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Observation of soluble electroactive intermediates during the anodic formation of conducting polypyrrole films
269
J. Electmanal. Chem., 296 (1990) 269-273 Elsevier Sequoia S.A., Lausanne
Preliminary note
Observation of soluble electroactive intermediates during the anodic formation of conducting polypyrrole films Daniel E. Raymond and D. Jed Harrison * De~a~~e~t of Chemistry, U~i~rsi~ 0~Aibert~ Edmonton, Alberta T6G 2GZ (~~a~a~ (Received 10 September 1990)
The formation of electronically conducting polypyrrole films by anodic oxidation acetonitrile was first reported by Diaz et al. [l]. A large literature has developed exploring the effect of oxidation conditions such as solvent, electrolyte, additives, functional substituents, and electrode potential on the properties of the polymer [2-121, as well as applications of these thin conducting films 112-161. However, the underst~d~g of the mechanism of polymerization has developed slowly [g-11, 17-211. Several authors have concluded polymer deposition occurs via a three-dimensional nucleation and growth process that involves soluble intermediates [lo-13,20,21], but no direct observation or identification of soluble intermediates has been reported. The common single electrode electrochemical techniques used to analyze intermediates are not well suited to cases where the product forms an electroactive film on the electrode. The rotating ring-disk electrode (RRDE) has been used to evaluate azulene and pyrrole polymerization and a reduction process at the ring electrode arising from protons released during polymerization is reported [22,23]. We have observed anodic current at the ring during oxidation of pyrrole at the disk that exhibits an extremely low collection efficiency. This is the first direct observation of a soluble intermediate formed during el~tropol~e~zation of pyrrole that involves a pyrrole moiety. A RRDE with a carbon disk and a platinum ring (Pine Instruments, Grove City, Pennsylvania) was used to examine the oxidation of pyrrole in dry acetonitrile + 0.1 M NaClO,. HPLC grade CH,CN (Burdick and Jackson) was distilled over CaH,, NaClO, was vacuum dried at 110 o C, and pyrrole was vacuum distilled and stored under Ar. All solutions were filtered through activated alumina (Woelm N-Super I, ICN Nutritional Biochemicals, Cleveland, Ohio, vacuum dried 250 o C) immediately in
* To whom correspondence should be addressed. ~22-0728/~/$03.SO
Q 1990 - Ekevier Sequoia S.A.
50 mM Pyrrole 0.1 M NaCl04 CH3CN 3600 rpm
4 i
5( I
;.$ 52 04 f 6 I
.-F lx I
ED=1.08 v ED=l.O V & ED=O.O V 0 ! 0.3
0.4
0.5
0.6
I
0.7
ERing/V vs SSCE
Fig. 1. The Pt ring electrode current is shown as a function of the ring electrode potential (100 mV/s, 3600 rpm) at several values of the disk potential, ED (f20 mV, since the disk was also swept at 5 mV/s). The inset shows the C disk electrode current for pyrrole oxidation versus disk potential (5 mV/s, 3600 rpm).
before use. The procedure involved rotating the electrode at 3600 rpm (Pine MRS rotator), sweeping the disk at 5 mV/s, and cycling the ring simultaneously at 100 mV/s from 0.30 to 0.70 V vs. SCE. A Pine RDE-4 bipotentiostat was used to control the potentials and sweep the ring, with a PAR 175 programmer used as a second sweep source for the disk electrode channel, and a Kipp and Zonen BD91 used as an x, y, y’ recorder. One complete ring cycle was obtained for every - 40 mV swept at the disk. At one point two separate Pine RDE-4 potentiostats were used, with separate reference and counter electrodes, one for the ring and one for the disk, and identical results were obtained. This was done to ensure the low ring currents observed were not a result of cross-talk between the two potentiostat channels of a single RDE-4. Potentials were measured relative to a saturated sodium chloride calomel electrode (SSCE) isolated with a double junction containing 0.1 M NaClO, in CH,CN, and CaH, powder to consume H,O derived from the SSCE. A Ag electrode in 0.010 M AgNO, + 0.1 M NaClO, was also used with identical results. A Pt gauze counter-electrode was isolated from the solution using a fine glass frit. The ring current versus ring voltage curves obtained at the ring in 50 mM pyrrole + 0.1 M NaClO, during polymerization at the disk can be seen in Fig. 1.
271
The inset shows the disk current as a function of disk voltage with an anodic current onset at about 0.8 V vs. SSCE. No significant current is observed at the ring until the disk reaches a potential of - 1.0 V vs. SSCE, a potential at which an inflection point is observed in the disk current-voltage curve. Above this disk potential a well-defined wave develops as the potential of the Pt ring is cycled, with a half-wave potential, E,,,, of 0.55 f 0.01 V vs. SSCE. On the current plateau for the ring a collection efficiency of 9 X low3 was obtained over a disk potential range of 1.08 to 1.2 V. The efficiency increased in a non-linear fashion as the square root of rotation rate was increased, but this effect has not yet been explored in detail. The collection efficiency is substantially lower than the value measured for ferrocene in 0.1 M tetrabutylammonium perchlorate in CH,CN, which was in agreement with the calculated value of 0.216 [24]. An identical set of experiments was performed while cycling the ring cathodically from 0.5 to - 1.5 V at 100 mV/s. A large reduction current is observed at the ring with an E1,2 of - 0.08 V vs. SSCE in agreement with the observations reported by Bruckenstein and Sharkey [22] for azulene, and this is assigned to H+ reduction. The small reduction current observed near +0.5 V vs. SSCE, Fig. 1, may be due to a pyrrole derived species, but it is not possible to separate this from the H+ reduction process. The cyclic voltammetric peak potential for bipyrrole in CH,CN was reported to be 0.51 V vs. SSCE [9], which suggests the observed product at the ring is the pyrrole dimer. Bipyrrole was synthesized according to literature methods [25-271 and was identified by NMR, giving spectra matching those reported in refs. 25 and 27. In a solution of the product (1 mM bipyrrole + 0.1 M NaClO, in dry CH,CN) a rotating Pt disk electrode (3600 rpm) exhibited an oxidative wave with an E,,2 of 0.55 V vs. SSCE at a sweep rate of 100 mV/s. When the electrode was not rotated an anodic peak potential of 0.52 V was obtained. A previously unreported second oxidation step was also observed with an E1,2 of 1.15 V. Oxidation at the higher potential resulted in deposition of conducting polypyrrole films similar to those obtained from low concentrations of pyrrole, in contrast to an earlier report [9]. Morse et al. [7] have shown that addition of pyridine or 2,6-di-t-butyl pyridine changes the profile and magnitude of pyrrole oxidation currents, while the onset potential is unchanged. Similarly, we found that neither of these bases changed the El,* of the first wave observed for bipyrrole. Addition of 100 mM of either base to a 50 mM pyrrole solution shifted the cathodic process at the ring of a RRDE to -0.7 V, consistent with H+ reduction, while the El,* for the anodic process shown in Fig. 1 was unchanged. There are several products of pyrrole oxidation that might be expected to be observed at the ring including the primary radical cation product, pyrrl’+, and secondary products such as bipyrrole or higher oligomers, or pyrrl-pyrrole adducts in various protonated states [11,17,20,21]. For many of these both a reductive and an oxidative current would be expected, but only, an anodic current is definitely observed. Secondly, the close agreement of E1,2 values for the observed species and for bipyrrole under the same conditions, and the fact that both give waves with an
212
E i,* independent of pH provides a compelling argument for identification of the intermediate as bipyrrole. Direct observation of bipyrrole as a soluble intermediate in the electropolymerization of pyrrole is consistent with the mechanistic conclusions deduced from indirect evidence. It is noteworthy that bipyrrole is not observed until the oxidation current at the disk reaches an inflection point at + 1.0 V. Presently we are evaluating the ring collection efficiency as a function of rotation speed to gain further insight into the potential dependence observed. The bipyrrole formed is obviously not a primary reaction product and the pathway to its formation involves a number of chemical and electrochemical steps that can be expected to lead to a complex dependence on rotation rate. The low collection efficiency for this stable species indicates that it is consumed rapidly in the subsequent polymerization steps, or that it is formed in low yield as a result of a side reaction during the polymerization. The fact that bipyrrole can form conducting films at sufficiently oxidizing potential, contrary to an earlier report, indicates that chain formation is not limited to oligomer coupling with oxidized monomer, and that coupling of oligomers in solution followed by precipitation represents a viable mechanism. Finally, the ability to observe a soluble intermediate using the RRDE creates an opportunity to develop an increased understanding of the polymerization process. The conditions of polymerization are likely to affect the yield of bipyrrole at the ring electrode, providing a means to probe the factors influencing physical and chemical properties of the polymer. ACKNOWLEDGEMENT
We thank the Natural Sciences and Engineering Research Council of Canada for support of this research. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
A.F. Diaz, K.K. Kanazawa and G.P. Gardini, J. Chem. Sot., Chem. Commun., (1979) 635. A.F. Diaz, A. Martinez and K.K. Kanazawa, J. Ekectroanal. Chem., 130 (1981) 181. R.M. Penner and CR. Martin, J. Electrochem. Sot., 133 (1986) 2206. G. Nagasubramanian, S. Distefano and J. Moacanin, J. Electrochem. Sot., 133 (1986) 305. L.F. Warren and P. Anderson, J. Electrochem. Sot., 134 (1987) 101. K. Naoi and T. Osuka, J. Electrochem. Sot., 134 (1987) 2479. N.J. Morse, D.R. Rosseinsky, R.J. Mortimer and D.J. Walton, J. Electroanal. Chem., 255 (1988) 119., F.T.A. Vork and L.J.J. Janssen, Electrochim. Acta, 33 (1988) 1513. A.F. Diaz, J. Crowley, J. Bargon, G.P. Gardini and J.B. Torrance, J. Electroanal. Chem., 121 (1981) 355. S. Asavapiriyamont, G.K. Chandler, G.A. Gunawardena and D. Pletcher, J. Electroanal. Chem., 177 (1984) 229. A.J. Downard and D. Pletcher, J. Electroanal. Chem., 206 (1986) 139. G.K. Chandler and D. Pletcher, Electrochemistry. Specialist Periodical Report, Vol. 10, Royal Society of Chemistry, London, 1985, p. 117. L.L. Miller, B. Zinger and Q.X. Zhou, J. Am. Chem. Sot., 109 (1987) 2267.
273 14 15 16 17 18 19 20 21 22 23 24 25 26 27
M. Josowicz, J. Janata, K. Ashley and S. Pons, Anal. Chem., 59 (1987) 253. R. Noufi, D. Tenth and L.F. Warren, J. Ehxtrochem. Sot., 128 (1981) 2596. M. Imtia and J. Wailer, Anal. Chem., 58 (1986) 2979. T.F. Otero, R. Tejada and AS. EIola, Polymer, 28 (1987) 651. G. Zotti, S. Cat&in and N. Comisso, J. Electroanal. Chem., 235 (1987) 259. R.E. Noftle and D. Pletcher, J. Electroanal. Chem., 227 (1987) 229. M.L. Marcos, I. Rodriguez and J. Gonzalez-Velasco, Electrochim. Acta, 32 (1987) 1453. C.K. Baker and J.R. Reynolds, J. EIectroanaI. Chem., 251 (1988) 307. S. Bruckenstein and J.W. Sharkey, J. Electroanal Chem., 241 (1988) 211. F. Beck and M. Oberst, J. Electroanal Chem., 285 (1990) 177. A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, p. 160. T. Itahara, J. Org. Chem., 50 (1985) 5272. T. Itahara, K. Kawalski and F. Ouseto, Bull. Chem. Sot. Jpn., 57 (1984) 3488. M. Farmer, S. Soth and P. Foumari, Can. J. Chem., 54 (1976) 1083.
J. Electmanal. Chem., 296 (1990) 269-273 Elsevier Sequoia S.A., Lausanne
Preliminary note
Observation of soluble electroactive intermediates during the anodic formation of conducting polypyrrole films Daniel E. Raymond and D. Jed Harrison * De~a~~e~t of Chemistry, U~i~rsi~ 0~Aibert~ Edmonton, Alberta T6G 2GZ (~~a~a~ (Received 10 September 1990)
The formation of electronically conducting polypyrrole films by anodic oxidation acetonitrile was first reported by Diaz et al. [l]. A large literature has developed exploring the effect of oxidation conditions such as solvent, electrolyte, additives, functional substituents, and electrode potential on the properties of the polymer [2-121, as well as applications of these thin conducting films 112-161. However, the underst~d~g of the mechanism of polymerization has developed slowly [g-11, 17-211. Several authors have concluded polymer deposition occurs via a three-dimensional nucleation and growth process that involves soluble intermediates [lo-13,20,21], but no direct observation or identification of soluble intermediates has been reported. The common single electrode electrochemical techniques used to analyze intermediates are not well suited to cases where the product forms an electroactive film on the electrode. The rotating ring-disk electrode (RRDE) has been used to evaluate azulene and pyrrole polymerization and a reduction process at the ring electrode arising from protons released during polymerization is reported [22,23]. We have observed anodic current at the ring during oxidation of pyrrole at the disk that exhibits an extremely low collection efficiency. This is the first direct observation of a soluble intermediate formed during el~tropol~e~zation of pyrrole that involves a pyrrole moiety. A RRDE with a carbon disk and a platinum ring (Pine Instruments, Grove City, Pennsylvania) was used to examine the oxidation of pyrrole in dry acetonitrile + 0.1 M NaClO,. HPLC grade CH,CN (Burdick and Jackson) was distilled over CaH,, NaClO, was vacuum dried at 110 o C, and pyrrole was vacuum distilled and stored under Ar. All solutions were filtered through activated alumina (Woelm N-Super I, ICN Nutritional Biochemicals, Cleveland, Ohio, vacuum dried 250 o C) immediately in
* To whom correspondence should be addressed. ~22-0728/~/$03.SO
Q 1990 - Ekevier Sequoia S.A.
50 mM Pyrrole 0.1 M NaCl04 CH3CN 3600 rpm
4 i
5( I
;.$ 52 04 f 6 I
.-F lx I
ED=1.08 v ED=l.O V & ED=O.O V 0 ! 0.3
0.4
0.5
0.6
I
0.7
ERing/V vs SSCE
Fig. 1. The Pt ring electrode current is shown as a function of the ring electrode potential (100 mV/s, 3600 rpm) at several values of the disk potential, ED (f20 mV, since the disk was also swept at 5 mV/s). The inset shows the C disk electrode current for pyrrole oxidation versus disk potential (5 mV/s, 3600 rpm).
before use. The procedure involved rotating the electrode at 3600 rpm (Pine MRS rotator), sweeping the disk at 5 mV/s, and cycling the ring simultaneously at 100 mV/s from 0.30 to 0.70 V vs. SCE. A Pine RDE-4 bipotentiostat was used to control the potentials and sweep the ring, with a PAR 175 programmer used as a second sweep source for the disk electrode channel, and a Kipp and Zonen BD91 used as an x, y, y’ recorder. One complete ring cycle was obtained for every - 40 mV swept at the disk. At one point two separate Pine RDE-4 potentiostats were used, with separate reference and counter electrodes, one for the ring and one for the disk, and identical results were obtained. This was done to ensure the low ring currents observed were not a result of cross-talk between the two potentiostat channels of a single RDE-4. Potentials were measured relative to a saturated sodium chloride calomel electrode (SSCE) isolated with a double junction containing 0.1 M NaClO, in CH,CN, and CaH, powder to consume H,O derived from the SSCE. A Ag electrode in 0.010 M AgNO, + 0.1 M NaClO, was also used with identical results. A Pt gauze counter-electrode was isolated from the solution using a fine glass frit. The ring current versus ring voltage curves obtained at the ring in 50 mM pyrrole + 0.1 M NaClO, during polymerization at the disk can be seen in Fig. 1.
271
The inset shows the disk current as a function of disk voltage with an anodic current onset at about 0.8 V vs. SSCE. No significant current is observed at the ring until the disk reaches a potential of - 1.0 V vs. SSCE, a potential at which an inflection point is observed in the disk current-voltage curve. Above this disk potential a well-defined wave develops as the potential of the Pt ring is cycled, with a half-wave potential, E,,,, of 0.55 f 0.01 V vs. SSCE. On the current plateau for the ring a collection efficiency of 9 X low3 was obtained over a disk potential range of 1.08 to 1.2 V. The efficiency increased in a non-linear fashion as the square root of rotation rate was increased, but this effect has not yet been explored in detail. The collection efficiency is substantially lower than the value measured for ferrocene in 0.1 M tetrabutylammonium perchlorate in CH,CN, which was in agreement with the calculated value of 0.216 [24]. An identical set of experiments was performed while cycling the ring cathodically from 0.5 to - 1.5 V at 100 mV/s. A large reduction current is observed at the ring with an E1,2 of - 0.08 V vs. SSCE in agreement with the observations reported by Bruckenstein and Sharkey [22] for azulene, and this is assigned to H+ reduction. The small reduction current observed near +0.5 V vs. SSCE, Fig. 1, may be due to a pyrrole derived species, but it is not possible to separate this from the H+ reduction process. The cyclic voltammetric peak potential for bipyrrole in CH,CN was reported to be 0.51 V vs. SSCE [9], which suggests the observed product at the ring is the pyrrole dimer. Bipyrrole was synthesized according to literature methods [25-271 and was identified by NMR, giving spectra matching those reported in refs. 25 and 27. In a solution of the product (1 mM bipyrrole + 0.1 M NaClO, in dry CH,CN) a rotating Pt disk electrode (3600 rpm) exhibited an oxidative wave with an E,,2 of 0.55 V vs. SSCE at a sweep rate of 100 mV/s. When the electrode was not rotated an anodic peak potential of 0.52 V was obtained. A previously unreported second oxidation step was also observed with an E1,2 of 1.15 V. Oxidation at the higher potential resulted in deposition of conducting polypyrrole films similar to those obtained from low concentrations of pyrrole, in contrast to an earlier report [9]. Morse et al. [7] have shown that addition of pyridine or 2,6-di-t-butyl pyridine changes the profile and magnitude of pyrrole oxidation currents, while the onset potential is unchanged. Similarly, we found that neither of these bases changed the El,* of the first wave observed for bipyrrole. Addition of 100 mM of either base to a 50 mM pyrrole solution shifted the cathodic process at the ring of a RRDE to -0.7 V, consistent with H+ reduction, while the El,* for the anodic process shown in Fig. 1 was unchanged. There are several products of pyrrole oxidation that might be expected to be observed at the ring including the primary radical cation product, pyrrl’+, and secondary products such as bipyrrole or higher oligomers, or pyrrl-pyrrole adducts in various protonated states [11,17,20,21]. For many of these both a reductive and an oxidative current would be expected, but only, an anodic current is definitely observed. Secondly, the close agreement of E1,2 values for the observed species and for bipyrrole under the same conditions, and the fact that both give waves with an
212
E i,* independent of pH provides a compelling argument for identification of the intermediate as bipyrrole. Direct observation of bipyrrole as a soluble intermediate in the electropolymerization of pyrrole is consistent with the mechanistic conclusions deduced from indirect evidence. It is noteworthy that bipyrrole is not observed until the oxidation current at the disk reaches an inflection point at + 1.0 V. Presently we are evaluating the ring collection efficiency as a function of rotation speed to gain further insight into the potential dependence observed. The bipyrrole formed is obviously not a primary reaction product and the pathway to its formation involves a number of chemical and electrochemical steps that can be expected to lead to a complex dependence on rotation rate. The low collection efficiency for this stable species indicates that it is consumed rapidly in the subsequent polymerization steps, or that it is formed in low yield as a result of a side reaction during the polymerization. The fact that bipyrrole can form conducting films at sufficiently oxidizing potential, contrary to an earlier report, indicates that chain formation is not limited to oligomer coupling with oxidized monomer, and that coupling of oligomers in solution followed by precipitation represents a viable mechanism. Finally, the ability to observe a soluble intermediate using the RRDE creates an opportunity to develop an increased understanding of the polymerization process. The conditions of polymerization are likely to affect the yield of bipyrrole at the ring electrode, providing a means to probe the factors influencing physical and chemical properties of the polymer. ACKNOWLEDGEMENT
We thank the Natural Sciences and Engineering Research Council of Canada for support of this research. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
A.F. Diaz, K.K. Kanazawa and G.P. Gardini, J. Chem. Sot., Chem. Commun., (1979) 635. A.F. Diaz, A. Martinez and K.K. Kanazawa, J. Ekectroanal. Chem., 130 (1981) 181. R.M. Penner and CR. Martin, J. Electrochem. Sot., 133 (1986) 2206. G. Nagasubramanian, S. Distefano and J. Moacanin, J. Electrochem. Sot., 133 (1986) 305. L.F. Warren and P. Anderson, J. Electrochem. Sot., 134 (1987) 101. K. Naoi and T. Osuka, J. Electrochem. Sot., 134 (1987) 2479. N.J. Morse, D.R. Rosseinsky, R.J. Mortimer and D.J. Walton, J. Electroanal. Chem., 255 (1988) 119., F.T.A. Vork and L.J.J. Janssen, Electrochim. Acta, 33 (1988) 1513. A.F. Diaz, J. Crowley, J. Bargon, G.P. Gardini and J.B. Torrance, J. Electroanal. Chem., 121 (1981) 355. S. Asavapiriyamont, G.K. Chandler, G.A. Gunawardena and D. Pletcher, J. Electroanal. Chem., 177 (1984) 229. A.J. Downard and D. Pletcher, J. Electroanal. Chem., 206 (1986) 139. G.K. Chandler and D. Pletcher, Electrochemistry. Specialist Periodical Report, Vol. 10, Royal Society of Chemistry, London, 1985, p. 117. L.L. Miller, B. Zinger and Q.X. Zhou, J. Am. Chem. Sot., 109 (1987) 2267.
273 14 15 16 17 18 19 20 21 22 23 24 25 26 27
M. Josowicz, J. Janata, K. Ashley and S. Pons, Anal. Chem., 59 (1987) 253. R. Noufi, D. Tenth and L.F. Warren, J. Ehxtrochem. Sot., 128 (1981) 2596. M. Imtia and J. Wailer, Anal. Chem., 58 (1986) 2979. T.F. Otero, R. Tejada and AS. EIola, Polymer, 28 (1987) 651. G. Zotti, S. Cat&in and N. Comisso, J. Electroanal. Chem., 235 (1987) 259. R.E. Noftle and D. Pletcher, J. Electroanal. Chem., 227 (1987) 229. M.L. Marcos, I. Rodriguez and J. Gonzalez-Velasco, Electrochim. Acta, 32 (1987) 1453. C.K. Baker and J.R. Reynolds, J. EIectroanaI. Chem., 251 (1988) 307. S. Bruckenstein and J.W. Sharkey, J. Electroanal Chem., 241 (1988) 211. F. Beck and M. Oberst, J. Electroanal Chem., 285 (1990) 177. A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, p. 160. T. Itahara, J. Org. Chem., 50 (1985) 5272. T. Itahara, K. Kawalski and F. Ouseto, Bull. Chem. Sot. Jpn., 57 (1984) 3488. M. Farmer, S. Soth and P. Foumari, Can. J. Chem., 54 (1976) 1083.