Journal of Power Sources 80Ž. 1999 142±148 Polythiophene-based supercapacitors Alexis Laforgue, Patrice Simon ), Christian Sarrazin, Jean-FrancËois Fauvarque Laboratoire d'Electrochimie Industrielle du CNAM, 2 rue Conte, 75003 Paris, France Abstract PolythiopheneŽ. Pth and polyparafluorophenylthiophene Ž PFPT . have been chemically synthesized for use as active materials in supercapacitor electrodes. Electrochemical characterization has been performed by cyclic voltammetry and an electrode study has been achieved to get the maximum capacity out of the polymers and give good cyclability. Specific capacity values of 7 mAh gy1 and 40 mAh gy1 were obtained for PFPT and polythiophene, respectively. Supercapacitors have been built to characterize this type of system. Energy storage levels of 260 F gy1 were obtained with Pth and 110 F gy1 with PFPT. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Supercapacitors; Polythiophene 1. Introduction thesized polymers, made by electro-oxidation of the monomer when dissolved in an electrolytewx 5±7 . These are In the research fields of energy storage, and more synthesized directly onto the current collector, but the specifically of supplying high powers, electrochemical su- thickness of these electrodes cannot exceed 100 mm. This percapacitors have been among the most studied systems way, the fabrication of large electrodes is not easy to for many yearswx 1±4 . achieve. It was therefore decided to chemically synthesize One of the possible applications is in electric vehicles. the polymers, in order to obtain powders that were easier We have been working on electronically conducting poly- to manipulate during the making of the electrodes. mers for use as active materials for electrodes in superca- pacitors. These polymers have the ability of doping and undoping with rather fast kinetics and have an excellent capacity for energy storageŽ 350 F gy1 for polypyrrole, 250 F gy1 for polythiophene. The aim of this work was to fabricate a high energy densityŽ 10 Wh gy1 . 3 V supercapacitor using electroni- cally conducting polymersŽ. ECP . Polythiophene Ž. Pth ap- peared to be capable of such a voltage value, as it could be negatively or positively doped. Previous work had shown that negative doping of PTh appeared at very negative 2. Experimental potential values, where electrolyte reduction occurred. So, it was decided to use one of its derivatives, polyparafluo- 2.1. Polymer synthesis rophenylthiopheneŽ. PFPT , as the negative electrode. Ž.yve PFPTrrelectrolyterrPth Ž.qve systems were then PolythiopheneŽ. Pth was synthesized by poly-con- studied, where PFPT cycled within its negative doping densation of 2,5-dibromothiophene in THF, in the presence Ž.n-doping potential range, and Pth in its positive doping of magnesium, and catalyzed by NiClŽ diphenylphos- Ž.p-doping domain. 2 phino ethane. wx 8 . The most important part of the studies on ECP de- Magnesium was introduced into a round bottom flask scribed in the literature have been focused on electrosyn- containing the THF, and the solution was stirred under an argon atmosphere. The dibromothiophene was added ) Corresponding author. Fax: q33-1-40-27-26-78; tel.: q33-1-40-27- slowlyŽ. without air contact and a Mg-insertion reaction 24-20; E-mail: [email protected] occurred at each halogen bondŽŽ.. Eq. 1 . Polymerization 0378-7753r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S0378-7753Ž. 98 00258-4 A. Laforgue et al.rJournal of Power Sources 80() 1999 142±148 143 was performed by carefully adding the catalyst to the already used to polymerize pyrrole, thiophene and arylthio- solution. The reaction began immediately and was com- pheneswx 11±14 . A suspension of anhydrous ironŽ. III pleted in one hour. After filtering to obtain the polymer, chloride in chloroform was prepared in the round bottom which appeared as a red powder, it was washed several flask. The suspension was placed under argon and stirred times with methanol to get rid of the oligomers and the at ambient temperatureŽ. 238C . The solution potential was catalyst. then q1.25 VrECSŽ the monomer oxidation occurs at The reaction yield was approximately 90% in weight. q0.9 VrECS. FPT was dissolved in chloroform and r s added in the medium with a molar ratio of nFeCl3n FPT 4. Polymerization was performed for 2 h before stopping it by addition of methanol, which neutralizes the FeCl3 still present in the solutionŽ probably by dimerization of the FeCl which had then no more oxidative power. wx 15 . The Ž.1 3 polymer was then filtered and washed several times with methanol to remove the residual chloride ions from the PFPT. The yields obtained for these reactions were near 80 wt.%. ParafluorophenylthiopheneŽ. FPT , the monomer of PFPT, was synthesized by a coupling reaction of 3- bromothiophene and 4-fluorophenylmagnesium bromide in THF with NiCl2 Ž. diphenylphosphino propane as catalyst wx9,10ŽŽ.. Eq. 2 . The bromothiophene was placed in a round bottom flask with the catalyst, in a glove box filled with argon. The solution was stirred and kept at y108C. Fluorophenyl Ž.3 magnesium bromideŽ. 1 M in THF was added to the solution with a syringeŽ. no air contact . The mixture was stirred for 12 h and then the reaction stopped by adding carefully 1 M HCl to neutralize the excess of fluorophenyl magnesium. The organic phase was filtered to obtain the monomer which appeared as a yellow powder. Recrystal- lization was performed by dissolving the monomer in methanol and precipitating it with water. FPT was a white powder. The yield from this reaction was always more than 90 wt.%. 2.2. Characterization As these polymers are quite insoluble in any known solvent, their chemical characterization was difficult. It was decided to test them in a way representative of their future use as active materials for energy storage devices: cyclic voltammetry was used to compare their electro- chemical properties. Ž.2 Cyclic voltammetry on the polymers was performed in qyq acetonitrile using different saltsŽ NEt4334 , CF SO , NEt , y CH33 SO. , in the n- and p-doping domain, for PFPT and Pth. The reference electrode used was AgqrAg, made of a y2 silver wire in a solution of 10 M AgNO3 in acetonitrile. Its potential of this electrode is q0.2 VrECS. The counter electrode was a 4 cm2 platinum foil for primary characteri- zation and an over-capacity polymer electrode bound onto metallic grids for the cyclability tests. This over-capacity electrode was used to settle the counter electrode potential FPT was polymerized by a direct oxidation with FeCl3 and thus prevent electrolyte degradation reactions on this as oxidant in chloroformŽ. Eq. 3 . This method had been electrode. 144 A. Laforgue et al.rJournal of Power Sources 80() 1999 142±148 The potentiostat used was an EG&G PAR Model 273A, the reverse scan, at y1.8 Vrref for both polymers and coupled to a computer. Galvanostatic cycling was achieved corresponds to electron extraction from the polymer, and with a Biologic MacPile and resistance measurements to the removal of cations. The polymer returns to its were made using a HP 4338B milliohmmeter or an neutral and isolating state. impedance meterŽ. Solartron Schlumberger 1255 at 1000 doping ) Hz. Ž0. q yqq Polymer e NEt 4 - Two types of working electrodes were used. The first undoping oneŽ. referred as `enclosed electrodes' was made of a Polymery,NEtq Ž.4 mixture of polymer and graphite powders, enclosed and 4 pressed into a metallic grid. This technology allowed the It is important to note that the n-doping process of the qualitative characterization of the polymers. Graphite was Pth occurs at more negative potentials than it does for used to maintain a good conductivity inside the electrode PFPT. As the electrolyte reduction begins at y2.3 Vrref in the range of potentials where the polymers show insulat- on a platinum electrode, it was decided to use the PFPT as ing properties. In the second type of electrode, the active the negative active material to minimize degradation of the materialsŽ. polymer and conductor were mixed with differ- electrolyte. The fluorophenyl groupŽ electron acceptor sub- ent bindersw carboxymethylcelluloseŽ. CMC , polyvinyl- stituent. confers to PFPT a higher and more stable n-dop- idene fluorideŽ. PVDF , polytetrafluororoethylene, Ž. PTFE x ing process leading to a better cyclability than Pth. and pasted onto metallic current collector grids, to quanti- For the positive doping, the same process of insertion of tatively characterize the polymers. All the tests were car- ions occurs, but the electrons are now extracted from the ried out in an argon atmosphere. polymer and anions are inserted in the electrodeŽŽ.. Eq. 5 . The p-doping of both polymers appears as an oxidation wave beginning at 0.3 Vrref and the undoping as a 3. Results and discussion reduction peak at 0.3 Vrref for PFPT and 0.2 Vrref for Pth. It represents the removal of anions while electrons are 3.1. Doping processes injected into the polymer. doping ) Fig. 1 presents the cyclic voltammetry of a Pth and a PolymerŽ0. qCF SOy 33- undoping PFPT electrode. The negative doping appears as a reduction wave at low q y q y Polymer ,CF33 SO e Ž.5 potentials: y1.8 Vrref for PFPT and y2.2 Vrref for Pth. It corresponds to the injection of electrons into the poly- Both polymers can be used as positive active material mer and to the insertion of cations from the electrolyte into for supercapacitors, since they have almost the same p- ŽŽ.. doping domains, but Pth is more capacitive than PFPT the electrode to preserve the electroneutrality Eq. 4 . y1 y1 This leads to a change in the electronic structure of the Ž250 F g and 110 F g , respectively.