J . Phys. Chem. 1991, 95, 9634-9635
9634
Permselectlvlty of Polypyrrole in Acetonitrile Gerri Lynn Duffitt and Peter G. Pickup* Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada AI B 3x7 (Received: September 12, 1991)
The overpotential required for the initial reduction of polypyrrole in acetonitrile has been linked to a large increase in the ionic conductivity of the polymer. It is proposed that the low mobility of counterions in the permselective oxidized polymer necessitates cation insertion during the initial reduction. Reoxidation is accompanied by anion insertion to produce a nonequilibrium state in which the polymer contains excess electrolyte. Relaxation to the low ionic conductivity equilibrium state is very slow in 0.1 M electrolyte solution but rapid in pure solvent.
Introduction Ion transport plays a key role in many of the applications of polypyrrole and other conducting polymers and has therefore attracted considerable interest. Progress in this area has been hampered by uncertainty concerning the concentrations of mobile ions in these materials. In particular, the interpretation of their ion transport behavior depends critically on whether or not they are permselective, and the currently available data on this subject are conflicting. Burgmayer and Murray’ have clearly shown that oxidized polypyrrole is permselective in 1 M aqueous KCI and that its ion transport properties are dominated by counterion (anion) transport. On the basis of this finding, we treated ion transport in polypyrrole as the migration of counterions.* For 1 M aqueous KCI electrolyte the results were in good agreement with those reported by Burgmayer and Murray,] and so it was assumed that the same treatment would be valid in acetonitrile containing 0.1 M Et4NC104.2 This assumption appeared to be justified when it was found that poly[ 1 -methyl(pyrrol-1-ylmethyl)pyridinium] is permselective in acetonitrile containing 0.5 M Et4NCI04.3 However, a recent report from Cai and Martin4 of convincing evidence that oxidized polypyrrole is not permselective in acetonitrile has caused us to reexamine this question. These authors found that the ionic conductivity of oxidized polypyrrole depends strongly on electrolyte (Et4NBF4) concentration. Extrapolation to zero concentration suggested a negligible contribution to the ionic conductivity from counterion transport. Our attempts to confirm these findings have revealed an electrochemically induced breakdown of permselectivity which leaves the polymer in a nonequilibrium state. Oxidized polypyrrole a t equilibrium and polypyrrole that has not been reduced both appear to be permselective in acetonitrile. Results Figure 1A shows a series of cyclic voltammograms of polypyrrole5 in acetonitrile +0.1 M Et4NCI04. The first voltammogram (--), recorded between -0.4 and +0.6 V, is featureless but shows a large charging current. The tilt of this voltammogram suggests a large uncompensated resistance (ca. 1 MQ based on the slope). When the lower limit of the potential scan was extended to -1 V (-). a sharp reduction peak occurred at E, = -0.68 V . On subsequent cycles (- - -) reduction of the polymer occurred at significantly less negative potentials, closer to the peak potential for reoxidation. Further insight into the transformation induced by the initial reduction was obtained by ac impedance spectroscopy.6 Before the initial reduction to -1 V, the polymer exhibited a relatively simple Warburg-type response (Figure 2A). A plot of (impedanceI2 vs (frequency)-’ was linear with a slope corresponding’ to a resistance of 1.5 MQ, similar to the value estimated above *To whom correspondence should be addressed.
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from cyclic voltammetry. After reduction to -1 V and reoxidation, the polymer’s ionic resistance (R,) had decreased by 3 orders of magnitude (Figure 2B). The Warburg-type region is now short due to the high ionic conductivity and finite thickness of the film. At low frequency the polymer behaves as a pure capacitance. The real component of the impedance becomes almost constant and corresponds to Rs + R1/3.8-9 From Figure 2B the ionic resistance of the polymer film is just 0.8 kQ.Io The resistance of the polymer increased to ca. 30 kQ when it was rinsed with acetone and acetonitrile (Figure 2C). However, after reduction again to -1 V, the low resistance observed after the initial reduction to -1 V was restored. Furthermore, the initial reduction peak after rinsing was sharper and occurred at a lower potential than on subsequent cycles (Figure 1 B). Many other polypyrrole films were studied using similar sequences of experiments to that described above and similar results were obtained.” Some additional pertinent observations on oxidized polypyrrole are briefly summarized here. I . The high ionic resistance of an as grown (oxidized) film did not change significantly with time (1 h) in 0.1 M Et4NCI04 solution. 2. The low ionic resistance of a film following potential cycling to -1 V in acetonitrile does not depend significantly on electrolyte concentration over the range 0.033-0.33 M . 3. After the large decrease in resistance induced by the first cycle, further potential cycling has little effect on a film’s ionic resistance. 4. The ionic resistance of a cycled film slowly increased with time in 0.1 M Et4NC104solution. 5. The ionic resistance of a rinsed film did not change significantly with time (50 min) in 0.1 M Et4NC10., solution.
( I ) Burgmayer, P.; Murray, R . W. J . Phys. Chem. 1984,88,2515-2521. ( 2 ) Paulse, C. D.; Pickup, P. G . J . Phys. Chem. 1988, 92, 7002-7006. (3) Pickup, P G J . Chem. Soc.. Faraday Trans. 1990, 86, 3631-3636. ( 4 ) Cai, Z . : Martin, C. R. J . Elecrroanal. Chem. 1991, 300,35-50. ( 5 ) Polypyrrole was prepared at a constant current density of 0.5 mA cm-* from an acetonitrile solution containing 0.1 M pyrrole and 0.1 M Et,NCIO,. ( 6 ) All impedance data are for the oxidized polymer at +0.3 V. In all cases the high-frequency real component of the impedance corresponded to the uncompensated solution resistance (Rs), indicating tha: the electronic resistance of the polymer was negligible. ( 7 ) Jakobs, R . C. M.; Janssen, L. J . J.; Barendrecht, E. R e d J . R. .Yetherlands. Cheni. Soc. 1984, 103, 275-28 1 (8) Albery, W . J.; Chen, Z.; Horrocks, B. R.; Mount, A. R.; Wilson, P. 1.; Bloor, D.; Monkman, A . T.; Elliot, C. M. Faraday. Discuss. Chem. SOC. 1989, 88, 247-259. (9) Albery. W . J.; Elliot, C. M.; Mount, A . R . J . Elecfroanal. Chem. 1990. 288. 15-34. ( I O ) The same value was obtained from analysis of the Warburg and
low-frequency regions. I I I ) Duffitt. G . L: Pickup. P. G.. manuscript in preparation.
Q 1991 American Chemical Societ)
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9635
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Figure 1. Cyclic voltammograms (20 mV/s) of a polypyrrole (0.25 pm) coated Pt electrode in acetonitrile 0.1 M Et,NCIO,. A: First cycle to -0.4 V (--), first cycle to -1 V (-), and second cycle to -1 V (-- -). B: First (-1 and second (- - -) cycles after rinsing with acetone and
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Figure 2. Nyquist impedance plots at +0.3 V for the system described for Figure I , before potential cycling (A), after two cycles to -0.4 V and two cycles to -1 V (B), and after rinsing (C). The dashed line in B is for a bare electrode.
6. Similar results were obtained with acetonitrile containing Et4NBF4. 7. As grown films exhibit a very low ionic resistance in aqueous 0.1 M Et4NCI04, which increases with potential cycling.
Discussion It is clear from the large change in ionic conductivity that accompanies the initial reduction of polypyrrole in acetonitrile, and the fact that this change can be reversed by washing the polymer with pure solvent, that there is an influx of electrolyte (salt) into the polymer during the initial reduction/oxidation cycle. Presumably cations are inserted during reduction and these are not all expelled during reoxidation. Cation insertion during reduction of polypyrrole doped with small anions has been demonstrated by X-ray photoelectron spectroscopyi2 and microgra~imetry.’~J~
The high, stable ionic resistances of as grown and rinsed films suggest that the equilibrium form of oxidized polypyrrole in acetonitrile is permselective and that the influx of cations during reduction is a result of the very low mobility of counterions ((IO4-). Reduction of the polymer is slow until a sufficient overpotential is applied to overcome the Donnan potential and force cations into the film. It then occurs rapidly to produce the sharp cathodic peak observed in cyclic voltammetry. After reduction, the presence of electrolyte in the film increases counterion mobility and counterion transport can become dominant during potential cycling, as indicated by quartz crystal microbalance studies.I4 The constant ionic conductivity following further potential cycling indicates that the salt content of the film remains approximately constant after the first cycle. This explanation of the initial reduction peak differs from that proposed by Warren and Anderson,ls who attributed its anomalous sharpness to an irreversible structural change in which the polymer becomes more disordered. Although it is possible that the influx of cations is caused by such a structural change, the observation of similar behavior for cycled films that have been rinsed makes this unlikely. The solvent plays an important role in this phenomenon. In contrast to observations in acetonitrile, the counterions of as-grown polypyrrole are very mobile in aqueous media. Consequently there is little or no cation insertion during reduction and the polymer remains permse1ective.I It thus appears that water solvates the polymer but acetonitrile does not. Presumably, solvent which enters the polymer with cations during reduction in acetonitrile is partially responsible for the improved ionic conductivity. However, the presence of salt is also important, as indicated by the increase in resistance following rinsing. A full understanding of this system requires quantification of the solvent and salt content of the polymer in each state, and this should be possible using the quartz crystal microbalance. It is clear from the results presented here that reduction and reoxidation of a polypyrrole film in acetonitrile produces a nonequilibrium state containing excess salt (and probably solvent). This observation supports Hillman et al.’s conclusion that salt and solvent transport are important in the equilibration of a conducting In the present case, the slow increase in the ionic resistance of the polymer following reduction/reoxidation indicates that the rate of equilibration is limited by transport of salt (and/or solvent) out of the film. Since cycled polypyrrole can exist in a nonequilibrium state with high ionic conductivity for a considerable time, the consequences of electrolyte loss are rarely noticed. However, the influx of electrolyte during the initial reduction and its subsequent loss are presumably responsible, at least in part, for the wide range of ion transport rates that have been reported for polypyrrole. Our failure to reproduce the trend with electrolyte concentration reported in ref 4 is presumably also a consequence of this phenomenon.” Furthermore, the low mobility of counterions in the equilibrium form of oxidized polypyrrole has serious implications for its application in batteries and other devices involving electrochemical switching.
Acknowledgment. Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Employment and Immigration Canada (SEED program) is gratefully acknowledged. (14) Naoi, K.; Lien, M.; Smyrl, W. H. J . Electrochem. Soc. 1991, 138, 440-445. (15) Warren, L. F.; Anderson, D.P. J . Electrochem. SOC.1987, 134, 101-105.
(12) Zhou, Q.-X.; Kolaskie, C. J.; Miller, L. L. J . Electroanab Chem. 198’1. 223, 283-286. (13) Reynolds, J. R.; Sundaresan, N. S.;Pomerantz, M.; Basak, S.; Baker, C . K.J . Elecrroanal. Chem. 1988, 250, 355-371.
( I 6) Hillman, A. R.; Swann, M. J.; Bruckenstein, S. J. Electroonal. Chem. 1990, 291, 147-162. (17) Hillman, A. R.; Swann, M. J.; Bruckenstein, S. J . Phys. Chem. 1991, 95, 3271-3277.
