ELSEVIER Synthetic Metals 101 (1999) 24 An EIS study of oxidized and reduced poly(2,5-di-(-2-thienyl)-pyrrole) E. BriIlasa’*, J. Carrascob, V. Fernhndeza, J.A. Garrido’, RM. Rodriguez’, P.L. Cabot”, E. PCrez”, F. Centellasa a LCTEM Departament de Q&mica Fisica, Universitat de BarceEona, Marti i FranquPs 1, 08028 Barcelona, Spain b Departament d’Enginyeria Quimica iMetaE.Etirgia, Vniversitat de Barcelona, Marti i Franquks I, 08028 Barcelona, Spain Abstract Poly(2,5-di-(-2-thienyl)-pynole) films anodically synthesized on Pt from 5 mM monomer + 0.1 M LiClO4 + CHaCNat 0.5 mA cti’were reduced in water + C&CN (3:2) with 0.1 M LiClOl at -200 mVvs. SSCE. Thecapacitance and chargestorage capacigof these reduced films andsome oxidized filmsin the same medium weredetermined by electrochemical impedance spectroscopy (EIS). Keywords : electrochemical generation,poly(2,5-di-(-2-thienyl)-pyrrole, electrochemical impedance spectroscopy Electrochemicalimpedance spectroscopy (EIS) has been appliedsuccessfully to characterize the charge storage capaci- ty of films of several polyheteroaromatics, as polypyrrole [l] polythiophenes [2] and poly(SSS) [3]. In this paper, we presenta comparative EIS study on insoluble oxidized and reduced films of poly(2,5-di-(-2-thienyl)-pyrrole) [poly(SNS)] in water + CHXN (3:2) with 0.1 M LiC104. Although good oxidized poiy(SNS) films can be generated on Pt &om LiClOl +propylene carbonate [4] and LiClOG + CHaCN[Sj, they are electrodissolved by cathodic reduction in these media, being necessary to addwater to obtain insoluble reduced iilms. Thesynthesis of SNS, chemicals and instruments have been reported previously [3,5]. All experiments were conductedin three-electrode two-compartment cellsof 25 ml capacity under Ar at 25°C. The working and counter electrodes were two Pt sheetsof 1 cm’ area. The reference electrode was a calomel electrode with a NaCl-saturated aqueous solution (SSCE). Thin films ofoxidized poly(SNS) were depositedon Pt from a5 mM SNS+ 0.1 M LiC104 + CH&N solution by applying 0.5 mA cm“ during electrogeneration times &om30 s to 240 s. All filmswere grown at ca. -580 mVvs. SSCE.Each coated electrode was rinsed with the electrolyte and water, dried with hot air, and immersed in a cell containing water + CHaCN (312) with 0.1 M LiClO, to bereduced at -200 mV vs. SSCEfor a time equal to that its electrogeneration. Reduced poly(SNS)wasinsoluble in this medium. Its films werefurther oxidized at 300,660, 1020or 1400 mVvs. SSCE, correspond- ing to anodicpotentials associated with its four consecutive oxidation processes detected by cyclic voltarnmetry. TheBode, Nyquist and complex plane admittance diagrams determined by EIS for all films in water + CH&N (3:2) with 0.1 M LiClO4 fit well with the theoretical predictionsof the cdl 1 I I Rf Fig, 1.Equivalent circuit for oxidized and reduced poly(SNS): %, solution resistance; Rt, charge transfer resistance; Cdl, doublelayer capacitance; W, Warburg; Rr, film resistance; Cr, fihn capacitance. 12 I I I I I 0.05 - &L--b--.‘* -200 mV o.ool I I I I I 0 60 120 180 240 3~00 electrogeneration time / s Fig. 2. Capacitance of poly(SNS) films at different potentials vs. electrogeneration time.Temperature 25’C. equivalent circuit ofFig. 1. Film capacitances calculated from this circuit (Fig. 2) increase strongly when passingf?om the reduced state at -200 mVto the first oxidation one at 300 mV. This canberelatedto the much higher charge storagecapacity of films at 300 mV due to the formation of positive charges (polarons) along the poly(SNS) chains, In this case, Cr varies linearly with electrogeneration time (proportional to fihn weight [5]). However,gradual oxidation from300 mVto 1020 mVcauses a simultaneous decayin Cr(Fig. 2). This suggests a decrease in charge storage capacity of filmsdue to increasing degradation and/or crosslinkingprocesses in the polymer. Acknowledgements The authors thank to DGICYT (MEC, Spain) for financial supportunderproject PB94-0883. References [l] T.F. Otero,E. Angulo, Synth. Met. 5 1 (1992) 87. [2] 0. Genz,M.M. Lohrengel, J.W.Schultze,Electrochim. Acta 39 (1994) 179. [3] E. Brillas, P.L. Cabot, J.A. Garrido, M. Mantilla, RM. Ro- driguez,J.Carrasco, J. Electroanal. Chem430 (1997) 130. [4] J.P. Ferraris, T.R. Hanlon,Polymer 30 (1989) 13 19. [5] E. Brillas, J. Carrasco, A. Figueras, F.XJrpi,T.F,Otero, .I, Electroanal. Chem 392 (1995) 55. ~~ 0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00979-5