Chemical Synthesis, Characterization, and Electrochemical Studies of

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Chem. Mater. 1999, 11, 262-268

Chemical Synthesis, Characterization, and Electrochemical Studies of Poly(3,4-ethylenedioxythiophene)/ Poly(styrene-4-sulfonate) Composites Mark Lefebvre, Zhigang Qi,† Danesh Rana, and Peter G. Pickup* Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X7 Received June 29, 1998. Revised Manuscript Received October 28, 1998

Poly(3,4-ethylenedioxythiophene)/poly(styrene-4-sulfonate) (PEDOT/PSS) composites have been prepared from aqueous and aqueous acetonitrile solutions of EDOT and NaPSS by oxidation using Fe(III) salts. Powders with PEDOT to PSS ratios ranging from 0.3 to 4.2 and electronic conductivities as high as 10 S cm-1 have been obtained in good yields. The PEDOT/PSS blends are cation exchangers and exhibit facile eletrochemistry in both aqueous and acetonitrile media. Impedance measurements have shown that 30 µm thick PEDOT/ PSS layers have proton conductivities as high as 0.03 S cm-1.

Introduction In recent years there has been increasing interest in the synthesis of conducting polymer particles by chemical oxidation.1-5 This method is more suitable for mass production than the more commonly used electrochemical methods.6 Our interest in chemically prepared conducting polymer particles stems from their potential as catalyst support materials for proton exchange membrane (PEM) fuel cells. By polymerizing pyrrole in the presence of a proton-conducting polymer [poly(styrene-4-sulfonate), PSS]7 and subsequently depositing catalytic Pt particles,8-10 we have been able to produce highly active catalysts. However, the poor long-term stability of polypyrrole (PPY) precludes its use in commercial fuel cells. Poly(3,4-ethylenedioxythiophene)11 (PEDOT) has been reported to exhibit greatly enhanced stability relative to polypyrrole12 and appears to be the most stable conducting polymer currently available.13 We have * To whom correspondence should be addressed. † Current address: H Power, 60 Montgomory St., Belleville, NJ 07109. (1) Sun, Z. C.; Geng, Y. H.; Li, J.; Jing, X. B.; Wang, F. S. Synth. Met. 1997, 84, 99-100. (2) Corradi, R.; Armes, S. P. Synth. Met. 1997, 84, 453-454. (3) Arribas, C.; Rueda, D. Synth. Met. 1996, 79, 23-26. (4) Nishio, K.; Fujimoto, M.; Ando, O.; Ono, H.; Murayama, T. J. Appl. Electrochem. 1996, 26, 425-429. (5) Boara, G.; Sparpaglione, M. Synth. Met. 1995, 72, 135-140. (6) Heinze, J. Synth. Met. 1991, 41-43, 2805-2823. (7) Qi, Z.; Pickup, P. G. Chem. Mater. 1997, 9, 2934-2939. (8) Qi, Z.; Pickup, P. G. In Quantum Confinement: Nanoscale Materials, Devices, and Systems; The Electrochemical Society Proceedings Series; The Electrochemical Society: Pennington, NJ, 1997; Vol. 97-11, pp 28-34. (9) Qi, Z.; Pickup, P. G. Chem. Commun. 1998, 15. (10) Qi, Z.; Lefebvre, M. C.; Pickup, P. G. J. Electroanal. Chem. 1998, in press. (11) Heywang, G.; Jonas, F. Adv. Mater. 1992, 4, 116-118. (12) Yamato, H.; Ohwa, M.; Wernet, W. J. Electroanal. Chem. 1995, 397, 163-170.

therefore used it to replace polypyrrole in our catalyst support.14,15 PEDOT has been attracting growing interest since it was first reported in 199211 and is currently being marketed by Bayer. In most cases, electrochemically prepared films have been used, although there are a number of reports of the oxidative chemical polymerization of EDOT.2,11,16 Our interest in using PEDOT as a catalyst support in PEM fuel cells requires that they have good cation (proton) conductivity. To achieve this, a polyanion, PSS, is used as the counteranion during synthesis.10,17 The anions then become fixed in place around the conducting polymer chains. Incorporation of an excess of the polyanion during synthesis, or partial reduction of the polymer (eq 1) will produce a cation exchanger with mobile cations.

(-EDOTn+-)n(poly-A-) + ne + nC+(solution) h (-EDOT-)n(poly-A-)nC+ (1) This strategy has been shown to be effective in the case of PPY/PSS composites, which exhibit proton conductivities as high as 25 mS cm-1.10 The PEDOT/ PSS composites exhibit similarly high proton conductivities, and when catalyzed with Pt, they provide significantly better activity for oxygen reduction in PEM gas diffusion electrodes.14,15 This paper describes the synthesis, characterization, and electrochemical studies of chemically synthesized polymer blends of PEDOT and the polyanion PSS. The (13) Winter, I.; Reese, C.; Hormes, J.; Heywang, G.; Jonas, F. Chem.Phys. 1995, 194, 207-213. (14) Qi, Z.; Pickup, P. G. Chem. Commun. 1998, in press. (15) Lefebvre, M.; Qi, Z.; Pickup, P. G. J. Electrochem. Soc. 1998, submitted. (16) Pei, Q.; Zuccarello, G.; Ahiskog, M.; Inganas, O. Polymer 1994, 35, 1347-1351. (17) Iyoda, T.; Ohtani, A.; Shimidzu, T.; Honda, K. Chem. Lett. 1986, 687-690.

10.1021/cm9804618 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/20/1999

PEDOT/PSS Composites

Chem. Mater., Vol. 11, No. 2, 1999 263

Table 1. Synthesis Conditions, Compositions, Yields, and Initial Conductivities PEDOT:PSS ratio ID

solvent

oxidanta

initial

obtainedb

reactn time

M2 M3 M4 MB MS2 MS1a MS1 Q1 Q2

AN/H2O AN/H2O AN/H2O AN/H2O AN/H2O AN/H2O H2O H2O H2Od

×5 ×10 ×4 ×5 ×10c ×10 ×10 ×5 ×5

2.5 2.5 7.5 1.5 1 1 1 5 5

1.1 1.4 2.1 1.5 0.3 1.0 0.5 0.4 4.2e

3 days 3 days 3 days 4 days 2 days 2 days 2 days 2h 2h

% yield

initial conductivity (S cm-1)

18 61 93 35 38 39 50 62 100f

0.3 1.5 1.3 1.0 6 × 10-3 2.5 0.3 0.4 9.9

a Fe(NO ) ‚9H O per mole of EDOT. b See the text. c FeCl . d Low-volume emulsion polymerization. See ref 14. e Estimated for PEDOT0.5+/ 3 3 2 3 PSS/NO3- assuming 100% incorporation of PSS. f Assumed.

Table 2. Elemental Analyses elemental analysis and calculated values (% by mass) sample

a

formula

M3

PEDOT1.4(NO3)0.22Fe0.18PSS‚3H2O

M4

PEDOT2.1(NO3)0.45Fe0.13PSS‚1.5H2O

MB

PEDOT1.5(NO3)0.15Fe0.14PSS‚1.5H2O

Q1

PEDOT0.42Fe0.26PSS‚1.5H2O

expl calcd expl calcd expl calcd expl calcd

C

H

N

O

S

Fea

43.20 43.0 45.27 45.8 45.64 46.6 45.46 44.6

3.26 3.89 3.11 3.41 3.39 3.67 3.78 4.12

0.69 0.68 1.21 1.17 0.49 0.49 0 0

31.61 33.2 27.18 29.8 25.71 29.2 27.68 30.1

15.38 16.8 17.09 18.4 17.88 18.3 14.50 16.0

2.4 2.2 0.90 1.4 2.1 1.8 4.8 5.2

Determined from the %S by mass and the Fe:S ratio from EDX.

deposition of precious metal catalysts on these materials and the electrocatalysis of oxygen reduction, hydrogen oxidation, and methanol oxidation by the resulting supported catalysts are described elsewhere.14,15 We are aware of only one previous report of the synthesis of PEDOT/PSS, and that described only the electrochemical formation of films on electrodes.12 The PEDOT/PSS blends described here are also attractive for applications in supercapacitors18 and batteries.19 Their high cation conductivities will produce large power densities and make them more compatible with lithium and lithium ion anodes than most other conducting polymers, including PEDOT itself. Results of a preliminary investigation of one of the PEDOT/PSS blends in Li+ containing aqueous and acetonitrile solutions is therefore included.

excess (up to 10:1 relative to EDOT) of the oxidant, in a small amount of water, was added. Oxidants used were Fe(NO3)3‚ 9H2O (BDH) and FeCl3 (anhydrous, Fluka). After various reaction times (from 2 to 100 h) deep blue powders were collected by filtration, washed, and then dried overnight in a vacuum oven at 25-40 °C. For synthesis of PEDOT/PSS in the absence of acetonitrile, the EDOT was first dissolved in warm NaPSS(aq). Fe(NO3)3‚ 9H2O in a small volume of water was then added to the cooled (room temperature) solution. EDOT dispersed much better in NaPSS(aq) than in pure water, and this assisted the formation of highly conducting PEDOT/PSS composites.14 The blend compositions given in Table 1 were determined from Fe:S ratios from energy-dispersive X-ray emission analysis (EDX) by using eq 2, which is based on a simple charge balance.

Experimental Section

where nFe is the average charge on the Fe3+/2+ ions, a is the ratio of other anions (NO3-) to PSS, and d is the doping level of the polymer (average charge per EDOT ring). A doping level of 0.50 and an Fe oxidation state of 3 were assumed, since these values gave the best fit with elemental analyses (see below). The parameter a was determined from the %N from elemental analysis or assumed to be zero for samples that were not analyzed. Elemental analysis was performed on selected samples on three separate occasions by two different companies (Canadian Microanalytical Service Ltd., Vancouver, Canada, and CE Instruments, Calgary, Canada). The most complete data set (Canadian Microanalytical Service) is given in Table 2. Repeat determinations of both carbon and sulfur differed by as much as 3%, making the C:S ratios unreliable for estimation of PEDOT:PSS ratios. The analyses gave unaccounted for masses ranging from 3.2 to 8.5%, and since all elements that could possibly be in the samples were quantified (only He, Li, Be, B, F, and Ne would not be detected by the combination of elemental and EDX analysis), these residuals must be due to errors in the analyses. The origin of these errors is unclear, but they are not due to inhomogeneity of the samples, because replicate elemental analyses performed at the same time gave good precision (