Inductance effect in the AC impedance of conducting polypyrrole films

Synthetic Metals, 55-57 (1993) 3637-3642

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INDUCTANCE EFFECT IN THE AC IMPEDANCE OF CONDUCTING POLYPYRROLE FILMS

YONGFANG LI, RENYUAN QIAN Institute of Chemistry, Academia Sinica, Beijing 100080, China

ABSTRACT It was found that a conducting polypyrrole (PPy) film strip with two Ag-paste electrodes exhibited an inductance in the AC impedance measurement. An equivalent circuit was proposed for an approximate analysis of experimental data. The effect of the physical dimensions on the parameters of the elements of the equivalent circuit is given. The parameters were found to be sensitive to the applied AC field strength. Although the DC conductivity of a conducting PPy film is isotropic in the film plane, the conductivity for the applied field in the thickness direction is some 4 orders of magnitude smaller and highly non-ohmic even at low fields of ow~,r 4Vmm -1. The anisotropy in the thickness direction of AC impedance is also very pronounced.

INTRODUCTION Rechargeable Li/polypyrrole (PPy) battery has been developed jointly by BASF and Varta[1]. Recently Naegele[2] developed a supercapacitor consisting two thin PPy films one in its oxidized form while the other in its reduced form and a separator film in between. A 20mm diameter, 2.5mm thick button cell showed a capacity of 10Farad and a 7mm diameter, <0. I mm thick cell showed a capacity of 0.1Farad. It is interesting enough that a dry PPy film in its oxidized form exhibits an inductance by AC impedance measurement when the field is applied in the film plane. Although the DC conductance and AC impedance of conducting PPy are isotropic in the film plane it is anisotropic when the field is applied in the thickness direction.

EXPERIMENTAL Conducting PPy films with CI- and TsO- as counter anion were prepared electrochemically from aqueous solution of 0.1M pyrrole, 0.2M KCI or TsONa adjusted to pH 3 by adding HC1 or TsOH respectively, using indium doped tin oxide (ITO) conducting glass plate or stainless steel plate as electrodes at a current density of lmAcm-2. The film was washed and dried. DC conductivity in the film plane was measured by the four-probe method using Ag paste as the contacts. AC impedance measurement of the PPy film with two Ag paste contacts painted on a 0379-6779/93/$6.00

© 1993- Elsevier Sequoia. All rights reserved

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film strip was performed on a Solartron Model 1250 Frequency Response Analyser with a Model 1286 Electrochemical Interface. For measurements with the field applied across the thickness of the film two Ag paste contacts were used. RESULTS AND DISCUSSION The inductance effect A Ppy(TsO-) film grown on ITO glass of 48pm thickness was cut into strips of 5ram wide and two Ag paste contacts were painted on the film at a inter-contact distance of 11ram. Results of impedance measurement from 10Hz to 64kHz are shown in the Niquist plot (Z" vs Z') of Fig, la, where Z' is the real part and Z" the imaginary part of the impedance Z, Le. Z = Z' + jZ", where j2=-1. The results for a PPy(CI-) film also grown on ITO glass, 6jam thick, 3.5ram wide and 8ram between the contacts are similarly shown in Fig, lb. In both cases the applied voltage of AC source was 100mV RMS. The data points fall nicely on a circular arc, but the center of the circle does not lie on the real axis but deviated in the negative direction of the imaginary axis. For an analysis of the experimental data the center of the circular arc is considered to be rotated in counter clockwise sense of the figure through an angle. This behavior can be simulated by an equivalent circuit shown in the cut of Fig, 1. -3

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Fig. 1. AC impedance measurements on (a) PPy(TsO-) and (b) PPy(CI-) films. solid curves - simulation by the equivalent circuit shown in the insert; x in (b) - simulation by a capacitor connected across X,Y in the equivalent circuit. This equivalent circuit contains a resistance R 3 in series with the inductance L. This R 3 is assumed to be proportional to frequency so that R3 = boo, where b is a constant and co the circular frequency, so as to result in the rotation of the center of the circular arc in the Z'-Z" plane. In this equivalent circuit R 1 represents the DC resistance of the specimen plus the contact resistance, R2 -1 signifies the energy loss independent of frequency while R3 signifies the energy loss proportional to frequency. Analysis of the circuit as given in the Appendix shows that the data points measured at various frequencies lie on a circular arc which cuts the real axis at zero frequency at (RI,0) and at infinite frequency at (R1 +R2,0) and the center of the circular arc lies on a radial line from (R 1,0) and rotated towards the negative direction of the imaginary axis by an angle of tan-l(b/L). However, this model is not really correct in the sense that the

3639

circular arc should terminate on the real axis at infinite frequency and the circuit should never be capacitive, i.e. Z" can not be negative. However experimental data in some cases do not conform to these requirements. In the context of this equivalent circuit approximation the values of R1, R2, L and b for the two PPy films mentioned above are: PPy(TsO-) PPy(CI-)

R 1 =39.1ohm, R2 =2.76ohm, L =23.21JH, b =14"10-6ohm.s R 1 =208.5ohm, R 2 =14.0ohm, L =ll5/JH, b =55.10 -6 ohm.s

Experimental data seem to extend smoothly into the capacitive quadrant of Fig, Ib at higher frequencies. Apparently a capacity should be involved in the equivalent circuit. In fact a capacity of 170pF connected across X,Y of the equivalent circuit for PPy(CI-) will roughly simulate the experimental data as also shown in Fig. lb leading to a Niquist plot extending into the capacitive quadrant at frequencies higher than 50kH~ Only a small adjustment of the circuit parameters was needed. Film dimension effect on the inductance A PPy(TsO-) film grown on Stainless steel of 251am thickness was cut into strips of 3.6mm width painted with Ag paste contacts at 8.2mm (A) and 24.6mm (B) distance. Another strip of 15.4mm width was painted with Ag paste contacts at 8.2mm distance (A'). Results of impedance measurements are shown in Fig, 2 and Table 1.

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Fig. 2. Film dimension effect on the AC impedance measurements on a PPy(TsO-) film, A, A' and B referring to Table 1. TABLE 1 Dimension effect on the inductance of a 25~um thick PPy(TsO-) film Specimen A A' B

width distance between mm contacts, mm 3.6 8.2 15.4 8.2 3.6 24.6

R1 R2 ohm ohm 16.5 3.26 3.1 1.00 41.9 11.I

L laH 14.3 3.1 34.6

b.106 ohm.s 1.56 0.46 2.90

The data indicate that the inductance increases in pace with the resistance when the dimension is changed but not in proportion. The energy loss of the inductor increases with the width of the film strip and decreases with the distance between the contacts.

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Effect Qf aonlied voltage It was found that the impedance behavior of PPy film was sensitive to the applied AC voltage across the contacts. For example, a PPy(TsO-) film grown on stainless steel of 48pm thickness, strip width 5ram, distance between contacts 11mm was applied with AC voltages of 10, 50 and 100mV RMS to give the results shown in Fig, 3 and Table 2. The applied field strength was rather small in the order of 10-3 to 10-2Vmm-1 yet it showed significant difference when the applied field was higher than 5.10-3Vmm-1.

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Fig, 3. Effect of applied voltage on the AC impedance measurements on a PPy(TsO-) film, o 10mY, 65Hz to 52kHz; + 50mV, 65Hz to 65kHz; • 100mV, 65Hz to 65kHz TABLE 2 Effect of applied AC voltage on a PPy(TsO-) film strip of 481am thick, 5mm wide and I lmm between contacts Applied voltage mV 10 50 100

R1 ohm 66 66 68

R2 ohm 2.10 2.54 3.66

L pH 7.68 7.67 8.19

b.106 ohm.s 2.32 2.96 5.20

Anisotroov in the thickness direction The conductivity of PPy as polymerized is isotropic in the film plane, however across the thickness the conductivity is much lower. This was first observed by Watanabe e t all 3] with two Pt contacts on the two surfaces of a PPy(CIO4-) film that the conductivity through the film was 4 orders of magnitude smaller than the conductivity in the film plane. This has its origin in the anisotropy in the structure of chain packing in the film, as studied by Mitchell e t aL[4] using X-ray diffraction. They showed that the PPy chains have the tendency of the planar conjugated segments of the chain lying parallel to the film plane. As the distribution of counter anions is strikingly uniform across the thickness direction[5], so this anisotropy in conductivity cannot be attributed to skin-core effect. Rather the anisotropy is a manifestation of the difference in the carrier transport properties in the conjugation plane and along the direction normal to the conjugation plane. This view point is clearly borne out by the following experimental result~ A PPy (TsO-) film of 1lpm thickness and an area of 3xSmm2 was painted with Ag paste contacts

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on both surfaces showed ohmic behavior in DC measurements with applied fields of 0.l-lVcm -I and a conductivity of 6.1.10-3Scm -1, while the conductivity measured by ordinary four-probe method, i.e. the applied field in the film plane, was 72Scm-1, an anisotropy of l04. The usual experience is that a thinner PPy film has higher conductivity than a thicker one. Recently Zinger eta/.[ 6] gave the concrete figures that a PPy(C104-) film polymerized in aqueous solution of 39~um thickness showed a conductivity of 177Scm-l as compared to a 38~ m thick film of 3.3Scm-1. This could only mean that in the thicker film the alignment of the conjugated segments parallel to the film surface will be poorer in a thicker film. This is perhaps also the reason why the conductivity of a PPy film is so much dependent on the conditions of polymerization, as Play polymerized at lower temperatures usually results in higher conductivity may be attributed to a better planar alignment of the conjugated segments[4]. Another PPy(TsO-) film of 25pm thickness and an area of 5.7x5.7mm with Ag paste contacts on both surfaces showed a volt-ampere characteristics as shown in Fig. 4.

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Fig. 4. Volt-ampere characteristics of a PPy(TsO-) film measured across the film thickness direction, 1V = applied field strength of 40Vmm -1. It shows an ohmic region for the applied field strengths less than 4Vmm-1, followed by a superlinear relationship at higher fields, typical of an organic semiconductor. The volt-ampere curve obtained is perfectly symmetric with respect to the polarity of applied voltages, It is believed therefore that the low conductivity observed across the thickness direction is hardly a contact effect but an intrinsic property of the PPy film. The pronounced deviation from ohmic behavior at such low applied field is really striking. In the ohmic region the conductivity observed was 5.9-10-4Scm-1. In AC impedance measurements with applied field in the thickness direction of a PPy(TsO) film of 50~m thickness and an area of 2x2mm 2 the locus of data points look very much different from measurements in the film plane. Z' (real part) values are much higher and only in very low frequencies Z" values are positive (inductive). At higher frequencies the data points fall in the capacitive quadrant as shown in Fig 5. Of course the electrode configuration for measurements across the film is different from the measurement in the film plane, it should exhibit considerable capacitance.

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Fig, 5. AC impedance measurements on a PPy(TsO-) film across the film thickness direction ACKNOWLEDGEMENTS The authors like to express their thanks to Profs. Wenzhi Yang and Huaqian Yang, Peking University, for the use of Frequency Response Analyzer. This work was supported by National Natural Science foundation of China. REFERENCES [1] R. Brittihn, G. Ely, F. Woeffler, Makromol. Chem. MacromoL Syrup. 8, 51 (1987). [2] D. Naegele, in H. Kuzmany, M. Mehring, S. Roth (EdS.),~ Properties of Conjugated Polvmers III,, Springer Verlag, Berlin, 1989, p.428. [3] A. Watanabe, S. Murakami, K. Mori, Y. Kashiwaba, Macromolecules 22, 4231 (1989). [4] G.R. Mitchell, A. Geri, J. P_h,v&D20, 1346 (1987). [51 B. Zinger, P. Sliaier, A. Zemel, Syntk MeL 40, 283 (1991). APPENDIX For the equivalent circuit shown in the cut of Fig, 1 when leaving out R1, which corresponds to shifting the orion of (Z',Z') plot to (R 1,0), we have the admittance Y = Y' -jY", where Y' = 1/R2 + b/~0(b2+L2), Y" = L/cO(b2+L2). In polar coordinates (r,0) we can write Y = exp(-jO)/r --(cos0+j sin0)/r, that is cos0/r=Y' and sin0/r=Y", or tan0= R2L/[60(b2 +L2)+b/R2]. Then r = ~ ( b 2 + L2)sin0/L = sinO/L(R2L/tanO- bR2) = R2 [cos0- (b/L)sin0]. Putting b / L = t a n ~ , we have r = (R2/cos~ ) cos($+~), which is an equation of a circle with its center on the radial line rotated through an angle from the Z" axis and a diameter of length R2/cosg.