JOURNAL

OF MATERIALS

SCIENCE

LETTERS

3 (1984)

745-747

Preparation of polypyrrole in aqueous media A.S.N. MURTHY*,SHRI PAL, K.S. REDDY Department of Chemistry, Indian Institute of Technology, New Delhi 110 016, India

Recently Diaz and co-workers [1] have prepared polypyrrole (PP), a conducting polymer ( ~ 100~~ -1 cm -1) by electrochemical polymerization of pyrrole in acetonitrile employing Et4NBF4 as a supporting electrolyte. Electrodes coated with PP have been considered useful for stabilizing semiconductor electrodes in photoelectrochemical cells [2--7]. Recently, Murthy and Reddy [8] have found that PP-coated platinum or SnO2 electrodes are selective to Fe2+/Fe 3+ couple rather than dye/reduced dye couple. The advantage of the selectivity has been used to improve the photo outputs of iron-thionine photogalvanic cell [8]. In contrast, the films of pyrrole polymer prepared by a chemical method are different from the electropolymerized PP films [9]. They are hydrogen rich, transpoarent and insulating. We report here the results on electrochemical oxidation of pyrrole in aqueous acidic, basic and neutral media as examined by cyclic voltammetry. The main objective was to see whether a conducting polymer film similar to PP prepared by Diaz and co-workers [1] in a nonaqueous medium could be obtained. The details of electrochemical measurements were previously described [8]. The electrochemical area of the working platinum electrode was 0.393cm 2. For conductivity measurements, the films ( ~ 2 c m 2 in area) were electrochemically deposited on a platinum electrode in an acid

medium containing 5.0 x 10-2M pyrrole at a constant potential of 0.92 V vs SCE for 5 rain. The films were stripped from the electrode surface after drying at 70°C. The classical four-probe method was employed to measure surface conductivity of the film. The thickness of the film used for the conductivity measurements was 0.45 mm, obtained from the difference in the weight of the platinum electrode before and after film deposition. Bulk conductivity measurements were carried out by mounting the sample between two copper blocks. The area of the copper electrode in contact with the film was 0.2 cm 2. The cyclic voltammograms of pyrrole (10 -2 M) at different scan rates in 0.1 M solutions of H2804 are shown in Fig. 1. Similar voltammograms were obtained in NaOH and KC1 solutions. The peak potentials (Vp) and peak currents (ip) in all the media at different scan rates are shown in Table I. It was observed that the anodic peaks were much broader in the KC1 medium than in the acidic or basic media. As can be seen from the table, the peak potentials are more positive in the KC1 medium than in the acidic or basic media, indicating that it is difficult to oxidize pyrrole in a KC1 medium than in acidic one. Polymerization, if any, is thus not favourable in the KC1 medium. Also, the current densities are larger in the acidic and smaller in the KC1 medium, thereby indicating that polymerization is more favourable in H2SO4. In the acid medium, the decrease in peak current at

TAB LE I The anodic peak potentials and currents for pyrrole (10 -2 M) oxidation in H2SO4, KC1 and NaOH media at different scan rates Scan rate (mV sec- 1) 10 25 50 100 150 200

Vp(mY)

ip (mA)

H2SO 4

KC1

NaOH

H~SO 4

KC1

NaOH

835 840 840 890 895 900

920 1000 1050 1100 1100 1100

850 860 850 850 870 885

1.60 3.65 4.90 7.60 6.45 5.02

0.85 1.27 1.75 2.70 3.35 4,01

0.98 1.70 2.00 3.01 4.00 4.75

*To whom all correspondence should be addressed. 0261 --8028/84 $03.00 + .12

© 1984 Chapman and Hall Ltd.

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V(SCE vsmV) Figure 1 Cyclic v o l t a m m o g r a m s o f pyrrole (10 -2 M) in 0.1M H~SO 4 on platinum electrode at different scan rates: (10 to 200 m V sec-t).

high scan rates (Fig. 1) is possibly due to the conductivity of the material and possibly the increase in the resistance of the film formed and course of the polymerization reaction. It has been also a decrease in the diffusion of monomer recently noted that, in the anodic polymerization of pyrrole in CH3CN with EtaNBF4 as supporting through the film. The thin films obtained on the electrode sur- electrolyte, it is the oxidation of BF4 that initiates face are blue black in colour in H2SO4 medium and the polymerization of pyrrole to PP [11]. We tenyellow in the basic medium. The resistivity of the tatively propose that, in an acid medium, the yellow film was observed to be greater than solvent water possibly undergoes oxidation and 101s~2 cm. It is interesting to note that the yellow the active oxygen facilitates the polymerization of film obtained in basic medium is similar to that pyrrole to give PP. Oxygen incorporation in the obtained by chemical method [9]. It was, there- film is probably responsible for its lower conducfore, not considered interesting to investigate the tivity. It may be concluded from the above studies structure and chemical composition of this film. No film formation has, however, been noticed in that electrochemical oxidation of pyrrole in aqueous acid medium gives conducting polyKC1 medium. In order to know whether the film material pyrrole, whereas in basic medium nonconducting obtained in the acid medium is similar to PP, film is obtained. obtained by Diaz and co-workers [1], conductivity measurements have been made. The surface References 1. A. F. DIAZ, K . K . KANAZAWA, J . J . CASTILLO conductivity of the film was determined to be and J. A. LOGAN, in "Conductive Polymers", edited 7.8 x 10-zf2-1cm -~ at 298K and bulk conducby R. B. Seymour (Plenum Press, New York, 1981) tivity to be 6.5 x lO-2~-~cm -1. Although these and references cited therein. values are lower than the range 10 to 100 ~2-1 cm -1 2. R. NOUFI, D. TENCH and L . F . WARREN, J. Eleetroehem. Soc. 127 (1980) 2310. reported by Diaz and co-workers [1], they are 3. R. NOUEI, A. J. F R A N K and A. J. NOZIK, J. Amer. comparable to the value of 1.6 x 10-2f2 -1 cm -l reChem. Soe. 103 (1981) 1849. ported by Watanabe and co-workers [10] for the PP 4. F . R . F . FAN, B. WHEELER, A . J . BARD and R. films prepared with LiC104 as the supporting elecNOUFI, J. Eleetrochem. Soe. 128 ( 1 9 8 l ) 2042. 5. T. SKOTHEIM, I. LUNDSTROM and J. PREJZA, J. trolyte. The nature of the supporting electrolyte Eteetroehem. Soc. 128 (1981) 1625. seems to be an important factor in determining the 746

6.

T. SKOTHEIM, I. LUNDSTROM, A. E. DELAHOY, F. 3. KAMPAS and P. E. VANIER, Appl. Phys. Lett. 40 (1982) 281. 7. T. SKOTHEIM, L.G. PETERSON, O. INGANAS and I. LUNDSTROM, d. Electrochem. Soc. 129 (1982) 1737. 8. A . S . N . MURTHY and K. S. REDDY, Electrochim. Aeta 28 (1983) 473. 9. M. SALMON, K. K. KANAZAWA, A. F. DIAZ and

M. KROUNBI, J. Polym. Soc. Polym. Lett. 20 (1982) 187. 10. A. WATANABE, M. TANAKA and J. TANAKA, Bull. Chem. Soe. Jpn. 54 (1981) 2278. 11. J. PREJZA, I. LUNDSTROM and T. SKOTHEIM, d. Eleetroehem. Soe. 129 (1982) 1685.

Received 24 January and accepted 21 February 1984

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