Synthetic Metals 123 (2001) 311±319

Chemical synthesis and characterization of ¯uorinated polyphenylthiophenes: application to energy storage Alexis Laforgue, Patrice Simon*, Jean-FrancËois Fauvarque Laboratoire d'Electrochimie Industrielle, CNAM, 2 Rue ConteÂ, 75003 Paris, France Received 29 November 2000; accepted 10 January 2001

Abstract Some ¯uorinated polyphenylthiophene have been chemically synthesized with good yields. The characterization of their positive and negative doping processes was performed by cyclic voltammetry and showed high capacities, but proved no real stability in cycling experiments. The application of the P-4-FPT as electroactive material in supercapacitor systems showed interesting properties in term of energy and power delivered. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polythiophenes; Chemical synthesis; Doping processes; Supercapacitors

1. Introduction Nowadays, electronically conductive polymers are the subject of many studies, due to their capability to go reversibly from an insulating state (also called undoped state) to a conductive state (or doped state), which corresponds also in many cases to an optical change [1±5]. The polythiophene and its derivatives possess two doped states (negative and positive). In the negative doping (n-doping), the conduction is realized by the electrons delocalization on the polymer chains, while in the positive doping (p-doping), the conduction is takes place by `positive holes' delocalization. The p-doping of the polythiophenes is generally easy to achieve. The observed charge densities vary from 20 to 30% (100% corresponding to the storage of one charge per monomer unit) [6,7]. On the contrary, the polythiophene n-doping is dif®cult to achieve, with low charge densities and poor stability [1]. One of the most probable cause is the very negative potential, where the n-doping occur. But when the thiophene ring is grafted with an electron-withdrawing substituent, the polymer doping potentials are shifted to more positive values (cf. Fig. 1) [4]. In this way, several polyphenylthiophene derivatives were proposed by Rudge et al. [5,8].

*

Corresponding author. Tel.: ‡33-1-40-27-24-20; fax: ‡33-1-40-27-26-78. E-mail address: [email protected] (P. Simon).

The work described is the synthesis and characterization of some ¯uorinated polyphenylthiophenes (cf. Fig. 2). Polymers were chemically synthesized to obtain powders, easier to manipulate than electrochemical ®lms, and can be used to make large and double-faced electrodes. 2. Experimental equipment Cyclic voltammetry experiments were performed using a PAR EG&G model 273A potentiostat controlled by a computer using the software M270 (version 4.0). Galvanostatic cycling was realized with a Biologic VMP. Impedance spectroscopy was performed using a Schlumberger Solartron 1255 frequency analyzer and a potentiostat Schlumberger Solartron 1286 controlled by a computer with the software ZPlot. IR spectra were enregistered with a Bruker Equinox IFS55 by diffuse re¯ection and golden gate techniques. 3. Synthesis The monomers were synthesized by a coupling reaction of 3-bromothiophene and each ¯uorophenylmagnesium bromide in THF with NiCl2 (diphenylphosphinopropane) as catalyst [9,10] (cf. Fig. 3). The monomers were then puri®ed several times by dissolution/recristallization in methanol/ water (solvent/non-solvent), and/or sublimated. The yields for this reaction varied from 41 to 92 wt.%, depending on the

0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 2 9 6 - X

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Fig. 3. Monomers synthesis scheme. First step: insertion of magnesium inside the carbon±halogen bound; second step: coupling reaction catalyzed by NiCl2(dppp). Fig. 1. Cyclic voltamogramms of polythiophene and P-4-FPT in acetonitrile.

dif®culty to isolate and purify the monomers. They appeared all as white powders. The monomers were then polymerized by a direct oxidation with anhydrous FeCl3 as oxidant in chloroform. This method had been already used to polymerize pyrrole, thiophene and arylthiophenes [11±15] (cf. Fig. 4). The polymers were then ®ltered and washed several times by methanol to remove the residual ferric chloride ions. The yields obtained for this reaction were around 80 wt.%. The global yield for both reactions was 73.6% for P-4-FPT, 62.2% for P-3,4DFPT, 56.9% for P-3,5-DFPT and 32.6% for P-3,4,5-TFPT. The polymers appeared as very ®ne powders from dark red to brown colors.

Fig. 4. Polymers synthesis scheme.

4. Chemical characterization As these polymers are quite insoluble in any known solvent, their chemical characterization was dif®cult. The main characterization was realized by elemental analyses and IR spectra. The elemental analyses (cf. Table 1) showed less than 5 wt.% of impurities, mainly chloride and iron, probably in FeCl4 form, which is the dopant from the polymerization. Small amounts were expected to be trapped into the polymer chains even after washing [16].

Fig. 2. The different fluorinated polyphenylthiophenes synthesized and characterized.

Table 1 Elemental analyses of the fluorinated polyphenylthiophenes (all values in wt.%) Polymer

P-4-FPT P-3,4-DFPT P-3,5-DFPT P-3,4,5-TFPT

C

H

S

F

Calculated

Measured

Calculated

Measured

Calculated

Measured

Calculated

Measured

68.18 61.86 61.86 56.6

67.13 61.47 61.13 55.15

2.84 2.06 2.06 1.42

2.83 1.92 1.89 1.63

18.18 16.49 16.49 15.09

18.05 16.25 16.53 14.05

10.79 19.59 19.59 26.89

10.25 18.28 18.69 25.46

Fe Measured

Cl Measured

0.25 0.32 0.41 0.46

1.49 2.22 1.26 2.46

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Table 2 IR spectra main attribution of the fluorinated polyphenylthiophenes IR peak wavenumber (cm 1)

Attribution

P-4-FPT

P-3,4-DFPT

P-3,5-DFPT

P-3,4,5-TFPT

3053 1892 1605

3074 1895 1603

3091

3079

nC±H Aromatic 20a±ba

1508

1517 1431 1373 1275

1621 1591 1521 1433 1385 1274

1615 1586 1528 1434 1370 1294 1242 1207

nC=C Phenyl 8aa nC±C Phenyl 8ba nC=C Thiophene nC±C Thiophene nC±C Thiophene nC±F Aromatic nC±F Aromatic nC±F Aromatic dC±H Phenyl 9aa dC±H Thiophene dC±H Phenyl 18ba Phenyl breath 1a Phenyl breath 12a dC±H Phenyl 9ba gC±H Phenyl 10aa gC±H Thiophene C±S±C deformation C±S±C deformation

1372 1230 1157 1091 1015 829 800 724

1219 1180 1117 1086

1217 1190 1120

947 875 821 771

988 858 847 758

709 681

681 624

625 602 573

574 556 536

454 a

689 670

1086 1046 852 844 808 731 708 643 599

532 510

Wilson notations.

The IR spectra (cf. Table 2) were rather complex, because of the presence of two kind aromatic rings that breath and have resonance vibrations; the thiophene rings resonate also through the polymer chain [17]. However, the main attribution was done, which shows the more important expected peaks: the major aromatic vibration peaks were found, referenced using the Wilson notations [18], and some strong peaks in the region 1200±1300 were measured, generally attributed to aromatic ¯uorines stretching vibrations [17].

Figs. 5 and 6 present the cyclic voltammetry of P-4-FPTbased electrodes containing different binders. The experiments were performed at 20 mV s 1 in acetonitrile with 1 M

5. Processing The electrochemical tests were performed on composite polymer electrodes. The electrodes were constituted by an active material laminated onto expanded metallic current collectors. To prepare the active material, the polymer powder was mixed together with a electronic conductive powder (acetylene black) to enhance the electronic conductivity. An organic binder was then added to the mixture to ensure mechanical properties to the active material paste. Several binders were studied to realize P-4-FPT-based electrodes: carboxymethylcellulose (CMC), polyvinylidene ¯uorid (PVDF) and polytetra¯uoroethylene (PTFE) [19].

Fig. 5. Cyclic voltammetry of 4 cm2 P-4-FPT electrodes with different binders; current collectors of expanded stainless steel (AISI 316L); electrolyte 1 M NEt4‡, CF3SO3 in acetonitrile; mpolym ˆ 9:0 mg; v ˆ 20 mV s 1.

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6. Electrochemical characterization

Fig. 6. Cyclic voltammetry of 4 cm2 P-4-FPT electrodes with different binders; current collectors of expanded stainless steel (AISI 316L); electrolyte 1 M NEt4‡, CF3SO3 in acetonitrile; mpolym ˆ 13:3 mg; v ˆ 20 mV s 1.

NEt4‡, CF3SO3 . PVDF allowed a good n-doping to the polymer, but only a few amount of charge was released after doping. Electrodes containing PTFE or CMC exhibited a poor capacity during the undo ping process (35 and 57% of coulombic reversibility, respectively). Meanwhile, when CMC and PTFE were used together, a synergy effect was observed, leading to higher coulombic reversibility (91%). This reproducible synergy effect has not been clari®ed yet. For the positive doping/undoping process (Fig. 6), the coulombic reversibility is high in each case and the improvement seems to be due to a lower electrode resistivity when CMC±PTFE is used.

The electrochemical properties of the ¯uorinated polyphenylthiophenes were characterized by cyclic voltammetry at 20 mV s 1 in the electrolyte NEt4‡, CF3SO3 1 M in acetonitrile. Although every experiment was performed under dry argon atmosphere, the salts and solvent were used as received. The water content was measured to be between 0.1 and 0.2% (1000±2000 ppm) with a Karl Fischer titrator. Cyclic voltammetry experiments were performed in a three-electrodes cell. The working electrode (WE) was made of a composite polymer-AB-CMC/PTFE sheet (around 200 mm thick) laminated on a 4 cm2 expanded stainless steel (AISI 316L). The polymer content in the electrode is 30 wt.%, an important content of acetylene black being used to ensure a good electrode conductivity. The counter electrode (CE) was made of activated carbon, bound on expanded stainless steel. This CE was overcapacitive to limit the potential shift of the electrode, avoiding the electrolyte degradation on the CE. The electrodes were assembled between two PTFE plates and separated with two PTFE sheets, each 25 mm thick. The system was kept under pressure with stainless steel clamps (4 bar cm 2) to ensure low internal resistance and the electrolyte was then added in an argon ®lled box. The reference electrode was placed above the electrodes as presented on Fig. 7. Figs. 8 and 9 present the negative and positive doping processes of the polymers. The electron withdrawing effect of the substituents can be observed on the peaks potential of the doping processes. The presence of ¯uorines increases the electron withdrawing effect: the differences appears clearly between the doping potentials of the PFPT, PDFPT and PTFPT.

Fig. 7. The three-electrode cell used for the electrochemical characterization.

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315

Fig. 8. Cyclic voltammetry of the different polymer n-doping processes; electrolyte 1 M NEt4‡, CF3SO3 in acetonitrile; mpolym ˆ 16:0 mg; v ˆ 20 mV s 1.

One method to evaluate the effect of substituents on the electronic properties of molecule is to calculate the Hammett constant of these substituents (s), which represent their electron withdrawing (s > 0) or donating (s < 0) in¯uence [1,20±23]. The ¯uorines located in meta and para have an electron withdrawing in¯uence (spara ˆ 0:06; smeta ˆ 0:35). According to several authors, in the case of multiple substituents, if there are no intramolecular interactions, the Hammett constants can be summed to evaluate the whole electronic effect [24,25]. Assuming this hypothesis, the constants associated to the polymers are then 0.06 for P4-FPT, 0.41 for P-3,4-DFPT, 0.70 for P-3,5-DFPT and 0.76 for P-3,4,5-TFPT.

Figs. 10 and 11 show the doping and undoping peak potentials versus the Hammett constant of the polymers. A linear relationship between the p-doping process potential of the polymers and the electronic effect of the substituent can be observed. The more important withdrawing effect and the more positive is the doping of the polymer. This relationship shows that the dif®culty to extract the electrons from the polymer backbone is directly correlated with the electronwithdrawing effect of the substituent. For the n-doping processes however, if the same relationship is observed for doping, the undoping peaks do not show the same tendency. It probably means that the n-undoping process is limited by ions diffusion instead of electronic transfer mechanism.

Fig. 9. Cyclic voltammetry of the different polymer p-doping processes; electrolyte 1 M NEt4‡, CF3SO3 in acetonitrile; mpolym ˆ 15:7 mg; v ˆ 20 mV s 1.

Fig. 10. The p-doping and p-undoping potential peaks vs. the Hammett constants of the polymers.

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Fig. 11. The n-doping and n-undoping potential peaks vs. the Hammett constants of the polymers.

The doping level does not seem to be directly in¯uenced by the withdrawing effect of the substituents as it was observed by Sarker and coworkers on electrochemical polymers [25] (cf. Table 3). For the n-doping process, the doping level varies from 0.16 to 0.2, which corresponds to the storage of one charge every 6.3±5 monomer units (mu). This values are rather high compared to the polythiophene's doping level (0.1, i.e. one charge/10 mu). The p-doping level varies from 0.23 to 0.26, which corresponds to the storage of one charge every 3.8±4.3 mu. These values are commonly observed for polythiophenes [3,25±28]. All these charge densities are approximately the same that those found for the electrochemically synthesized polymers [25]. Fig. 8 shows that the coulombic reversibilities of the ndoping processes are far from unity. This means that a charge-trapping phenomenon occurs when the polymers are reduce [29]. This phenomenon does not appear for the p-doping process. The stability of the doping processes is a key parameter for the application to energy storage. Cyclic voltammetry experiments have then been carried out on each doping process. Figs. 12 and 13 present different cycling experiments of the P-4-FPT n-doping. There is a sharp degradation of the polymer electroactivity: 80% of capacity was lost in 20 cycles. All the polymers exhibited the same behavior. This can be explained by different phenomena: breaks of the polymer chains due to the steric stress provoked by the ions

Fig. 12. The 21st cycles of a P-4-FPT electrode in the n-doping potential range; electrolyte 1 M NEt4‡, CF3SO3 in acetonitrile; mpolym ˆ 15:7 mg; v ˆ 20 mV s 1.

Fig. 13. The 500 cycles of a P-4-FPT electrode in the n-doping potential range with a doping 1eve1 of 0.05e /mu; electrolyte 1 M NEt4‡, CF3SO3 in acetonitrile; mpolym ˆ 14:8 mg; v ˆ 20 mV s 1.

insertion/desinsertion, chemical attack of the solvent, the doping ions or the water contained in the electrolyte, etc. [30±32]. The phenomenon has not been really clari®ed yet. To obtain some stability, it was decided to partially dope the

Table 3 Doping levels and capacities of the fluorinated polyphenylthiophenes, obtained from the voltamogramms presented in the Figs. 8 and 9 Polymer

n-Doping levela

p-Doping levela

n-Doping capacitya (mAh g 1)

p-Doping capacitya (mAh g 1)

P-4-FPT P-3,4-DFPT P-3,5-DFPT P-3,4,5-TFPT

0.16 0.2 0.2 0.17

0.26 0.23 0.25 0.23

24.1 27.3 27.5 21.7

40 31.8 34.6 29.1

a

All values measured from the undoping peak at the first cycle.

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Fig. 14. Doping level of the polymers during multi-cycling between and 2 V/Ag‡/Ag.

1

polymers limiting the potential to reach a doping level of approximately 0.05 charge/mu. This partial doping allows a better stability and a higher coulombic reversibility (over 90%). It means that degradation and charge-trapping occur at deep doping. Fig. 14 shows the variation of the doping level during this partial cycling. The P-4-FPT appears as the more stable polymer. The p-doping process is generally more capacitive and more stable than the n-doping one. Fig. 15 presents 500 cyclic voltammetries performed on P-4-FPT, and Fig. 16 shows the doping level evolution during the p-doping cycling for all the polymers. When doped totally, the polymers exhibited a p-doping process more stable than the n-doping one. However, there is still some degradation of the electroactivity, mainly in the 40

Fig. 15. The 500 cycles of the P-4-FPT p-doping process; electrolyte 1 M NEt4‡, CF3SO3 in acetonitrile; mpolym ˆ 11:9 mg; v ˆ 20 mV s 1.

317

Fig. 16. Doping level of the polymers during p-doping multi-cycling.

®rst cycles, without stabilization. As for the n-doping, the P4-FPT seems to be the more stable polymer, may be due to a more open structure, which allows ions to be inserted with less steric stress. 7. Application to energy storage The application to energy storage has been made with the P-4-FPT, as it seems to be the more stable polymer [19]. Fig. 17 shows the synthetic principle scheme for polymer based supercapacitors. When the system is charged, the polymer of the positive electrode is oxidized (p-doped), while the polymer of the negative electrode is reduced (n-doped). The discharge corresponds to the undoping of each polymer. The electrodes were assembled in the same way as the electrochemical characterization (cf. Fig. 7). Fig. 18 presents the galvanostatic cycling of a P-4-FPT/ P-4-FPT system. To obtain some stability, the potential of the negative electrode was limited to 2 V/Ag‡/Ag. The system was

Fig. 17. Principle scheme for polymer based supercapacitors.

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A. Laforgue et al. / Synthetic Metals 123 (2001) 311±319 2 †) (W s), where C is the system capa Emax ˆ 1=2…CUmax city (F) and Umax the potential at the end of charge.  Ereal ˆ UIt (W s), where U ˆ …U max ‡ U min †=2 (V), I the current applied (A) and t the time of discharge (s).  Pmax ˆ U02 =…4  R† (W), where U0 is the potential at the beginning of discharge (after the ohmic drop) and R the internal resistance (O). It represents the instant power (0 s of discharge).

Fig. 18. Galvanostatic cycling of a P-4-FPT/P-4-FPT system at 1 mA cm 2; electrolyte 1 M Net4‡, CF3SO3 in acetonitrile; mpolym ‡ ˆ 8:3 mg; mpolym ˆ 34:3 mg; current collector 4 cm2 expanded stainless steel (AISI 316L).

then cycled between 2 and 3 V. The polymer used in the positive electrode reached 31 mAh g 1 of polymer instead of the 40 mAh g 1 obtained in cyclic voltammetry. This is due to the polymer content in the electrode that was increased to 80 wt.%. The electronic percolation was then less ef®cient and a little loss of capacity was observed. The negative electrode reached 7 mAh g 1 of polymer (with a polymer content in the electrode of 65 wt.%). Fig. 19 presents the Nyquist plot of the system. The system shows a capacitor-like behavior at rather high frequency (13 Hz) with a small limiting diffusion. The internal resistance measured was 8 O cm2 at 1000 Hz. The performances of the system were measured in terms of maximum energy, real energy and maximum power, to be comparable with other systems, calculated with the following formula.

In the beginning of cycling, this system reached a maximum energy of 40.1 Wh kg 1 of polymer, a real energy of 15.3 Wh kg 1. The maximum power reached was 21.1 kW kg 1 of polymer. Of course, the energy could be strongly enhanced with a complete doping of the negative electrode. Meanwhile, the cycling behavior has still to be improved, as the system follows the polymer deactivation. 8. Conclusions and perspectives Four ¯uorinated polyphenylthiophenes have been synthesized and electrochemically characterized. The doping capacities obtained are rather high (from 21.7 to 27.5 mAh g 1 for the n-doping and from 29.1 to 40 mAh g 1 for the pdoping). However, the doping processes were found unstable during cycling. The use of P-4-FPT as electroactive material in supercapacitors showed interesting performances of energy and power and could be improved by doping completely the polymer in the negative domain. To improve their stability, the polymers will be tested in very pure media (less than 10 ppm of water and oxygen) in different electrolytes. Experiments will be carried out to understand the degradation of electroactivity in each doping domain (electron scanning microscopy, several spectroscopy techniques, elemental analyses, etc.). References

Fig. 19. Nyquist plot of the system P-4-FPT/P-4-FPT from 65 kHz to 60 mHz with a ac signal amplitude of 15 mV.

[1] J. Roncali, Chem. Rev. 92 (1992) 711. [2] K. Gurunathan, A. Vadivel Murugan, R. Marimuthu, U.P. Mulik, D.P. Amalnerkar, Mater. Chem. Phys. 61 (1999) 173±191. [3] P. NovaÂk, K. MuÈller, K.S.V. Santhanam, O. Haas, Chem. Rev. 97 (1997) 207±281. [4] J. Roncali, Chem. Rev. 97 (1997) 173±205. [5] A. Rudge, I. Raistrick, S. Gottesfeld, J.P. Ferraris, Electrochim. Acta 39 (1994) 273±287. [6] J.P. Ferraris, M.M. Eissa, I.D. Brotherson, D.C. Loveday, A.A. Moxey, J. Electroanal. Chem. (1998) 57±59. [7] C. Arbizzani, M. Catellani, M. Mastragostino, C. Mingazzini, Electrochim. Acta 40 (1995) 1871. [8] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J.P. Ferraris, J. Power Sources 47 (1994) 89. [9] J.P. Montheard, J.F. Delzant, M. Gazard, Synth. Commun. 14 (3) (1984) 28s9. [10] K. Tamao, S. Kodama, I. Nakajima, M. Kumada, Tetrahedron 38 (1982) 3354. [11] R. Sugimoto, S. Takeda, H.B. Gu, K. Yoshino, Chem. Exp. 1 (1986) 635.

A. Laforgue et al. / Synthetic Metals 123 (2001) 311±319 [12] M. Shimomura, M. Kaga, N. Nakayama, S. Miyauchi, Synth. Metals 69 (1995) 313. [13] S. Mashida, S. Miyata, Synth. Metals 31 (1989) 311. È sterholm, J. Laakso, P. Nyholm, Synth. Metals 28 (1995) 435. [14] J.E. O [15] T. Olinga, B. FrancËois, Synth. Metals 69 (1995) 297. [16] S. Kitao, T. Matsuyama, M. Seto, Y. Maeda, S. Masubushi, S. Kazama, Synth. Metals 9 (1995) 371±372. [17] E. Pretsch, C. Clerc, J. Seibl, W. Simon, Tables of Spectra Data for Structure Determination of Organic Compounds, 2nd Edition, Springer, Berlin, 1989. [18] E.B. Wilson, J. Phys. Chem. 45 (1934) 706. [19] A. Laforgue, P. Simon, C. Sarrazin, J.F. Fauvarque, J. Power Sources 80 (1999) 142±148. [20] L.P. Hammett, J. Am. Chem. Soc. 59 (1937) 96. [21] M.J.S. Dewar, The Electronic Theory of Organic Chemistry, Oxford University Press, Oxford, 1949. [22] T. Lowry, K.S. Richarson, Mechanism and Theory in Organic Chemistry, Harper & Row, New York, 1976.

319

[23] Y. Gofer, J.G. Killian, H. Sarker, T.O. Poehler, P.C. Searson, J. Electroanal. Chem. 443 (1998) 103±115. [24] P. Garcia, J.M. Pernaut, P. Hapiot, V. Wintgens, P. Valat, F. Garnier, D. Delabouglise, J. Phys. Chem. 97 (1993) 513±516. [25] H. Sarker, Y. Gofer, J.G. Killian, T.O. Poehler, P.C. Searson, Synth. Metals 88 (1997) 179±185. [26] M. Mastragostino, C. Arbizzani, R. Paraventi, A. Zanelli, J. Electrochem. Soc. 147 (2000) 407±412. [27] H. Tang, L. Zhu, Y. Harima, K. Yamashita, Synth. Metals 110 (2000) 105±113. [28] T. Fukuhara, S. Masubushi, S. Kazama, Synth. Metals 69 (1995) 359±360. [29] G. Zotti, G. Schiavon, S. Zecchin, Synth. Metals 72 (1995) 275±281. [30] J. Wang, Electrochim. Acta 39 (1994) 417±429. [31] E.W. Tsai, S. Basak, J.P. Ruiz, J.R. Reynolds, K. Rajeshwar, J. Electrochem. Soc. 136 (12) (1989) 3683±3689. [32] J. Wang, Electrochim. Acta 42 (1997) 2545±2554.