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Morphology of electropolymerized aniline films modified by para-phenylenediamine
289
J. Electroanal. Chem, 262 (1989) 289-295 Ekevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note
Morphology of electropolymerized by para-phenylenediamine
aniline films modif ied
C. Mailhe-Randolph Institut de Chimie Physique, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne (Switzerland)
J. Desilvestro Paul Scherrer Institut, 5303 Wiirenlingen (Switzerland) (Received 19 December 1988; in revised form 16 February 1989)
INTRODUCTION
Polyaniline (PANI) has emerged as one of the promising candidate materials in the fast-growing field of conductive polymers [l]. It is stable in air, can accept high levels of doping and is readily prepared in water [2]. Electropolymerization of an acidic aqueous solution of aniline yields thin, electroactive PAN1 films in a reproducible manner [3]. The electrochemical behavior of such PAN1 films has been investigated extensively and two main redox systems have been identified [2,4]. However, the characteristics of the cyclic voltammograms depend on the synthesis conditions, especially on the potential limit used in oxidation, and some additional middle peaks have been reported [5]. These middle peaks have been attributed tentatively to the formation of oligomers, cross-linking reactions, formation of phenazine rings or hydrolysis of the imine yielding the corresponding quinone [6]. The purpose of this note is to report the effect of copolymerization with para-phenylenediamine (PPDA), on the electrochemical behavior and morphology of PAN1 films. EXPERIMENTAL
Aniline (Fluka) was freshly distilled and the colorless liquid was kept under argon in darkness at 5 o C. Para-phenylenediamine was used as received. A typical three-electrode cell was used, with a 2 cm* platinum or tin oxide electrode, a saturated calomel reference electrode (WE) and a platinum counter electrode. The platinum working electrode was cleaned in HNO, and polished with alumina prior to use. The potentiostat was a PAR model 274 potentiostat/ galvanostat. All potentials are quoted versus SCE. 0022-0728/89/$03.50
6 1989 Elsevier Sequoia S.A.
290 The electrooxidation of aniline was carried out in 1 M HCl containing 0.2 M aniline. The PANI films were prepared by cycling the potential continuously between -0.15 and +0.78 V at a scan rate of 50 mV/s until the desired quantity of PAN1 was formed. The cell was kept under argon during the electropolymerization. For SEM examination (Cambridge instrument model 250), the sample was removed from the cell at 0.44 V (in the emeraldine state), rinsed in 1 M HCl and dried in air.
RESULTS AND DISCUSSION
Figure 1 shows initial cyclic voltammograms obtained during aniline electropolymerization. Aniline oxidation takes place at potentials higher than 0.62 V and the resulting PAN1 film exhibits a first oxidation peak at 0.16 V. The oxidation limit during the film synthesis was set to 0.78 V in order to limit the current densities associated with aniline oxidation and to avoid possible side reactions such as hydrolysis to quinones, which may occur at higher potentials. It was also confirmed, as reported in the literature, that no additional middle peak is observed in the 0.5 V region when the upper potential limit does not exceed 0.8 V during the electropolymerization [5]. The PAN1 films were grown to 60 mC/cm2 (the charge was calculated by taking into account only the first electrochemical wave). The microstructures of the resulting PAN1 films, grown on platinum, are shown in Figs. 2 and 3. Tin oxide was also used as a substrate and did not induce noticeable differences in the microstructure. At the beginning of the deposition (up to a thickness of 10 mC/cm*), the polymer exhibits a rather uniform globular microstructure. For thicker deposits (60 mC/cm2), a fibrous structure develops, as shown in Fig. 3. These fibers are about 0.2 pm thick and exhibit some branching. Similar microstructures have been reported previously for PAN1 films prepared in the presence of H,SO, or HCl [5,7]. In contrast to the initial globular morphology,
T0.5m~
I
E/mV
-600
400
200
0
Fig. 1. Consecutive cyclic voltammograms of a PAN1 film electropolymerized from a 0.2 M aniline+ 1 M HCl aqueous solution, at a scan rate of 50 mV/s, on a platinum substrate.
291
Fig. 2. Initial stages of PANI fihn formation (10 mC/cm*) on a platinum electrode; fiber growth on a globular microstructure.
the fibrous deposit is not entirely uniform over the electrode. This seems to indicate that the fibers are preferred sites for further polymer growth. These irregularities were discernable macroscopically in the case of films as thick as 0.5 C/cm*. Dried, thick films undergo noticeable swelling when immersed in the electrolyte solution. Figure 4 shows the influence of PPDA on the cyclic voltammogram during aniline electropolymerization. The PPDA concentration was 5.5 x lop4 M, in a 0.2 it4 aniline + 1 M HCl starting solution. An additional peak appears at 0.45 V and the film growth rate is increased significantly. In order to keep the anodic current densities at values comparable to those used in the case of pure aniline, the oxidation potential limit was reduced further to 0.62 V during the deposition. The cyclic voltammogram of the same film in 1 M HCl exhibits a peak at 0.45 V as well. However, it should be noted that PPDA, when electropolymerized alone, is oxidized above 0.50 V and gives rise to poorly adherent, non-electroactive films. Also, for concentrations of PPDA greater than lop3 M in the starting aniline solution, polymer growth occurs rapidly and the resulting polymer film is poorly adherent. Therefore, PPDA addition must be controlled carefully. Visual inspection and micrographs (cf. Fig. 5a) show that PPDA-modified PAN1 films (60 mC/cm*) are distributed over the electrode surface much more evenly than pure PAN1 films (Fig. 3a). Films as thick as 0.5 C/cm* are readily obtained in the presence of PPDA and appear, at least visually, very uniform. PPDA addition appears, therefore, to facilitate the fabrication of such films, which could prove very useful for battery applications. The higher-magnification micrograph (Fig. 5b) reveals that the fibers are much thinner and exhibit more branching. PPDA appears to modify the polymer film morphology and to act as a cross-linking agent. For example, a PPDA molecule
292
Fig. 3. SEM micrographs of the fibrous structure of a PANI fii electrode, at two different magnifications.
(60 mC/cm’),
grown on a platinum
could provide bridges between neighboring fibers by coupling PAN1 units at the o&o and metu positions. PPDA may also modify the kinetics of the film growth and promote nucleation sites, leading to thinner fibers and more branching points. The middle peak present in the cyclic voltammogram may be attributed to a cross-linking site induced by the diamine during the polymerization. The SEM pictures of the films substantiate this hypothesis. However, this middle peak could also correspond to oxidation product of the diamine, e.g. a partial hydrolysis to a quinone. PPDA is more oxidizable than aniline and is oxidized above 0.5 V. It should be noted that another peak arises at 0.5 V when these PPDA-modified films
293
600400!2000
Fig. 4. Consecutive cyclic voltammograms of a PPDA-modified PAN1 film prepared from a 0.2 M aniline + 5.5 x low4 M PPDA solution in 1 M HCl at a scan rate of 50 mV/s, on a platinum electrode.
are cycled up to 0.9 V in 1 M HCl, which may correspond to hydrolysis to the imino quinones or benzoquinones. Several types of defects, or different types of linkage between aniline molecules (other than the puru coupling), as well as hydrolysis products, are likely to give rise to peaks in the 0.5 V region. The effect of the other isomers, o&o-phenylenediamine (OPDA) and meta-phenylenediamine (MPDA), on the morphology of polyaniline films, under the same preparation conditions, was also investigated. Preliminary studies showed that they did not induce such a dramatic change as in the case of PPDA. Pure OPDA has been reported to electropolymerize to yield a conductive “ladder” type of polymer [8]. However, in our case, the voltammogram in 1 M HCl of the resulting OPDA-modified film was identical to that of pure PAN1 and did not exhibit any redox couple in the -0.1 V region, characteristic of poly-OPDA. This could indicate that the proportion of poly-OPDA formed, if any, is too small to be detected. Red, soluble oxidation products originating from OPDA oxidation were observed during the film preparation in the vicinity of the electrode. SEM examination of the resulting fiim revealed a fibrous structure which was similar to pure PANI, although somewhat less regular. In the case of MPDA, the resulting film had a dual microstructure with fibrous domains very similar to pure PAN1 and denser regions resembling PPDA-modified
294
Fig. 5. SEM micrographs of a PPDA-modified PAN1 film (60 mC/cm*), Film grown on a platinum substrate.
at two different magnifications.
PANI. However, MPDA seemed to slow down the polymer growth and therefore appeared less attractive for film fabrication.
CONCLUSIONS
Additions of small concentrations of PPDA cause dramatic changes in the morphology and growth rate of polyaniline films. PPDA appears to act as a cross-linking agent for aniline polymerization, leading to a denser and more uniform
295
film structure. This may prove useful for battery applications, the polymer structure for optimal electrode performance.
and allow tuning of
ACKNOWLEDGEMENTS
Brian Senior is thanked for his help with the SEM pictures and Dr. Otto Haas for helpful discussions. The present work was supported by the Swiss “Nationaler Energie Forschungs-Fond” (NEFF Project 382). REFERENCES 1 Reviews: J.R. Renolds, Chemtech, 18 (1988) 441; A.F. Diaz and J.C. Lacroix, New J. Chem., 12 (1988) 171; M. Bryce, Chem. Br., 24 (1988) 781. 2 E.M. Genies, M. Lapkowski and C. Tsintavis, New J. Chem., 12 (1988) 181. 3 A.F. Diaz and J.A. Logan, J. Electroanal. Chem., 111 (1980) 111. 4 E.M. Genies and M. Lapkowski, Synth. Met., 24 (1988) 61; D.E. Stilwell and S.M. Park, J. Electrochem. Sot., 135 (1988) 2254; A.G. MacDiarmid, J.C. Chiang, A.F. Richter and A.J. Epstein, Synth. Met., 18 (1987) 285. 5 J.C. Lacroix and A.F. Diaz, J. Electrochem. Sot., 135 (1988) 1457. 6 E.M. Genies, M. Lapkowski and J.F. Penneau, J. Electroanal. Chem., 249 (1988) 97. 7 S. Taguchi and T. Tanaka, J. Power Sources, 20 (1987) 249. 8 K. Chiba, T. Ohsaka, Y. Ohnuki and N. Oyama, J. Electroanal. Chem., 219 (1987) 117; C. Barbero, J.J. Silber and L. Sereno, J. Electroanal. Chem., 263 (1989) 333.
J. Electroanal. Chem, 262 (1989) 289-295 Ekevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note
Morphology of electropolymerized by para-phenylenediamine
aniline films modif ied
C. Mailhe-Randolph Institut de Chimie Physique, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne (Switzerland)
J. Desilvestro Paul Scherrer Institut, 5303 Wiirenlingen (Switzerland) (Received 19 December 1988; in revised form 16 February 1989)
INTRODUCTION
Polyaniline (PANI) has emerged as one of the promising candidate materials in the fast-growing field of conductive polymers [l]. It is stable in air, can accept high levels of doping and is readily prepared in water [2]. Electropolymerization of an acidic aqueous solution of aniline yields thin, electroactive PAN1 films in a reproducible manner [3]. The electrochemical behavior of such PAN1 films has been investigated extensively and two main redox systems have been identified [2,4]. However, the characteristics of the cyclic voltammograms depend on the synthesis conditions, especially on the potential limit used in oxidation, and some additional middle peaks have been reported [5]. These middle peaks have been attributed tentatively to the formation of oligomers, cross-linking reactions, formation of phenazine rings or hydrolysis of the imine yielding the corresponding quinone [6]. The purpose of this note is to report the effect of copolymerization with para-phenylenediamine (PPDA), on the electrochemical behavior and morphology of PAN1 films. EXPERIMENTAL
Aniline (Fluka) was freshly distilled and the colorless liquid was kept under argon in darkness at 5 o C. Para-phenylenediamine was used as received. A typical three-electrode cell was used, with a 2 cm* platinum or tin oxide electrode, a saturated calomel reference electrode (WE) and a platinum counter electrode. The platinum working electrode was cleaned in HNO, and polished with alumina prior to use. The potentiostat was a PAR model 274 potentiostat/ galvanostat. All potentials are quoted versus SCE. 0022-0728/89/$03.50
6 1989 Elsevier Sequoia S.A.
290 The electrooxidation of aniline was carried out in 1 M HCl containing 0.2 M aniline. The PANI films were prepared by cycling the potential continuously between -0.15 and +0.78 V at a scan rate of 50 mV/s until the desired quantity of PAN1 was formed. The cell was kept under argon during the electropolymerization. For SEM examination (Cambridge instrument model 250), the sample was removed from the cell at 0.44 V (in the emeraldine state), rinsed in 1 M HCl and dried in air.
RESULTS AND DISCUSSION
Figure 1 shows initial cyclic voltammograms obtained during aniline electropolymerization. Aniline oxidation takes place at potentials higher than 0.62 V and the resulting PAN1 film exhibits a first oxidation peak at 0.16 V. The oxidation limit during the film synthesis was set to 0.78 V in order to limit the current densities associated with aniline oxidation and to avoid possible side reactions such as hydrolysis to quinones, which may occur at higher potentials. It was also confirmed, as reported in the literature, that no additional middle peak is observed in the 0.5 V region when the upper potential limit does not exceed 0.8 V during the electropolymerization [5]. The PAN1 films were grown to 60 mC/cm2 (the charge was calculated by taking into account only the first electrochemical wave). The microstructures of the resulting PAN1 films, grown on platinum, are shown in Figs. 2 and 3. Tin oxide was also used as a substrate and did not induce noticeable differences in the microstructure. At the beginning of the deposition (up to a thickness of 10 mC/cm*), the polymer exhibits a rather uniform globular microstructure. For thicker deposits (60 mC/cm2), a fibrous structure develops, as shown in Fig. 3. These fibers are about 0.2 pm thick and exhibit some branching. Similar microstructures have been reported previously for PAN1 films prepared in the presence of H,SO, or HCl [5,7]. In contrast to the initial globular morphology,
T0.5m~
I
E/mV
-600
400
200
0
Fig. 1. Consecutive cyclic voltammograms of a PAN1 film electropolymerized from a 0.2 M aniline+ 1 M HCl aqueous solution, at a scan rate of 50 mV/s, on a platinum substrate.
291
Fig. 2. Initial stages of PANI fihn formation (10 mC/cm*) on a platinum electrode; fiber growth on a globular microstructure.
the fibrous deposit is not entirely uniform over the electrode. This seems to indicate that the fibers are preferred sites for further polymer growth. These irregularities were discernable macroscopically in the case of films as thick as 0.5 C/cm*. Dried, thick films undergo noticeable swelling when immersed in the electrolyte solution. Figure 4 shows the influence of PPDA on the cyclic voltammogram during aniline electropolymerization. The PPDA concentration was 5.5 x lop4 M, in a 0.2 it4 aniline + 1 M HCl starting solution. An additional peak appears at 0.45 V and the film growth rate is increased significantly. In order to keep the anodic current densities at values comparable to those used in the case of pure aniline, the oxidation potential limit was reduced further to 0.62 V during the deposition. The cyclic voltammogram of the same film in 1 M HCl exhibits a peak at 0.45 V as well. However, it should be noted that PPDA, when electropolymerized alone, is oxidized above 0.50 V and gives rise to poorly adherent, non-electroactive films. Also, for concentrations of PPDA greater than lop3 M in the starting aniline solution, polymer growth occurs rapidly and the resulting polymer film is poorly adherent. Therefore, PPDA addition must be controlled carefully. Visual inspection and micrographs (cf. Fig. 5a) show that PPDA-modified PAN1 films (60 mC/cm*) are distributed over the electrode surface much more evenly than pure PAN1 films (Fig. 3a). Films as thick as 0.5 C/cm* are readily obtained in the presence of PPDA and appear, at least visually, very uniform. PPDA addition appears, therefore, to facilitate the fabrication of such films, which could prove very useful for battery applications. The higher-magnification micrograph (Fig. 5b) reveals that the fibers are much thinner and exhibit more branching. PPDA appears to modify the polymer film morphology and to act as a cross-linking agent. For example, a PPDA molecule
292
Fig. 3. SEM micrographs of the fibrous structure of a PANI fii electrode, at two different magnifications.
(60 mC/cm’),
grown on a platinum
could provide bridges between neighboring fibers by coupling PAN1 units at the o&o and metu positions. PPDA may also modify the kinetics of the film growth and promote nucleation sites, leading to thinner fibers and more branching points. The middle peak present in the cyclic voltammogram may be attributed to a cross-linking site induced by the diamine during the polymerization. The SEM pictures of the films substantiate this hypothesis. However, this middle peak could also correspond to oxidation product of the diamine, e.g. a partial hydrolysis to a quinone. PPDA is more oxidizable than aniline and is oxidized above 0.5 V. It should be noted that another peak arises at 0.5 V when these PPDA-modified films
293
600400!2000
Fig. 4. Consecutive cyclic voltammograms of a PPDA-modified PAN1 film prepared from a 0.2 M aniline + 5.5 x low4 M PPDA solution in 1 M HCl at a scan rate of 50 mV/s, on a platinum electrode.
are cycled up to 0.9 V in 1 M HCl, which may correspond to hydrolysis to the imino quinones or benzoquinones. Several types of defects, or different types of linkage between aniline molecules (other than the puru coupling), as well as hydrolysis products, are likely to give rise to peaks in the 0.5 V region. The effect of the other isomers, o&o-phenylenediamine (OPDA) and meta-phenylenediamine (MPDA), on the morphology of polyaniline films, under the same preparation conditions, was also investigated. Preliminary studies showed that they did not induce such a dramatic change as in the case of PPDA. Pure OPDA has been reported to electropolymerize to yield a conductive “ladder” type of polymer [8]. However, in our case, the voltammogram in 1 M HCl of the resulting OPDA-modified film was identical to that of pure PAN1 and did not exhibit any redox couple in the -0.1 V region, characteristic of poly-OPDA. This could indicate that the proportion of poly-OPDA formed, if any, is too small to be detected. Red, soluble oxidation products originating from OPDA oxidation were observed during the film preparation in the vicinity of the electrode. SEM examination of the resulting fiim revealed a fibrous structure which was similar to pure PANI, although somewhat less regular. In the case of MPDA, the resulting film had a dual microstructure with fibrous domains very similar to pure PAN1 and denser regions resembling PPDA-modified
294
Fig. 5. SEM micrographs of a PPDA-modified PAN1 film (60 mC/cm*), Film grown on a platinum substrate.
at two different magnifications.
PANI. However, MPDA seemed to slow down the polymer growth and therefore appeared less attractive for film fabrication.
CONCLUSIONS
Additions of small concentrations of PPDA cause dramatic changes in the morphology and growth rate of polyaniline films. PPDA appears to act as a cross-linking agent for aniline polymerization, leading to a denser and more uniform
295
film structure. This may prove useful for battery applications, the polymer structure for optimal electrode performance.
and allow tuning of
ACKNOWLEDGEMENTS
Brian Senior is thanked for his help with the SEM pictures and Dr. Otto Haas for helpful discussions. The present work was supported by the Swiss “Nationaler Energie Forschungs-Fond” (NEFF Project 382). REFERENCES 1 Reviews: J.R. Renolds, Chemtech, 18 (1988) 441; A.F. Diaz and J.C. Lacroix, New J. Chem., 12 (1988) 171; M. Bryce, Chem. Br., 24 (1988) 781. 2 E.M. Genies, M. Lapkowski and C. Tsintavis, New J. Chem., 12 (1988) 181. 3 A.F. Diaz and J.A. Logan, J. Electroanal. Chem., 111 (1980) 111. 4 E.M. Genies and M. Lapkowski, Synth. Met., 24 (1988) 61; D.E. Stilwell and S.M. Park, J. Electrochem. Sot., 135 (1988) 2254; A.G. MacDiarmid, J.C. Chiang, A.F. Richter and A.J. Epstein, Synth. Met., 18 (1987) 285. 5 J.C. Lacroix and A.F. Diaz, J. Electrochem. Sot., 135 (1988) 1457. 6 E.M. Genies, M. Lapkowski and J.F. Penneau, J. Electroanal. Chem., 249 (1988) 97. 7 S. Taguchi and T. Tanaka, J. Power Sources, 20 (1987) 249. 8 K. Chiba, T. Ohsaka, Y. Ohnuki and N. Oyama, J. Electroanal. Chem., 219 (1987) 117; C. Barbero, J.J. Silber and L. Sereno, J. Electroanal. Chem., 263 (1989) 333.