ILs affect their conductivity which is of 2.0, 2.6, and 7.4 mS cm−1 at room temperature for PYR14 Tf, PYR14 TFSI, and PYR13 FSI, respectively. The conductivity dramatically drops below 0.1 mS cm−1 at ca. 10 ◦ C for PYR14 Tf and −5 ◦ C for PYR13 FSI and PYR14 TFSI, i.e. approaching the Tm , thus marking the lower temperature limit for the use of these ILs in supercapacitors. The linear sweep voltammetries in the investigated ILs reported in Fig. 1 indicate that their ESW at RT is wide up to 5 V and, given that the pMeT p-doping/undoping typically takes place at ca. 1 V vs. Fc/Fc+ and that a negative carbon electrode could be polarized down to −3 V vs. Fc/Fc+ without overcoming the cathodic limit of the ILs, the use of PYR13 FSI and PYR14 Tf in carbon/IL/pMeT hybrid supercapacitors could allow to reach cell voltages even approaching 4.0 V and high specific energy provided that high specific capacitance is attained. With the aim of developing polymer electrodes of high performance in PYR13 FSI and PYR14 Tf, we performed a study on pMeT electropolymerization in these ILs. Voltammetric polymer growths were carried out at first and Fig. 2 reports the cyclic voltammetries
Fig. 1. LSVs at 5 mVs−1 and RT of platinum electrodes in (a) PYR13 FSI, (b) PYR14 Tf, and (c) PYR14 TFSI ionic liquids.
Fig. 2. Voltammetric polymerization on GC electrode at 50 mV s−1 and RT of (a) pMeT-a in PYR13 FSI–1 M MeT and (b, c) pMeT-b in PYR14 Tf–1 M MeT; the dashed lines are the CVs of the bare GC in neat (a) PYR13 FSI and (b,c) PYR14 Tf ILs.
(CVs) at 50 mV s−1 and RT that provided pMeT-a (Fig. 2a) and pMeTb (Fig. 2b and c). On the basis of the charges involved during the anodic and cathodic voltammetric scans, 60 cycles were necessary to yield in PYR13 FSI–1 M MeT 3 mg cm−2 of undoped pMeT, value calculated excluding any contribution to the loading of IL eventually trapped in the polymer; during the last cycle the polymer reached a doping level of 25%, thus indicating an electrochemical stoichiometry typical for electrosynthesis of pMeT in conventional electrolyte solutions, which is of ca. 2.3 F/mol of polymerized monomer unit. Fig. 2b shows that in the case of the polymerization in PYR14 Tf–1 M MeT the CV currents were lower than those recorded in PYR13 FSI when almost the same potential range with the anodic limit of 1.3 V vs. Fc/Fc+ was used. This cannot be ascribed only to the conductivity of the PYR14 Tf which is lower than PYR13 FSI’s, rather it has to be related to its higher polarity which reflects on the stability of the radical cation that propagates the electropolymerization. The radical cation MeT•+ produced at the electrode surface can be stabilized by the IL anions which are present at a high concentration of ca. 4 mol L−1 in the polymerization bath, and this may be more marked when the most polar IL, i.e. PYR14 Tf, is used. Such phenomenon may favor the diffusion of MeT•+ far from the electrode surface, so that radical coupling may take place not only on the electrode surface but also in solution, bringing to an electrochemical polymerization yield lower than that attainable in PYR13 FSI, as also confirmed by the slow current increase with cycles of the CVs reported in Fig. 2b. The Fig. 2c shows that the shift of the anodic scan limit up to 1.6 V vs. Fc/Fc+ is effective for high pMeT polymerization yield and rate. Indeed, by this shift the concentration of the radical cation at the electrode surface increases and 20 cycles are enough to provide ca 3.4 mg cm−2 of undoped pMeT, as evaluated
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on the basis of the charges involved upon the anodic and cathodic scans. In order to investigate the effect of the different ILs on the properties of pMeT polymerized at the same rate, galvanostatic synthesis conditions GLV were adopted and the Fig. 3 reports the working electrode potential profiles upon the polymer growth of pMeT-a in PYR13 FSI–1 M MeT (Fig. 3a) and pMeT-b in PYR14 Tf–1 M MeT (Fig. 3b) at 10 mA cm−2 and the subsequent undoping at −2.5 mA cm−2 . In both cases the doping level was 28%, confirming the results of the previous voltammetric study, and thus 4.7 mg cm−2 of undoped pMeT was obtained. Notably, such GLV polymerizations were performed without stirring because we have evidences that it is deleterious for the polymerization yield, thus further confirming the importance of a high radical cation concentration at the electrode surface. The pMeT-a and pMeT-b electrodes were tested by CVs at 20 mV s−1 and 60 ◦ C in the neat PYR13 FSI and PYR14 Tf and the results are reported in Fig. 4. Fig. 4a shows that in the case of pMeT-a tested in PYR13 FSI the p-doping onset is at more positive potentials and the doping/undoping peaks are less separated with respect to the case of pMeT-b tested in PYR14 Tf. From the plots reported in Fig. 4b, specific capacitance values of 255 F g−1 and 210 F g−1 are evaluated for pMeT-a and pMeT-b, respectively. These are quite interesting results that demonstrate that PYR13 FSI and PYR14 Tf provide polymer electrodes performing in the same ILs like and even better than pMeT grown and tested in PYR14 TFSI [7,8], with the advantage of being electrolytes of lower weight and, in the case of PYR13 FSI, of higher conductivity in wider temperature range than PYR14 TFSI, thus more promising for hybrid supercapacitor applications. The different performance of pMeT-a and pMeT-b electrodes can be explained with the different nature of the ILs used for the tests and a different “quality” of the polymer. The two electrodes have the same charge storage capability and can deliver up to 60 mAh g−1 corresponding to a 20% doping level. However, Fig. 4b shows that such charge is delivered in electrode potential ranges narrower in PYR13 FSI than in PYR14 Tf, thus indicating that the pseudocapacitive response is faster in the former IL than
Fig. 4. (a) CVs at 20 mV s−1 and 60 ◦ C reported in terms of specific current (I) and (b) corresponding plots of the electrode potential vs. integral over time of the discharge specific current (Q) of pMeT-a tested in PYR13 FSI (solid lines, 4.7 mg cm−2 of undoped pMeT electropolymerized in PYR13 FSI) and of pMeT-b tested in PYR14 Tf (dashed lines, 4.8 mg cm−2 of undoped pMeT electropolymerized in PYR14 Tf).
in the latter. This can be explained with the higher conductivity of PYR13 FSI (13.2 mS cm−1 at 60 ◦ C) than PYR14 Tf (5.5 mScm−1 at 60 ◦ C), and, thus, with the higher mobility of the counterions involved in the p-doping/undoping processes. In order to further investigate the effect of the IL on the pMeT performance, we also cycled pMeT-a and pMeT-b in the same PYR14 TFSI IL at 60 ◦ C and we found that the specific capacitance of each electrode decreased by a 10% with respect to the values exhibited in PYR13 FSI and PYR14 Tf, respectively, with the capacitance of the former being always higher than the latter’s. This cannot be related to the conductivity of PYR14 TFSI that at 60 ◦ C is of 6.0 mS cm−1 and almost the same as PYR14 Tf’s. Rather, it can be argued that PYR13 FSI and PYR14 Tf, being incorporated in the polymer matrix upon the electropolymerization, may act as templates and affect the “quality” of the pMeT-a and pMeT-b electrodes. Particularly, the use of these ILs as growing media may provide tailored polymer morphologies which favor the transport of the FSI− and Tf− counterions required for the doping process with respect that of the larger TFSI− anion. 4. Conclusions
Fig. 3. Voltage profiles of the working electrode upon galvanostatic electropolymerization on GC at RT of (a) pMeT-a in PYR13 FSI–1 M MeT and (b) pMeT-b in PYR14 Tf–1 M MeT at 10 mA cm−2 followed by undoping at −2.5 mA cm−2 ; from the polymerization and undoping charges the undoped polymer loading is 4.7 mg cm−2 .
The PYR13 FSI and PYR14 Tf ILs display conductivities, ESWs and formula weights which are of interest for application in safe and high specific energy supercapacitors, particularly in those operating above RT. Furthermore, they are very effective media for the electropolymerization of pMeT electrodes which can deliver in the same ILs more than 200 F g−1 at 60 ◦ C. The pseudocapacitive response of the pMeT electrodes depends on the ILs used for their
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electropolymerization and for the electrochemical tests. Indeed, irrespective of the IL used for the capacitance test, pMeT performs better when it is synthesized in PYR13 FSI than in PYR14 Tf and this suggests that ILs may have a templating effect that tailors polymer morphology for a favorable transport of the counterions upon the p-doping process. On the other hand, also the different conductivity of the ILs, affecting the p-doping/undoping rate, contributes to the higher pseudocapacitive response of the polymer in PYR13 FSI than in PYR14 Tf. Hence, this study indicates that PYR13 FSI and PYR14 Tf ILs and pMeT electrodes electropolymerized in such electrolytes could be used to develop carbon/IL/pMeT hybrid supercapacitors operating with high cell voltages up to 4.0 V even above RT and work is in progress in our laboratory to prove the feasibility of this strategy with attention to cycling stability. Acknowledgements Work partially funded by the European Commission in the Sixth Framework Programme, Sub-programme Sustainable Surface Transport, under contract no. TST4-CT-2005-518307 (Project ILHYPOS “Ionic Liquid-based Hybrid Supercapacitor”). Dario Cericola is acknowledged for his contribute on the determination of solvent polarity of ILs.
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