Jun 15, 1989 - Katsuhiko Naoi, William H. Smyrl, Boone B. Owens and Mary M. Lien. Prepared for Publication in. Extended Abstracts, 175th National Meeting ...
Lfn OFFICE OF NAVAL RESEARCH Contract N00014-88K-0360 R&T Code B41CO04DAROI Technical Report No. I
Capacitive Behavior in Conducting Polymers: AC Impedance and Quartz Crystal Microbalance Studies by Katsuhiko Naoi, William H. Smyrl, Boone B. Owens and Mary M. Lien
Prepared for Publication in Extended Abstracts, 175th National Meeting of the Electrochemical Society, Los Angeles, California, 1989
Corrosion Research Center Department of Chemical Engineering and Materials Science 221 Church St. SE University of Minnesota Minneapolis, MN 55455
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Capacitive Behavior in Conducting Polymers:
AC Impedance and Quartz Crystal Microbalance S
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Katsuhiko Naoi, William H. Smyri, Boone B. Owens and Mary M. Lien
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175th Meeting of the Electrochemical Society, Extended Abstracts, May 1989 17
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IS. SUBJECT TERMS (Continue in reverie if necessary and identify by block number)
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Conducting polymers, capacitance, microbalance, r I
'
.
impedance
........
9g ABSTRACT (Continue on'reverie if necessary and identify by block number)
-'Aow-frequency capacitive behavior as well as ionic transport of anions in electropolymerized conducting polymer filmas, such as polypyrrole, were investigated by using AC impedance analysis. From the impedance spectra at two different frequency regions, the diffusion coefficient of-the dopant (anions) and the redox capacity for the polymer filmss were estimated. The mass and volume changes during the charge-discharge process were also analyzed with a quartz crystal microbalance and phase detection interferoimetric microscopy. -
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Boone B. Ovens 00 FORM 1473.84 -MAR
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Capacitive Behavior in Conducting Polymers:
CL'I
AC Impedance and Quartz Crystal Microbalance Studies
-
Katsuhiko Naoi. William H. Smytl. Boone B. Owens. and farv M. Lien Corrosion Researcn Center. Departirent of Cherrucal Engineering and Materials Science. University of Minnesota. 221 Church St SE Minneapolis. Min 54 Conducting polymers ilikQ pofypyrrole) show super Capacitve behavior (100-1000 F/ iO) at low frequencies( ;4f. In practical application. hese materals(thin films may be utilized in high energy capacitors. The low-frequency capacitve behavior in such films is ill defined and still unclanfied because of the superposition of capacitive currents on the redox process which occurs at electroactive sites, The behavior has been explained in several ways. for exampie. loutile-layer formauon(l.21. overdoping of anions(31. and non-Nernstian redox processes(31. In the pTsent investigation. AC impedance analysis was adopted to clarify this behavior in electrically conducting polymers(4-61. From :he impedance spectra at two different frequency regions. the diffusion coefficient. D. of dopant anions and the redox Capacity. CL. for inoe.mer films were esnmatedf The variations in redox capacity and diffusivity of anions for polypyrrole and other films are compared with the mass and volume changes that occur during the charge-discharge process. The electrochemical ceils incorporated solutions of various lithium salts in either acetonitmle or propylene carbonate. The polymer doping process was facilitated by the transport of the anions across e eiec -,, t e ec tr e interface during the andic reaction. AC Impedance Analysis The impedance spectra of polymers exhibits the behavior typical of thin redox and electronically conductive polymer filmsilike polyvinylferrocene). as shown in Fig. 1[4-61. At nigh frequenctes region A). charge transfer domination is observed with a semi- circle and. at lower frequencies(region B) diffusion of the anion in the polymer film dominates the impedance results. Finally at the lowest frequencytregion C), the finite film thickness limits the extent of diffusion behavior. and the locus rises vertically, owing to the saturation of resistance and capacitance components. T1'
CLCL f - 0
Fig.2 Re~anonshtp between Zumg5 Uco At very low frequencies(< Ca. 30 mz,. the locus for each film becomes vertical, and plots of -Z, vs. 1/ca become linear asshown in Fig. 2. From the slope of the curves in Fig. 2. the redox capacitance (CL) of each film was calculated by using Eq. (2J. At interrt, ediate frequencies(ca. 40 mHz < f < -10 mHz). each locus was linear with unit slope. In this region, a plot of Z 2 vs. u-11 showed a straight line in agreement with Eq.Il. By combining the results for values of CL witn the Z VS.WIr2 plots. values of the diffusion coetficienttD) were obtained. &.
QCM & PDiM4Analyuis
The mass changes during both electropolvmenzation and charge-discharge processes were monitored by using a Quartz Crystal MicrobalanceiQCM). In order to check the volume change in the polymer film during the redox process. Phase Detection Interteromemc Microscopy (PDL.I) was applied, and its dependence on charge/discharge rates was examined. C;'arge-DischargeChtaracteristics For example. polypyrrole behaved more like a capacitor A.,th the linear increase/decrease in the chrgedischarge curmes, while polyazulene had a fiat discharte. The tatter behavior is due to : e higher diffusivityiD) and higher redox potential(Epu) of polvazuleneiD.7.5 x 10- 4 cm: s-1- Ep-,ca. 3.05 V) compared to that of poiypyrrole(D-l.0 X104 cm- s'l; Epac-ca. 3.35 V).
Acknowledgement The authors wish to acknowledge the financial suoport of Defense Advanced Research Projects Agency and the Office of
Nthe A ----
4. C
Naval Research.
Re ferences
-3
Z real Fig.l Typical Cole-Cole plot for a conducting polymer electrode. In the diffusion contolled region, where the impedance X14, the magnitude of the impedance is given by angle_4 phase Eq.[I]
1) S. W. Feldberg.J 4m. Chem. Soc. 106. 4671 (19841. 2) B. 1. Feldman. P. Burgermayer and R. W Murray, ibid.. 107. 372 (1985). 3) . Tanguy. N. Mermillotd and 'A. Hoclet. I Elecrrochem. Soc.. 134. 795 (1987). 4) C. Ho. I. D. Raismck and R. A. Huggins. .bd. 127, 343 ( 1980). T. B. Hunter. P. S. Tyler. W H. Smyrl and H. S. White. 5) T. ibid.. 134, 2198 (1987). 6) T. Osaka and K. Naoi. Bull. Chem. Soc. Jpn., 5S. 36
where CL, D. and L ae the tow frequency redox capacitance, diffusion coefficient and the polymer film thickness, respectively. Values of CL were estimated from the low frequency impedaice data (charge saturation region: a< L2
BY .r -utor./
In tis range, the phase
angle approached m2andCLwascalculatedusingEq[21.
/
/ -.
D.
Av:ilability Codes
pt-h
Avl1 and/or
ca I
