A study of the electrochemical properties of conducting polymers for application in electrochemical capacitors

0013~4686p4 $6.00 + OJIO Q 1993. Perpmon Prcu Ltd.

E&c&oc~ Acur Vol. 39. No. 2. pp. 273-287.1994 Printi io Gnu: Britain.

A STUDY OF THE ELECTROCHEMICAL PROPERTIES OF CONDUCTING POLYMERS FOR APPLICATION IN ELECTROCHEMICAL CAPACITORS ANDY RUDGE,*IAN RAISTNCK,*SHIMSHONGO-ITE~FIZLD* and JOHN P. FERR_mst * Electronics Research Group, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. t Department of Chemistry, University of Texas at Dallas, Richardson, TX 75080, U.S.A. (Received 5 April 1993)

Altstract~nducting polymers can be doped and dedoped rapidly to high charge density and as a result are potential active materials for use in electrochemical capacitors. We discuss three schemes by which conducting polymers can be utilized in electrochemical capacitors and in the third of these, which employs a conducting polymer that can be both n- and p-doped, high energy and power densities are demonstrated. Polythiophenes can be both n- and p-doped reversibly and we have tested a number of poly-3-arylthiophenes in order to achieve the best n-doping at thick polymer fihns. We have found that poly-3-+fluorophenyl)-thiophene, in a solution of 1 mot drne3 tetramethylammonium trifluoromethanesulfonate in acetonitrile, can be n- and p-doped, both reversibly and to high charge density. Such key properties for high power, energy storage devices have not been demonstrated before with conducting polymer active materials. We propose that the improvement in polymer ndopability that results from derivitixation of thiophene with aryl substituents in the 3-position, is probably achieved thanks to electron transfer from the negatively charged polythiophene backbone to the aryl substituent. This proposal is discussed in the light of modeling information and of voltammetric data. Further sup porting information is provided by electron microscopy, impedance spectroscopy and electrochemical quartz crystal microbalance results. Key words: ultracapacitors, supercapacitors, n-doped polymers, conducting polymers, electric vehicles, regenerative braking.

1. INTRODUCPION

An electrochemical capacitor is basically the same as a battery in terms of general design, the difference being that charge storage is capacitive in nature rather than Faradaic[l-31. Because of this, the maximum rates of charging and discharging are dictated only by ionic and electronic transport rates within the active material, and this results in high power densities. Historically, electrochemical capacitors were referred to as “double-layer capacitors”, because the dominant technology was based on double-layer type charging at high surface area carbon electrodes[2, 4-61. More recently, however, other active materials, notably noble metal oxides[3, 7, 81 and conducting polymers[l, 3, g-131, have been put forward for use in these devices. Higher energy densities can be achieved in the latter materials because charging occurs through the volume of the material rather than just at the outer surface of particles. Hence, these devices are now better referred to as “electrochemical capacitors”, rather than double-layer capacitors. Energy densities of electrochemical (currently lcapacitors 5 Wh kg-‘) are much higher than those of conventional capacitors, but typically lower than those of advanced batteries. However, compared to batteries, higher power densities (> 500W kg-‘) and longer cycle life (> 10’ cycles) have been either demonstrated or projected. These latter advantages over batteries are achievable because no rate-determining

and life limiting phase transformations take place at the electrode-electrolyte interface. Interest in electrochemical capacitors has accelerated recently because of their predicted suitability as a power source for an electric vehicle (EV). The EV capacitor will operate in parallel with the battery, acting as a load leveler by providing the power needed for acceleration and hill climbing. Recharging will be accomplished either from the battery when travelling at constant speed, or by regenerative braking while decelerating. By removing the need for pulses of peak power from the EV battery, it is expected that the battery size will be reduced and battery life extended. Electrochemical capacitors are already in use as power back-up for computers and other future applications that require rapid charge/dischargeability and extended cycle life have been proposed[3,4,8]. Conducting polymers have been investigated for use in electrochemical capacitors because of a combination of high charge density with predicted low materials cost. Doping levels as high as one electron per two monomeric building blocks can be achieved in the case of polyaniline (even higher levels can be achieved in a carefully selected environment[ 141) and this corresponds to a charge density of 5oOCg- ‘. High surface area carbon electrodes can be manufactured cheaply, however charge densities are low (< SOCg-i). Conversely, noble metal oxide based devices exhibit high charge density (> 500 C g- ‘), however materials cost (ruthenium oxide in particular) is high. If satisfactory power 273

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density, cycle life and thermal stability can be demonstrated with conducting polymers, then these materials could represent the most attractive active materials currently available for electrochemical capacitors. The distinction between a battery and an electrochemical capacitor is becoming more vague as higher energy density capacitor materials and higher power density battery components are being sought. With this in mind, it should be noted that a number of systems based on conducting polymer active materials have been referred to as batteries in the literature, where they may alternatively have been described as electrochemical capacitors[lS-221. In this paper we describe efforts to achieve the maximum energy and power performance from a conducting polymer based electrochemical capacitor. The key to achieving higher energy density in an electrochemical capacitor is to expand the cell voltage. This aim is particularly worthwhile because the energy of an ideal capacitor scales with the square of the cell voltage (E = 1/2CV2). This explains our reasons for investigating conducting polymers which are both n- and p-dopable, enabling cell voltages higher than 3V in a carefully selected electrolyte. We have previously described three distinct schemes in which electronically conducting polymers can be applied to electrochemical capacitors, and these we refer to as Types I, II and 111[23]. A Type I capacitor is based on a symmetric configuration, with identical p-dopable (only) conducting polymer active layers (eg polypyrrole) on each of the two electrodes in a cell. A Type II capacitor has an asymmetric configuration, with two different p dopable active materials, (eg polypyrrole and polythiophene) on each of the cell electrodes. The third, and we believe most promising, scheme for the application of conducting polymers to electrochemical capacitors, is based on a conducting polymer that can be both n- and p-doped electrochemically. In this Type III scheme, the charged capacitor consists of one electrode in the fully ndoped state and the other in the fully p-doped state. After discharge, both polymer films will be in the undoped state. Apart from the increase in cell voltage to about 3V and the ability to release the total charge within a doping regime, there are also two further advantages of the Type III capacitor, over Types I and II. When the Type III capacitor is charged, both conducting polymer films are doped and, therefore, both have a high electronic conductivity. In contrast, in a charged capacitor of Type I or II, one of the polymer films will be in the undoped form, associated with low electronic conductivity. Hence, it is expected that this low electronic conductivity will diminish the instantaneous power density that can be achieved on discharge of a Type I or II capacitor, but not so in the case of Type III. The second advantage of Type III is that, because of the separation in potential between the regions of n-doping and pdoping, all the charge is released at high voltage (in the case of polythiophenes, between 3 and 2V). In this regard, the discharge of a Type III capacitor resembles more closely that of a battery, leading to the description of Kaneto’s polythiophenepolythiophene energy storage device as a

battery[17]. This would be advantageous in a practical device, because charge stored at too low voltages may not be useful in the EV power system. A more detailed comparison of Type I, II and III electrochemical capacitors can be found in ref.[23]. Only a limited number of conducting polymers can be electrochemically n-doped, usually at highly reducing potentials, and for this reason electrochemists have concentrated more on p-dopable conducting polymers, such as polypyrrole[24-281 and polyaniline[24, 25, 29, 301 where aqueous solutions can be used. The initial interest in n-doping of conducting polymers came in the work of MacDiarmid and co-workers on polyacetylene[31-361. It was demonstrated that this conducting polymer was a potential active material for use as both anode[3740-J and cathode[41-481 in batteries. Investigations soon expanded to include poly-pphenylene, which has a higher band-gap than polyacetylene, but can also be n-doped electrochemically[49-511. However, polyacetylene and poly-p-phenylene are poorly suited as capacitor active materials because of the nature of the doping processes. Very high impedances are associated with n-doping of polyacetylene, for example when electrochemically n-doped with either potassium, sodium or lithium counter ions[52-541. This irreversibility is equally evident in the separation in the peak potentials for doping and dedoping in cyclic voltamograms[39, 52, 55, 56). In the cases of potassium and sodium ions, the irreversibility has been ascribed to a phase transformation from amorphous to crystalline, when either polyacetylene or poly-p-phenylene are n-doped, as observed using X-ray diffractionC51, 55, 57, 581. For lithium, this irreversibility is also ascribed to the need to remove the large solvation shell that accompanies lithium ions in acetonitrile or propylene carbonate, in order to enable ionic penetration into the active film. Because of this high-impedance to ndoping, it is not expected that these materials will have satisfactory power density or energy cycle et& ciency for use in electrochemical capacitors. There are reports of much more reversible n- and p-doping with polythiopheneC17, 59-65-J together with observations of improved n-doping by derlvatixing the thiophene monomer in the 3-position with a phenyl group[66-681 or by introducing an electron withdrawing group to an ether baaed substituent[69,70]. These materials therefore offer much greater potential for use in electrochemical capacitors. Improved n-doping at more positive potentials has also been achieved in polymers made from bridged dimers of thiophene, where the bridging unit is designed to reduce the HOMO-LUMO separation of the polymer[71]. ac impedance spectroscopy is a commonly used tool for the study of conducting polymers because it is able to provide valuable information on charge transport phenomena in porous 8lms[72]. The technique has been applied to 8lms of polyaniline[73753, polypyrroleC13, 76-821, polythiophene[83], and other polyacetylene[52-541 conducting polymers[84_8fl. Impedance analysis is also an informative technique in application to electrochemical capacitors[l, 3, 7, 883 and capacitive electrode reactions[89]. The high frequency real part of the

Electrochemical

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impedance provides a direct measure of the combination of electronic and ionic resistances within the current collection components and the electrolyte solution, respectively. The component of the impedance which is associated with the capacitive charge transport within the active material is obtained from the ac impedance spectrum, as the difference between the real parts of the impedance at low and high frequencies. When applied to doped conducting polymer electrodes, the region of the impedance spectrum associated with high capacitance is often interpreted in terms of a distributed impedance associated with ionic penetration to the charging sites throughout the film. Ionic transport is predicted to be more rate limiting than electron transport because the ionic conductivities of typical liquid electrolyte solutions are significantly lower than the electronic conductivity of conducting polymers in their doped forms (typically lo-lOOScm_ ‘). In some cases, a semi-circle is observed at high to medium frequencies in the complex impedance plot of a conducting polymer film[52, 54, 74, 78, 80, 84, 863, and this is usually interpreted as the result of significant ion or electron transfer resistance in parallel with “double layer” capacitive charging at the film/electrolyte interface. Another technique which is becoming increasingly popular among conducting polymer workers, including those investigating n- and p-dopable conducting polymersC61, 631, is the electrochemical quartz crystal microbalance (EQCM) technique for monitoring small changes in the mass at an oscillating quartz crystal[90]. The technique is used most effectively in the context of conducting polymers to differentiate between anion and cation transport during doping and dedoping processes. It can also identify transport of solvent and ion pairs that accompany the dopant ions[63]. We describe in this paper the development of pand n-dopable derivatives of polythiophene, and of

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appropriate electrolytes which together enable improved, reversible n-doping. We have characterized these new electrochemical systems with cyclic voltammetry, ac impedance and electrochemical quartz crystal microbalance techniques, highlighting properties relevant to their potential as electrochemical capacitor active materials.

2. EXPERIMENTAL Electrochemical experiments were performed in either a standard three electrode cell or in a five electrode cell of the design described in Fig. 1. This type of cell enabled polymer growth, single electrode cyclic voltammetry, potentiostatic setting of specific initial doping levels and constant current discharge between two polymeric electrodes, without alterations to the electrode assembly as it is transferred between growing, washing and cycling solutions. The mass of undoped polymer was obtained by weighting the electrode after it had been dedoped potentiostatically to remove dopant anions and washed to remove solvent and electrolyte. The working electrodes were fabricated from 3 mil (75pm) thick fibrous carbon paper (Toray -8pm fibers, porosity 075%) into the spade shape shown in Fig. lb, allowing only the lower square of carbon paper to be immersed in the working solution. The porous carbon paper electrodes were chosen to enable the deposition of a significant volume of polymer per unit geometric area of electrode without necessitating the formation of a very thick conducting polymer film which might reduce accessible power densities. It should be noted that geometric current and charge densities are always quoted per cm* of carbon paper, not per cm* of real carbon surface area. The counter electrodes were made of thicker carbon paper. Working and counter carbon paper lb) Carbon Spade Working Electrode

Fig. 1. (a) The electrochemical cell configuration used for testing the electrochemical characteristics of different conducting polymer materials electrodeposited on carbon paper and (b) scheme of the 3mil (75 q) thick carbon paper working electrodes.

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electrodes were connected to platinum wire (above the liquid level in the cell) employing silver loaded epoxy, with an outer coat of regular epoxy. The reference electrode was a standard Ag/Ag+ electrode for non aqueous systems, comprising of a silver wire immersed in the respective working solution, with silver nitrate added to the level of O.O2moldm-‘. The reference compartment was separated from the working solution by a porous glass plug. Polymer 6lms were grown onto the carbon paper electrodes at constant anodic current from a solution containing the relevant monomer. This galvanostatic polymer growth technique enabled accurate control of the amount of a particular polymer deposited, because the mass of electrodeposited polymer was found to be a linear function of growth charge, as shown for poly-3-(4-fluorophenyl)-thiophene (PFPT) in Fig. 2. All the cyclic voltammograms reported here have been iR corrected, by employing the positive feedback option of the PAR 273 potentiostat. Non-aqueous, acetonitrile solutions (< 50 ppm H,O) were used throughout and all experiments were performed in an argon controlled atmosphere glove box. A new electrolyte was used in this work, Tetramethylammonium Trifluoromethanesulfonate (Me,NCF,SO,), and the reason for this choice is described later in section 3. Me,NCF,SO, was synthesized by titrating an aqueous solution of tetramethylammonium hydroxide with 2 mol dm- 3 trifluoromethane-sulfonic acid until the pH just passes through 7. The water and small amount of excess acid were removed by evaporation. The crude salt was first dissolved in acetone and the residue filtered off, followed by four recrystallizations from acetone/hexane. The melting point of the new material was 383-384°C and elemental analysis supported the expected empirical content of Me,NCF,SO, as evident in Table 1. Figure 3 shows the dependence of ionic conductivity of solutions of

Table 1. A comparison of the expected and observed elcmental proportions in the new electrolyte employed in this work, tetramethylammonium trifluoromethanesulfonate, Me,NCF,SO, Calculatedcomposition Element

% by mass

Observed composition % by maa

carbon Hydrogen Nitrogen

26.91 5.42 6.28 25.53 14.3

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Me,NCF,SO, in acetonitrile on electrolyte concentration. In the experiments reported here, 1 moldmW3 solutions of Me,NCF,SO, were used, and this was found to have a conductivity of 0.025 S cm - ‘. Tetraethylammonium tetrafluoroborate (Et,NBF,) was recrystallized four times from acetone and tetrabutylammonium hexafluorophosphate (Bu,NPF,) was recrystallixed four times from ethanol/water. For experiments with the n- and pdopable conducting polymers, the aryl substituted thiophenes were prepared according to literature procedures via a nickel-catalyzed coupling of 3-bromothiophene with the appropriate aryl magnesium bromide[91, 921. The aryl substituted monomers were typically white solids that were purified by sublimation, and all displayed satisfactory physical and spectroscopic properties according to previous reportsC66, 93, 941. Pyrrole was purified by filtering through a column of activated alumina. All the chemicals listed above were purchased from Aldrich in the purest available grade. Impedance experiments were performed in a standard three electrode cell configuration using a Solartron 1250 Frequency Response Analyzer in conjunction with a Solartron 1286 Electrochemical Interface. Impedance spectra were recorded in the frequency range 65 kHz down to 0.1 Hz using an ac

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TOTAL GROWTH CHARGE - C/cm’ Fig. 2. The dependence of the mass of electrodeposited polymer on total charge passed during galvanostatic growth of PFPT from a solution of 0.1 moldm-” FPT, 1 moldmm3 Et,NBF, in acctonitrile.

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Fig. 3. The variation in ionic conductivity of acetonitrile solutions of Me,NCF,SO, as a function of electrolyte concentration. voltage amplitude of 20mV. Polymer 6lms for EQCM experiments were grown onto 0.5cm2 gold film (which had been evaporated onto a 1” diameter 5MHz quartz crystal disc in the key-hole design commonly employed in this type of experiment[90]). The counter electrode in these experiments was a platinum gauze and the reference electrode was Ag/Ag+. Relatively thick films of conducting polymer (>O.l pm) of high porosity were grown onto the quartz crystal, and so we have not attempted to use the Sauerbrey equation[95] to calculate mass changes at the electrode from the shifts in the quartz crystal resonant frequency.

3. RESULTS

AND DISCUSSION

3.1. The n-dopability of polythiophene and the aryl derivatives

Thin films of polythiophene (PT) have been reported to reversibly p and ndope using tetraalkylammonium based electrolytes[17, 59-65-J. Indeed the symmetric PT battery reported by Kaneto et al.Cl7-J was only able to utilize 0.75pm films of PT and in their paper the authors commented on the desirability of thicker films. Our experiments have shown that, while the ability to p-dope PT is not significantly reduced at thicker films, ndoping is severely compromised for films over 1 pm thick. Indeed a film of PT electrodeposited galvanostatically onto the 75 pm thick carbon paper at a growth charge of 7.2 Ccmm2, showed no ndopability whatsoever in 1 mol drne3 Bu,NPF, in acetonitrile. Sato and co-workers have reported that phenyl substitution of thiophene at the 3-position improves the n-dopability of the resulting polymer over PT, whilst p-doping appears unaffected[66-681. These authors attributed the improvement to the electron withdrawing ability of the substituent through inclusion of the phenyl group into the con-

jugated system of the polymer. However, this conclusion is not totally supported because the positions of the peaks for both p- and n-doping in the cyclic voltammograms of poly-3-phenylthiophene (PPT) do not differ greatly from those of PT. As improved ndoping in thicker films is precisely the characteristic we require for our Type III electrochemical capacitor, we further investigated PPT and then expanded our experiments to include other aryl thiophenes, which are described in terms of their structure, monomer name and polymer acronym in Table 2. As an indicator of satisfactory lihn thickness, we expect that the polyarylthiophenes would provide sufficient energy density for electrochemical capacitors when grown onto the 75pm thick carbon paper at a growth charge of 5 Ccmm2 or more, provided the resulting polymer is able to n-dope to the same high charge density as it p-dopes. We first synthesized 3-phenylthiophene, electropolymerized the monomer galvanostatically onto carbon paper, and examined the polymer’s n- and p-dedoping charges as a function of thickness in 1 moldm-3 Et,NBF, in acetonitrile. We chose to examine dedoping charges, because it is these which are released during discharge of a capacitor and therefore are representative of the potential energy content of the device. While the n-dopability of PPT is indeed greatly improved over PT, the ratio of ndedoping to p-dedoping charges still dropped from 53 to 21% as the film growth charge increased from 0.6 to 7.2 C cm- 2. We subsequently tested our other aryl-substituted thiophenes in the hope of improving n-dopability further. Poly-3-p-tolylthiophene (PTT) and poly-3-(4-tbutylphenyl)-thiophene (PBPT) both appear to ndope/dedope and p-dope/dedope to a similar degree to PPT. Poly-3-(4-trifluoromethylphenyl~thiophene (PTFMPT) on the other hand did appear to exhibit equivalent charges for n- and p-doping. However this last material suffered very badly from a different problem that diminishes the performance of a Type

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Table 2. The different monomers, and the acronyms used for the coresponding polymers investigated in this work, including those based on p-substituted 3-phenyl-thiophene (general molecular structure shown above) Monomer name

Polymer acronym

pyrrole thiophene 3-phenylthiophene 3+tolylthiophene) 3-(4-t-butylphenylhthiophene 3+fluorophenyl)-thiophene 3-(4-trifluoromethylphenyl)-thiophene

PPY PT PPT P-l-r PBPT PFPT PTFMPT

Substituent X H %I,), F CF,

III capacitor, “charge-trapping”. Charge-trapping refers to a deficiency in the amount of charge released during dedoping relative to that which was stored during doping, and this has been reported and discussed previously for PT[61]. We have observed charge-trapping to some degree with all these polymers, but none more so than PTFMPT. The effect can be seen in Fig. 4, which shows a cyclic

voltammogram of a relatively thin film of PTFMPT, grown onto carbon paper at 0.5Ccm-*. This figure demonstrates the significant asymmetry of the doping and dedoping charges in both the positive and negative voltammetric peaks. Charge-trapping appears to occur as a result of regions of doped polymer becoming electronically isolated from the electrode substrate (carbon) during the dedoping process. Therefore the residual charge is not actually lost, but remains in the polymer as the potential is scanned through the insulating region and is then subsequently released as the polymer becomes conducting again in the opposite doping regime. The low energy cycle efficiency that could result from high levels of charge trapping in a capacitor device is

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likely to make PTFMPT an unsuitable electrode material. By far the best results for an electrochemical capacitor, were achieved with PFPT. The n-dopability of this material was far superior to both PT and PPT. Figure 5 shows cyclic voltammograms recorded for films of PT, PPT and PFPT that p-dope comparably to illustrate the improvements in n-doping. The electrolyte solution was 1 mol dm-’ Et,NBF, in acetonitrile. Some charge-trapping was also observed with PFPT, but this was much smaller in magnitude than that observed with PTFMPT. At a growth charge of 7.2 C cm-*, the n- to p&doping ratio is about 60% for PFPT in the EthNBF, solution. This is a significant improvement, however further approach to unity ratio is clearly desirable. We therefore investigated further ways to improve n-dopability to create the best system for our Type III capacitor. We achieved this by optimizing the electolyte. We have observed that, for a given polymer film, the ndopability was better at higher electrolyte concentrations. Furthermore, for a given polymer film, the n-dopability was also significantly better in a

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POTBNTIAL (V VI Af#Ag+) Fig. 4. Cyclic voltammogram of PTFMPT in 1 moldm-3 Et,NBF, in acetonitrile at SOmVs-’ showing a high degree of charge-trapping The polymer film was grown galvanostatically onto 0.2 em2 x 75 q carbon paper at 5 mA for 20 s, out of a solution of 0.1 mol dm- 3 3-(4-trifluoromethylphenyl)-thiophene, 1 mol dm- ’ Et,NBF, in acetonitrile.

Electrochemical properties of conducting polymers ‘or

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Fig. 5. Cyclic voltammograms at 25 mV s -’ for films of PT (thin full line), PPT (dashed line) and PFPT (thick full line), in 1 moldme Et,NBF, in acetonitrile. The polymer tilms were grown galvanostatically onto l.Ocmz x 75 w carbon paper at 2mA for 3OMs (PT) or 2000s (PPT and PFPT), out of 0.1 mol dm- 3 monomer, 1 mol dm- 3 Et,NBF, in acetonitrile.

Et,NBF, solution than in a Bu,NPF, solution of the same ionic strength. This latter observation can be explained in terms of easier penetration of the Et,N+ cation into the 8hn due to the ion’s smaller size. Borjas and Buttry[61] proposed that the identity of the cation does not play a major role in determining charging rates for n-doping at polythiophene, as concluded from a comparison of results with Bu,NPF, and with Me,NPF,. However, these authors investigated films only up to 0.1 pm thick, so on the time scale of their experiment, complete ndoping with tetrabutylammonium ions was apparently possible. We hoped to achieve further improvement in n-doping at our very much thicker polymer films (
those with larger tetraalkylammonium cations. Indeed, up to a polymer growth charge of 20Ccm-2, we saw a ratio of n- to pdedoping charges of PFPT tihns very close to 100%. Figure 6 shows cyclic voltammograms for a 6hn of PFPT in solutions of Me,NCF3S03 and 1 moldm-’ Bu,NPF, in acetonitrile, to illustrate the effect of cation size on n-doping. The two stages of improvement that we have achieved over the system of PPT in Et4NBF, acetonitrile solution, reported by Sato and co-workers[66-681, are summarized in Fig. 7. This figure shows how the ratio of n- to p-&doping charges depends upon polymer growth charge for : (i) Sato’s system; (ii) PFPT in 1 moldm-3 Et,NBF, in acetonitrile; (iii) PFPT in 1 mol dm- 3 Me,NCF,SO, in acetonitrile. The equivalence in the n- and pdoping levels that we have now achieved in the latter of these systems is not only beneficial in terms of energy and power densities, but is also advantageous since now equal amounts of polymer can be deposited on each electrode in the device, leading to easier bipolar plate fabrication and the ability to reverse the polarity of the device without affecting performance. We have investigated the influence of growth current density on n- and p-doping in PFPT. There appears to be little influence of growth current on the total charge densities for n- and p-doping, however the degree of apparent charge-trapping is affected. Analysis of charge-trapping can be achieved by integrating the current with respect to time in a cyclic voltammogram to obtain the charge--potential dependence. Such a curve is presented in Fig. 8, together with the original voltammogram, for a film of PFPT in 1 mol drne3 Me,NCF,SO, in acetonitrile. The total oxidation and reduction charges are equal over a complete cycle, as evidenced by a lack of any long term drift of the charge-potential plot. Hence, there is no net contribution from parasitic and decomposition reactions. There is, however, a charge imbalance within each of the individual doping/dedoping regions and this reflects the phenomenon of trapped-charge. Hence, there appears to be

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Fig. 6. Cyclic voltammograms at 25mVs-’ for a film of PFPT in l.0moldm-3 Me,NCF,SO, in ace.tonitrile (thick line) and in 1.0moldm-3 Bu,NPF, in acetonitrile (thin line). The PFPT lilm was grown galvanostatically onto 1.0 cm2 x 75 pm carbon paper at 2 mA for 4000 s out of a solution of 0.1 mol dm- 3 FPT, 1 mol dm- 3 Me,NCF,SO, in acetonitrile.

some component of either the p-doping charge, the n-doping charge or both, which is not released within the respective dedoping region/s. Figure 9 shows how Qr/Qu, the proportion of trapped charge to total doping charge, decreases at higher growth current densities for fihns deposited at the same overall growth charge. The relatively small degree of charge-trapping evident in Fig. 8 is thus explained by the high current density employed in this case for film growth. This dependence seems to suggest that the degree of charge trapping depends on polymer morphology, determined by the rate of 6lm growth.

3.2. Theoretical investigation into the effect of aryl substituents on the electrochemistry of polythiophenes In order to facilitate future design of conducting polymer materials with even better performance in high power energy storage devices, it is critical to understand the reason for the improvements we have achieved thus far in n-dopability. The dominant issue is the relative contributions of steric and electronic [including inductive (I) and resonance (M)] influences of the 3-position substituent on the properties of the conducting polymer, in particular

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TOTAL POLYMER GROWTR CAARGE - C/cm* Fig. 7. The improvements in ndopability, measured as the ratio of ndedoping charge to p&doping charge in a 50mV s-’ cyclic voltammogram, for different polymer thicknesses of (i) PPT in 1 moldmT3 Et4NBF, in acetonitrile (circles); (ii) PFPT in 1 moldme Et,NBF, in acetonitrile (triangles); and (iii) PFPT in lmoldm-” Me,NCF3S03 in acetonitrile (squares); In each of the three cases, at a given polymer growth charge, the pdedoping charges are approximately the same.

Electrochemical properties of conducting polymers

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Fig. 8. (a) Cyclic voltammogram and (b) the corresponding chargeqotential curve obtained by integration, for a film of PFPT in 1 moldm-3 Me,NCF,SO, in acetonitrile. The polymer film was grown galvanostatically onto 0.2cm2 x 75pm carbon paper at 5mA for 200s out of 0.1 moldmm3 FPT, 1 moldm-3 Me,NCF,SO, in acetonitrile. Also indicated are the sum of the absolute values of the doping charges, Qn, and the corresponding trapped charge, QT. as obtained from the integrated cu. The high current density used in this case for polymer growth results in a relatively low fraction of trapped charge. its capability

to n-dope. significantly

and reversibly.

Our intention in testing PFPF was that the electron withdrawing fluorine would shift oxidation and reduction waves anodically, which could therefore

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result in more facile n-doping. The electron withdrawing effect of a trifluoromethyl group is expected to be larger than that of a fluorine and so we hoped there be further improvement in PTFMPT. As evident in Fig. 4, this was not the case. Furthermore, the gain in using PFPT rather than PPT seems to be more of an improvement in the amount of n-doping that can take place, rather than in the potential at which n-doping takes place. Since the para substituents on the phenyl group are far removed from the polythiophene backbone, we expect that their intramolecular effects are primarily electrical. However, even though fluorine is more electron withdrawing (-1) than hydrogen, it is electron donating by resonance ( + M). In such cases, it is difficult to predict which factor, if any, predominates. The different monomers chosen were selected to present a variety of contributions of steric, inductive and resonance effects. Thus, in comparison to PPT, PFPT is {-I, + M}, but is very similar in terms of steric influence (there is little difference between the physical sixes of Ph-F and Ph-H). The CH, group in PIT is weakly { +I, + M}, but presents a greater intermolecular steric demand than hyrogen and fluorine, and the steric effect is exaggerated further by the t-butyl { +I} of PBPT. The CF, substituent in PTFMPT is strongly {-I} and has a comparable steric influence as CH, . One measure of a substituent’s electron donating or withdrawing ability is its Hammett u constant which includes the total electronic effects of that group attached to a benzene ring. Since the substituents on the phenyl group in the monomers we tested are in the pma position, correlations with upn constants were deemed most appropriate, where u.g.._is the sum of the inductive and resonance contn uttons (a1 + ux). Table 3 gives the values of a, and us for the various substituents, together with the experimental electrochemical data of monomer oxidation potential, and the peak potentials for n- and p-doping of the

30

25

20 SC 2

l5

o* 10

5

0

1

2

3

4

5

6

7

8

9

GROWTH CURRENT - mA Fig. 9. The influence of current employed for polymer growth on the ratio of trapped charge to total doping charge, QT/Qo. Polymer tilms were grown to the same total growth charge onto 0.2m’ x 75~ carbon paper, out of 0.1 mol dm- ’ 1 mol dmS3 Me,NCF,SO, in acetonitrile.

A. Runoa et al.

282

Table 3. A comparison of the tbaoratical electron withdrawing/donating abilities of the substituant, X, at tlte gara position of a phcnyl group, and the observed electrochemical poteutials for monomer oxidation (onset), polymer p-doping (peak), and polymer n-doping (peak) of the 3-aryltbiophenas we tested (referto Table 2) Substituent X H Me t-Bu

a1 Ip7l

% c97l

g,, (onmt) V vs. Ag/Ag+

&-&I @cab) V vs. Ag/Ag+

qc.c. (P=w V vs. AgJAg+

0.00 SO %O 0.5 0.45

0.00 -0.11 (-0.11) - 0.45 0.08

1.09 1.02 1.01 1.00 1.25

0.72 0.69 0.73 0.72 0.95

- 2.00 -2.12 -2.12 -2.04 -1.94

polymer. An investigation similar to this has already heen performed on the p-dopability of polythiophenes derivitized in the 3-position with nonaryl substituents such as halogens and nitriles[98].

In general, the trends are as expected, with the electron donating substituents (upr. < 0) shifting the monomer onset potentials and the doping peaks towards less positive potentials, and the electron withdrawing groups (epr. > 0) shifting them to more positive potentials. The effects on the electrochemical characteristics, however, are relatively small, with the exception of the CF, group. This may be due to diminishing resonance effects, resulting from large values of the dihedral angle (9) between the phenyl group and the thiophene. Molecular mechanics and MOPAC calculations on the monomers gives 9 = 2%30”, still allowing for some degree of communication between the two x systems (PCModel [Serena Software Inc.], Chem 3D+ [Cambridge Scientific Computing Inc.] and CAChe [Cache Scientific Inc.]). This calculation, however, only accounts for intramolecular steric interactions within a single monomer. Much greater inter- and intramolecular influence can be expected within a solid polymer film and these could well force the substituent bearing rings out of coplanarity with the polythiophene backbone. Preliminary MM2 calculations (CAChe) indicate that this could increase values of 9 to over 60”. This would explain why the voltammetric peak positions for n- and p-doping in PFPT are the same as those for PPT. Hence if the n-system of the substituent phenyl group is communicating only very poorly with the polythiophene backbone, then resonant electronic interactions cannot be used to explain why the n-dopability of PFPT is so much better in terms of charge density than PPT. We believe that the improvement in n-doping at PFPT may occur as a result of intramolecular electron transfer, by electron hopping or tunnelling from the polythiophene backbone into the substituent fluorophenyl group. An interesting result which may support this theory has recently been presented by Roncali and co-workersC69, 703, although these authors interpret their results differently. Roncali and co-workers observed an improvement in the ndoping of substituted polythiophenes when a fluorophenyl group was attached to the thiophene ether monomer via a -(C,H,)-0-(CH,)when an linkage, relative to the n-doping unsubstituted phenyl group was attached to the end of the same ether chain. In this case, direct electronic

communication from the fluorophenyl group into the polythiophene is totally prevented by the additional ether linkage. Although these authors propose that differences in the structural geometry of the two polymers explains the difference in n-doping, we suggest alternatively that it may be that the fluorophenyl group (and to a lesser degree the phenyl group) on its own has a reduction potential which allows it to reduce as the polythiophene backbone becomes n-doped. This may then also explain the very high degree of charge-trapping that we observed with PTFMPT (see Fig. 4). Because of the larger u_ value for -CF, than for -F, it can be assumed that the reduction potential for trifluoromethylphenyl is more positive than for fluorophenyl. Thus, when the polymer backbone n-dedopes and therefore becomes insulating, a high density of charges may remain on the trifluoromethylphenyl groups, trapped until the polymer backbone becomes conducting again in the early stages of p-doping. This theory requires further verification, however if it is indeed the case, then it may be possible to tune the reducibility of the substituent to exactly the potential range where the polymer backbone n-dopes and dedopes, and thus maximize the charge density of n-doping, while minimizing charge-trapping. 3.3. EQCM analysis ofPFPT In 1 moldme Et,NBF, in acetonitrile, the processes of p-doping and n-doping in a PFPT film are associated with anion insertion and cation insertion, respectively. This can be concluded from the results obtained in our EQCM experiments, where we studied a film of PFPT electrodeposited onto a gold 8lm on a quartz crystal. The dependencies of current, charge and quartz crystal resonant frequency shift, measured simultaneously during a cyclic voltammetry experiment with this system, are presented in Fig. 10. The data were recorded during a series of consecutive scans, so that the electrode was already cycling at the start of data collection. These results are in good agreement with those of Borjas and Buttry for polybithiophene (PBT) in a 0.1 moldm-’ solution of Me,NPF, in acetonitrileC611. An interesting feature of both our results for PFPT, presented in Fig. 10, and those of Borjas and Buttry for PBT, is the presence of two pre-peaks which lead the main peaks for p- and n-doping. Borjas and Buttry believe that these pre-peaks are both explained as the delayed release of charge that was trapped

283

Electrochemical properties of conducting polymers

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HYL’ENTIAL - V vs. AdAg+ Fig. 10. Simultaneous measurements of (a) current, (b) charge and (c) crystal resonant frequency shit% during a voltammetric experiment at 2OmV s-’ on a 6lm of PFPT in 1 mol dm-’ Et,NBF, in acctonitrile. The polymer film was deposited galvanostaticaUy at 0.5 mA for 150s out of 0.1 mol dm-’ FPT, 1 mol dm- 3 Et,NBF, in acetonitrile, onto a gold layer evaporated onto the quartz crystal. The active electrode area was 0.5cm’. during the previous doping cycle. However, our EQCM results for PFPT, shown in part (c) of Fig

10, suggest that the anodic prepeak at around + 0.3 V can be explained in terms of delayed dedop ing, but not so the cathodic prapeak at around - 1.7V. Figure llc shows that the process of

cationic unloading, which begins during the positive

potential scan between -2.1 and -1.7V, is completed much later in conjunction with the anodic pre-peak at +0.3 V. In contrast, the anionic mass increase brought about by p-doping between +0.4 and + 0.8 V is completely reversed by around 0 V on

284

A. RUIXSEet al.

POTENTlAL

- V vs. Ag/Ag+

Fig. 11. Repeat of the cyclic voltammogram in Fig. lla, with the voltage limits of the 20mV s-’ scan restricted se as to exclude the major doping peaks. This figure shows that the pre-peaks are observed independently of the doping peaks.

the negative potential scan, and therefore no residual mass needs to be unloaded at more negative potentials. Thus the cathodic prep& at - 1.7 V does not appear to correspond to the de-trapping of p-charge, as suggested by Borjas and Buttry. Figure 11 shows that the two pre-peaks are observed even when the limits of the potential scan are restricted to include neither of the main doping regions. We have observed that the presence of one pre-peak is dependent on the presence of the other (in the previous potential sweep). This confirms that the processes relating to these peaks cannot both be the release of trapped-charge. We suggest there are two possible explanations for the pre-peaks. Firstly the pre-peak on the negative scan at - 1.7 V, and the charge and EQCM data which accompany it, can be satisfactorily explained as an n-doping pre-peak, observed as a result of the gradual increase in electronic conductivity of the polymer film. Film reduction, which had been delayed by poor electron transport through an insulating medium, can take place at faster rates as the electronic conductivity of the film increases. This would then be expected to result in a voltammetric prepeak such as that observed here. This theory has been proposed previously by Gottesfeld et al. to explain the voltammetric pre-peaks sometimes observed in other systems, including that on the first electro-oxidation sweep at polyaniline electrodes[99]. The anodic prepeak on the positive potential scan at + 0.3 V would then be the release of this initial n-type charge, which seems to be trapped. The quartz crystal frequency changes (Fig. 1Oe) are compatible with this suggestion. A second possibility is that the pre-peaks may be caused by solution contaminants that are constantly being generated at the counter electrode through oxidation and reduction of the solvent. In all of our experiments the counter electrode was not separated

from the working electrode, and, in our EQCM experiments, the counter was positioned close to and above the working electrode. We have observed that the pre-peaks increase in size during repeated cycling through both doping regimes. Furthermore, less significant pre-peaks were observed in the n- and pdoping of polythiophene by Kaneto and co-workers[59, 653 who took the precaution of employing a polythiophene counter electrode to minimize the generation of contaminants. The sharpness of the peaks can be explained as a result of the sharp increase in electronic conductivity of the polymer film at the start of the doping cycles, allowing the redox processes of the contaminant to take place electrochemically. Because the pre-peaks are dependent upon each other, and are also retained when the electrode is transferred to fresh eletrolyte, the redox active contaminants would have to become adsorbed on or absorbed within the polymer film. Further experiments are required to identify conclusively the origin of these pre-peaks. From Fig. lob, the charge associated with the pre-peaks accounts for about 40% of the overall apparent trapped-charge. Therefore, release of trapped-charge not accounted for by the pre-peaks, is apparently spread out over the opposite doping region. Chargetrapping remains a potential problem in application of PFPT to electrochemical capacitors. 3.4. Impedance analysis of PFPT In order to elucidate the potential power density of an electrochemical capacitor based on our new conducting polymer system, we performed impedance experiments on a film of PFPT electrodeposited onto carbon paper. The electrolyte solution was 1 mol drnm3 Me,NCF,SO, in acetonitrile. Two impedance spectra obtained from this film are presented in Fig. 12, one corresponding to the n-doped state (- 2.0 V) and the other to the p-doped state

Electrochemical properties of conducting polymers

285

lyte and dividing the energy density of the system by this characteristic time[23]. The power density thus evaluated is 35 kW kg-’ of active material on both electrodes. This is signiiicantly in excess of current goals for an electrochemical capacitor in application to electric vehicles[loO]. We stress that this power density has been demonstrated for active lilms which have the thickness required to provide, at the same time, the volumetric energy density requirement from a capacitor device for transportation applications (65 J cm- ‘).

4. CONCLUSIONS We have been able to show that it is possible to improve the doping of polythiophenes by substituting the thiophene monomer in the 3position with different aryl groups. In general, the potentials for oxidation and reduction of the resulting polymers appear to follow the trends predicted by model cal0 culations based on both inductive and resonant elec22 24 26 28 tronic effects. However, the overall electronic effects observed in terms of doping potential shifts caused Rorl 2 (cl) Fig. 12. Impedance spectra of a film of PFPT in by derivitixation are rather small. We believe that in 1.0 mol dm- 3 Me,NCF,SO, in acetonitrile, in the n-doped poly-3-arylthiophenes, the aryl group is substantially state at -2.OV vs. Ag/Ag+ (full squares), and in the out of coplanarity with the connecting thiophene, p-doped state at +0.8V vs. Ag/Ag+ (hollow squares). The and therefore the influence of resonance factors to polymer tilm was electrodeposited onto a 0.2cm2 x 75~ any substituent effect is expected to be severely thick carbon paper electrode out of O.lmoldm-” FPT, restricted. In view of this we have proposed that the l.0m01dm-3 Me,NCF,SO, in acetonitrile, at 1 mA for significant improvement in n-doping of PFPT, rela2ooos. tive to PPT and PT, occurs as a result of intramolecular electron transfer from the polythiophene backbone to the fluorophenyl substituent. In order to maximixe ndopability of these conducting poly(+0.8 V). We found only relatively small variations in the impedance distribution and in the low fre- mers, we have synthesized a novel electrolyte, quency capacitance at different potentials within the tetramethylammonium trifluoromethanesulfonate This material is unique in its two doping regimes. The results displayed in Fig. 12 (Me,NCF,SO,). are very promising, because they indicate that the properties that relate to ndoping of polythiophenes, because it combines a small cation, Me,N+, with doping processes at PFPT in the novel electrolyte high solubility in acetonitrile (1.4 mol dm- “) and system employed are associated with low impedgood electrochemical stability to both oxidation and ances. The 23 R of high frequency impedance evident in Fig. 12 corresponds to iR losses in the layer of reduction. Furthermore, we have described a novel electrolyte solution between the working and refer- electrode fabrication method based on electroence electrodes. This component of impedance will polymerization onto a low weight, porous carbon not be evident in a capacitor device, where iR losses support that permits easy electrolyte access to a large volume of polymer. This may be advantageous will be restricted to a very thin layer of electrolyte between the two electrodes (ionic) and to the elec- in terms of device performance and fabrication. We have shown that, in application to electrotrode support materials (electronic). Electronic and chemical capacitors, n- and pdopable conducting ionic charge injection throughout the active layer polymers are able to provide greater energy density itself, in both the n- and p-doped films, is associated with an impedance of only about 2Qcm*, as seen than conducting polymers that can only be p-doped. Our new system of PFPT in Me,NCF,SO, in acefrom the difference between low and high frequencies in the real component of the impedance, Fig. 12. to&rile, represents a significant achievement for This impedance, associated with film charging, is high power, energy storage devices. Energy densities very much lower than those reported previously for such as those reported here have only been achieved previously with much thinner films of polyacetylene, polypyrrole[ 13, 76-821, polyacetylene[52-541, poly-p-phenylene and polythiophene. The ability to polythiophene[83], poly-3-methylthiophene[86] and reversibly n- and p-dope PFPT to high charge poly-3-octylthiophcneC87J. Indeed only polyaniline density at films over 10~ thick is unique among in acid solution appears to exhibit such low impedconducting polymers. If preliminary costing predicances associated with tihn charging[73-751. From tions are achievable and, more importantly, if satisour results, we have obtained an approximate value factory cycle life can be demonstrated, then these for the effective power density for a device by c&umaterials should have practical use for capacitor lating the RC time constant (B 4 s) for charging and devices. discharging a pair of PFPT electrodes in this electro-

286

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AcknowledgementsThis work was supported by the Advanced Industrial Materials (AIM) Program and by the Electric/Hybrid Propulsion Division, both of the U.S. Department of Energy.

REFERENCES 1. B. E. Conway, J. electrochem. Sot. 138,1539 (1991). 2. A. F. Burke, in Preprints of the Annual Automotive Technology Development Contractors Coordination Meetina. Vol. 2: Deerborn. Michiaan, October (19911. . ,. publ. U.S. Dept: of Energy.. 3. I. D. Raistrick, in The Electrochemistry of Semiconductors

and

Electronics-Processes

and

Devices

(Edited by J. McHardy and F. Ludwig), p. 297. Noyes, New Jersey (1992). 4. S. T. Mayer, R. W. Pekala and J. L. Kaschmitter, J. electrochem. Sot. 140,446(1993). 5. T. Nogami, H. Mikawa and M. Nawa, J. Phys C3 44, 573 (1983). 6. I. Tanahashi, A. Yoshida and A. Nishino, Jpn. Kokai Tokkyo Koho JP 64 01, 218; 5 January 1989 (Chem. Abs. No. 111: 89017s). I. J. R. Miller, in Proceedings of the Tenth International Seminar on Primary and Secondary Battery Technology and Applications; Deerfield- Beach, -Florida, March (1993): (Edited bv S. P. Wolskv and N. Marincic). Florida BducaGonal Seminars Inc., Boca Raton. 8. S. Sarangapani, P. Lessner, J. Forchione, A. Griflith and A. B. LaConti, Proceedings of the 25th Intersociety Energy Conversion Engineering Conference, Vol. 3, Rena, Nevada, August (1991). (Edited by P. A. Nelson, W. W. Schertx and R. H. Till). American Institute of Chemical Engineers, New York. 9. R. H. Baughman, Makromol. Chem., Macromol. Symp. 51, 193 (1991). 10. S. Gottesfeld, in Proceedings of the First International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Deertield Beach, Florida,

December (1991). (Edited by S. P. Wolsky and N. Marincic). Florida Educational Seminars Inc., Boca Raton. 11. E. Hannecart, E. Destryker, J. Fauvarque, A. De Guibert and X. Andrieu, Eur. Pat. Appl. EP 413,382; 20 February (1991) (Chem. Abs. No. 115: 75334~). 12. D. Naegele, Springer Ser. Sol. St. Sciences 91 (Electronic Properties of Conjugated Polymers III), 428 (1989). 13. K. Naoi,M. M. Lien, W. H. Smyrl and B. B. Owens, ADD. Phvs. Commun. 9. 147 (1989). 14. D.*Ofer; R. M. Crooks a& M.’ S. Wrighton, J. Am. them. Sot. IQ7869 (1990). 15. E. Dalas, Sol. St. Commun. 77.63 (1991). 16. E. Dalas, J. Mater. Sci. 27,453 (1992). 17. K. Kaneto, K. Yoshino and Y. Inuishi, Jap. J. Appl. Phys. 2 22, L567 (1983). 18. A. Yasuda, T. Nagai and T. Yamahira, Jpn. Kokai Tokkyo Koho JP 01 06,383; 10 January (1989) (Chem. Abs. No. 110: 216331~). 19. N. Li, J. Y. Lee and H. Ong, J. appl. Electrochem. 22, 512 (1992). 20. T. Nagatomo, T. Honma, C. Yamamoto, K. Negishi and 0. Omoto, Jap. J. Appl. Phys. 22, L275 (1983). 21. T. Nagatomo, C. Ichikawa and 0. Omoto, J. electrothem. Sot. 134,305(1987). 22. Y. Kobayashi, T. Shishikura, H. Nakamura, H. Konuma and K. Fuxita, Synth. Met. 18,619 (1987). 23. A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld and J. P. Ferraris, J. Power Sources 47,89 (1994). 24. J. Heinxe, Top. Curr. Chem. 152, 1 (1990).

25. G. K. Chandler and D. Pletcher. Electrochemistry 10, 117 (1985). 26. A. F. Diax J. I. Castillo, J. A. Loaan and W-Y. Lee. J. electroanal. Gem. 129,115(1981): 28. 27. T. F. Otero. R. Teiada and A. S. Elola., Polvmer _ 651(1987). 28. G. B. Street, T. C. Clarke, R. H. Geiss, V. Y. Lee, A. Naxxal, P. Plluger and J. C. Scott, J. Phys. C3 44,599 (1983). 29. S. Gottesfeld, A. Redondo and S. W. Feldberg, J. electrochem. Sot. 134,271(1987).

30. M. Kalaji, L. M. Peter, L. M. Abrantes and J. C. Mesquita, J. electroanal. Chem. 274,289 (1989). 31. P. J. Nigrey, D. MacInnes Jr., D. P. Naims, A. G. MacDiarmid and A. J. Heeger, J. electrochem. Sot. 128, 1651(1981). 32. K. Kaneto, M. Maxtield, D. P. Naims, A. G. Mao Diarmid and A. J. Heeger, J. them. Sot. Forad. Trans. I 78 3417 (1982). 33. A. 6. MacDianhid, R. B. Kaner, R. J. Mammone and A. J. Heeaer. J. Phvs. C3 44.543 (1983). 34. R. B. Kane; and -A. G. MacDi&nid, J. them. Sot. Farad. Trans. I 80,2109 (1984). 35. M. Maxfield, S. L. Mu and A. G. MacDiarmid, J. electrochem. Sot. 132,838 (1985). 36. R. B. Kaner and A. G. MacDiarmid, Synth. Met., 14,3 (1986). 37. J. Caja, R. B. Kaner and A. G. MacDiarmid, J. electrochem. Sot. 131,2744 (1984). 38. F. Ignatious, B. Fran9ois and C. Math& Polymer 31, 1108 (1990). 39. M. Fouletier, P. Deaott and M. B. Armand. Solid State Ionics g, 165 (1983). 40. R. B. Kaner. S. J. Porter. D. P. Naims and A. G. MacDiarmid, J.&m. Phys. bo,5102 (1989). 41. T. Nagatomo, C. Ichikawa and 0. Omoto, Synth. Met. 18,649 (1987). 42. P. J. Nigrey, D. MacInnes Jr., D. P. Naims, A. G. MacDiarmid and A. J. Heeger, J. electrochem. Sot. 128,1651(1981).

43. M. Armand, J. Phys. C3 44,551(1983). 44. A. F. Diax and T. C. Clarke. J. electroanal. Gem. 111. 115 (1980). 45. F. Gl Will, J. electrochem. Sot. 132,2351(1985). 46. A. Padula B. Scrosati. M. Schwarx and U. Pedretti. J. electrochem. Sot. 131,276l (1984). 47. K. H. Dietz and F. Beck, J. appl. Electrochem. 15, 159 (1985). 48. G. C. Farrington, B. Scrosati, D. Frydrych and J. DeNuxxio, J. electrochem. Sot. 131, 7 (1984). 49. L. W. Shacklette, J. E. Toth, N. S. Murthy and R. H. Baughman, J. electrochem. Sot. 132,1529 (1985). 50. J. F. Fauvarque, M. T. Descarsin and S. Sadki, Fr. Demande FR 2,661, 280; 25 October (1991) (Chem. Abs. No. 116: 238840n). 51. R. H. Baugman, L. W. Shacklette, N. S. Murthy, G. G. Miller and R. L. Elsenbaumer, Mol. Cryst. Litb Cryst. 118,253(1985). 52. T. R. Jow and L. W. Shacklette, J. electrochem. Sot. 135,541(1988). 53. M. Schwarxenberg, K. Jobst and L. Sawtschenko, Electrochim. Acta 35,403 (1990). 54. G. Nagele, G. E. Nauer, H. Kuxmany and J. Kiirti, Synth. Met. 21, 293(1987). 55. N. S. Mm-thy, L. W. Shacklette and R. H. Bauahman. _ , Phys. Rev. B-h,3708 (1990). 56. T. R. Jow. L. W. Shacklette and M. Maxfield. Ext. Abst. 166th Meeting Electrochem. Sot. 84-2. ~1 _ 902. New Orleans, October (1984). 57. N. S. Murthy, R. H. Baughman, L. W. Shacklette, H. Fark and J. Fink, Sol. St. Commun. 7g, 691 (1991). 58. L. W. Shacklette and J. E. Toth, Phys. Rev. B 32, 5892 (1985).

Electrochemical properties of conducting polymers 59. K. Kaneto, S. Ura, K. Yoshino and Y. Inuishi, Jap. J. Appl. Phys. 23, L189 (1984). 60. J. Roncali, Chem. Rev. 92,711(1992). 61. R. Botjas and D. A. Buttry, Chem. Mater. 3, 872 (19911. 62. k. MI Crooks, 0. M. R. Chyan and M. S. Wrighton, Chem. Mater. 1.2 (1989). 63. A. R. Hillman, M. J. Swarm and S. Bruckenstein, J. electroanal. Chem. 291, 147 (1990). 64. M. Mastranostino and L. Soddu, Electrochim. Acta 3% 463 (1990). 65. K. Kaneto and K. Yoshino, Polym. Prepr. (Am. Chem. Sot., Div. Polym. Gem.) 25,255 (1984). 66. M.-A. Sato, S. Tanaka and K. Kaeriyama, Makromol. Chem. 190,1233 (1989). 67. M.-A. Sato, S. Tanaka and K. Kaeriyama, J. them. Sot. Chem. Commun. 1725 (1987). 68. K. Kaerivama S. Tanaka, M.-A. Sato and K. Hamada, !?ynth: Met. 2% C61i (1989). 69. J. Roncali. H. K. Youssoufi. R. Garreau. F. Gamier and M. iemaire, J. them. sot. Chem. ~ommun. 414 (1990). 1 K. Youssoufi, R. Garreau, F. Gamier, M. Lemaire 70. H. and J. Ron&, Synth. Met. 41-43,2916 (1991). 71. I. P. Ferraris and T. L. Lambert, J. c/tern. Sot. Chem. Commun. 1268 (1991). 72. M. M. Musiani; Ele&ochim. Acta 35, 1665 (1990). 73. I. Rubinstein, E. Sabatani and J. Rishpon, J. electrothem. Sot. 134,3078 (1987). 74. P. Fiordiponti and G. Pistoia, Electrochim. Acta 34, 215 (1989). 75. S. H. Glarum and J. H. Marshall, J. electrochem. Sot. 134,142 (1987). 76. R. A. Bull, F.-R. F. Fan and A. J. Bard, J. electrochem. Sot. 129, 1009 (1982). 77. J. Tanguy and N. Mermilliod, Synth. Met. 21, 129 (1987). 78. S. Panero, P. Prosperi, S. Passerini, B. Scrosati and D. P. Perlmutter, J. electrochem. Sot. 136,3729 (1989). 79. M. J. van de; Sluijs, A. E. Underhill and B..N. Zaba, J. Phvs. D: ADDS.Phvs. 20. 1411(19871. 80. N. I&rmillio~, J. fangu; and ‘F. Petiot, .I. electrothem. Sot. 133,1073 (1986). 81. G. L. Duffit and P. G. Pickup, J. phys. Chem. 95,9634 (1991).

287

82. F. Trinidad, J. Alonso-Lopez and M. Nebot, J. appl. Electrochem. 17,215 (1987). 83. T. F. Otero and E. De Larreta, J. electroanal. Chem. 244,311 (1988). 84. X. Ren and P. G. Pickup, J. electrochem. Sot. 139, 2097 (1992). 85. G. L&ng and G. In&, Electrochim. Acta 36, 847 119911 86. J. Tanguy, J. L. Baudoin, F. Chao and M. Costa, Electrochim. Acta 37, 1417 (1992). 87. J. Bobacka, M. Grzeszauk and A. Ivaska, Electrochim. Acta 37, 1759 (1992). 88. B. V. Tilak, C.-P. Chen and S. K. Rangarajan, J. electroanal. Chem. 324,405 (1992). 89. C. Ho, I. D. Raistrick and R. A. Huggins, J. electro&em. Sot. 127,343 (1980). 90. D. A. Buttry, Electroanal. Chem. 17, 1 (1991). 91. M. Lemair~; R. Garreau, D. Delabouglise, J. Roncali, H. K. Youssoufi and F. Gamer. New J. Chem. 14,359

ww.

92. J.-P. Monthkard, J. F. Delzant and M. Gazard, Synth. Commun. 14,289 (1984). 93. H. Wynber& H.‘ van’ Driel, R. M. Kellogg and J. Buter, J. Am. them. Sot. 89,3487 (1967). 94. C. V. R. Sastri, A. K. Marwah, P. Marwah, G. S. Rao and D. R. Shridhar, Synthesis 1024 (1987). 95. G. Sauerbrey, 2. Phys. 155,206 (1959). 96. V. D. Pokhodenko and V. A. Krylov, Synth. Met. a-43.533 (1991). 97. T. H. Lowry and K. S. Richardson, Mechanism and Theory in Organic Chemistry, p. 143. Harper and Row, New York (1987). 98. R. J. Waltman, A. F. Diaz and J. Bargon, J. electro&em. Sot. 131, 1542 (1984). 99. S. Gottesfeld, A. Redondo, I. Rubinstein and S. W. Feldberg, J. electroanal. Chem. 265, 15 (1989). 100. A. F. Burke, in Proceedings of the Second International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Dccrtield Beach, Florida,

December (1992) (Edited by S. P. Wolsky and N. Marincic). Florida Educational Seminars Inc., Boca Raton.