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The effect of anions of supporting electrolyte on the electrochemical polymerization of aniline and the properties of polyaniline
Synthetic Metals, 13 (1986) 329 - 334
329
Short Communication
The Effect of Anions of Supporting Electrolyte on the Electrochemical Polymerization of Aniline and the Properties of Polyaniline BAOCHEN WANG, JINSONG TANG and FOSONG WANG Changchun Institute of Applied Chemistry, Academia Sinica, Changchun, Jilin (The People’s Republic of China) (Received August 29,1985; accepted September 30,1985)
Introduction Polyaniline (PAN) is an electroconductive organic polymer and shows many interesting properties such as electrochemical redox behaviour [ 11, electrochromic [ 21 and catalytic activities [ 31. It can be prepared by chemical or electrochemical polymerization of aniline [4]. Although there have been some papers concerning the mechanism of anodic polymerization of aniline and the electrochemical properties of polyaniline [5, 61, little is known about the effect of anions of supporting electrolytes on the rate of polymerization and properties of the products. This communication presents preliminary results on the anodic polymerization of aniline on a Pt electrode in H2S04, HCl and HC104 supporting electrolytes. Experimental The electrochemical polymerization was carried out in a conventional three-electrode cell, containing HZS04, HCl or HC104 (pH = 1) solution. A Pt plate was the counter electrode and SCE served as the reference electrode. The working Pt electrode (surface area 3 cm2) was treated successively in concentrated H2S04 and HNOs solutions, and was then rinsed with twicedistilled water. Before polymerization, the working electrode was cycled between -0.2 V and +0.8 V several times until reproducible voltammograms were obtained. Purified aniline was then added to the acid solution to form 0.1 M aniline solution. The potential cycling polymerization was performed in the potential range -0.2 V to +0.8 V with a sweep rate of 50 mV/s. The films obtained from H,SO,, HCl and HC104 are abbreviated to PAN,, PAN, and PAN,, respectively. Electrochemical polymerization and electrochemical measurements were carried out on a DH-1 Universal Potentiostat. The morphology of the PAN films was recorded on a JXA-840 Scanning Electron Microscope. 0319-6719/86/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
330
All measurements were conducted at room temperature (15 “C). The electrolytic solutions were deaerated under N2 gas and all samples taken from solutions were kept under a purified Nz atmosphere before SEM were recorded. Results and discussion Figure 1 shows the voltammograms recorded during anodic polymerization of aniline on a Pt electrode in HzS04 solution. This voltammogram is similar to that obtained by Kitani [l]. After the addition of aniline the hydrogen adsorption peak is suppressed. This indicates that adsorption of aniline may occur on the Pt electrode surface. When the potential is scanned to +0.6 V (H,S04), the oxidation of aniline begins and a high oxidation current is obtained. Similar results were obtained in the cases of HCl and HC104 (Figs. 2 and 3). Another feature that can be seen from Figs. 1 - 3 is that with potential cycling in the range -0.2 V to 0.8 V, three redox couples were observed on
Qx s *-?
+1.0
+d-
+0.8
+a3 .
+0.6
+a1 -
+04
2 E
+0.2
‘f
+al -
o
-
0
-0.l -
- 0.2
-OS -
-
0.4
-a3
-a2
0
+a2
E (VJ Fig. 1. Voltammograms recorded during the anodic polymerization of electrode. The numbers on the curves indicate the number of potential HzS04 (pH = 1) +O.l M a n i l i n e . Sweep rate: 50 mV/s. Dotted curve: voltammogram of the Pt electrode.
+a4
*a6
+a8
vs. SCE aniline on a Pt cycles. Solution: the background
Fig. 2. Voltammograms recorded during the anodic polymerization of aniline on a Pt electrode. The numbers on the curves indicate the number of potential cycles. Solution: HCl (pH = 1) +O.l M aniline. Sweep rate: 50 mV/s. Dotted curve: the background voltammogram of the Pt electrode.
-0.2
-
0.3
-
0.1
0
ro.1
+ 0.4
W.6
40.8
Erv) vs. SLE Fig. 3. Voltammograms recorded during the anodic polymerization of aniline on a Pt electrode. The numbers on the curves indicate the number of potential cycles. Solution: HC104 (pH = 1) +O.l M aniline. Sweep rate: 50 mV/s. Dotted curve: the background voltammogram of the Pt electrode.
the voltammograms. The positions and relative intensities of these three peaks were effected by the nature of the anion of the supporting electrolyte. In the case of HzS04, three redox couples are recorded at +0.075 (the first couple), +0.42 V (the second) and +0.64 V (the third) on the voltammograms. The intensities of the first and third couples increase faster than the second with increasing cycling number. In HCl solution the positions of the first and second couples are very close to those in H,S04, but the third one is 75 mV more negative. In contrast to the H,S04 system, the second peak increases with the cycling number; after 50 cycles it is still higher than the other two. Quite different results were obtained in HC104 solution. The first and the third peaks appear only as shoulders, but the second one grows considerably . Suppose that the aniline polymerization process on a Pt electrode could be separated into two stages. The first stage is polymerization on the bare Pt electrode or partially blocked electrode by polyaniline. The second stage i s polymerization on the completely covered Pt electrode by polyaniline. At the first stage the Pt electrode surface is expected to be one of the determining factors of the polymerization rate. When the deposited polyaniline film blocks the Pt electrode surface, the aniline polymerization becomes more difficult, so the rate of polymerization decreases as shown in Figs. 1 - 3. Evidently, further polymerization of aniline (second stage) would depend to
(a)
(b)
Fig. 4. Scanning electron micrographs of PAN: (a) PAN,; (b) PA-N,; (c) PAN,.
some extent on the morphology of films formed. It is expected that for a polyaniline film with a pore-rich structure, the polymerization rate would be increased. This was confirmed by SEM, as shown in Fig. 4. It is evident from Fig. 4 that the morphology of polyaniline prepared in H,S04 is granular and richer in pores than that of PAN, and PAN,. Therefore the increase of polymerization rate for PAN, is more significant than that for PAN, and PAN,. The higher polymerization rate in the case of HzS04 was further confirmed by redox charge measurements. In general the redox charge at the same cycle can be used to compare the relative average rate of film formation (or polymerization). As is shown in Table 1, the increase of redox charge in the case of HzS04 is faster than in the other two acids and increases with the cycling number. For example, at the fifth cycle the redox charge for PAN, is 1.68 times that of PAN, and 1.26 times that of PAN,, while at the 50th cycle it becomes 8.0 times and 6.3 times faster respectively. Figure 5 shows the voltammograms of PAN, and PAN, in the corresponding acids with or without aniline in solution. The aniline in solution exerts only a small influence on the voltammograms, but the effect of the anions of the supporting electrolytes is still evident It was found that the polyaniline sample taken from the electrolysis solution without any treatment shows the presence of a crystalline salt
333 TABLE 1 The effect of anions on the redox charge of deposited films at different cycles Solution
Redox charge (mc/cm2) Number of cycles
HzS04
HCl HC104
5
10
20
30
40
50
0.78 0.46 0.61
1.61 0.62 0.92
4.26 0.96 1.54
7.91 1.45 2.12
14.19 2.03 2.59
20.66 2.59 3.27
b
I. -0.2
n
I
.
*
L
1
+o.g
+o.g
-0.2
E
(V)
-02
+08
vs. SCE
Fig. 5. Voltammograms of PAN in acids (pH = 1). (a) HzS04; (b) HCl; (c) HC104. Solid curve, without aniline; dashed curve, with aniline.
formed between aniline and the acids, which disappears after rinsing with H,O and acids. On other hand, the different treatment of the polyaniline sample with H20, acid or alkaline solutions may shift the acid-base equilibrium of the protonated polyaniline formed in acid solutions. From the above results it seems reasonable to postulate that the anions of the supporting electrolyte may take part in the protonation and intercalation of polyaniline. The reduced state of protonated polyaniline could be represented by.
The intercalation of anions in the oxidized state of protonated polyaniline could be expressed as follows:
334
Detailed results on the electrochemical behaviour of polyaniline will be reported in a separate paper. Acknowledgements This work was supported by a Fund of the Chinese Academy of Sciences. References 1 A. Katani and J. Izumi, Bull. Chem. Sot. Japan, 57 (1984) 248. 2 T. Kobayashi and H. Yoneyama, J. Electroanal. Chem. Interfacial Electrochem., 16 (1984) 419. 3 N. Oyama and Y. Ohnuki, Chem. Lett., (1983) 1759. 4 A. G. MacDiarmid, J. C. Chiang and M. Halpern, Polym. Prepr., (1984) 248. 5 R. N. Adams, Electrochemistry at Solid Electrodes, Marcel Dekker, New York, 1969, p. 327. 6 D. M. Mohilner and R. N. Adams, J. Am. Chem. Sot., 84 (1962) 3618.
329
Short Communication
The Effect of Anions of Supporting Electrolyte on the Electrochemical Polymerization of Aniline and the Properties of Polyaniline BAOCHEN WANG, JINSONG TANG and FOSONG WANG Changchun Institute of Applied Chemistry, Academia Sinica, Changchun, Jilin (The People’s Republic of China) (Received August 29,1985; accepted September 30,1985)
Introduction Polyaniline (PAN) is an electroconductive organic polymer and shows many interesting properties such as electrochemical redox behaviour [ 11, electrochromic [ 21 and catalytic activities [ 31. It can be prepared by chemical or electrochemical polymerization of aniline [4]. Although there have been some papers concerning the mechanism of anodic polymerization of aniline and the electrochemical properties of polyaniline [5, 61, little is known about the effect of anions of supporting electrolytes on the rate of polymerization and properties of the products. This communication presents preliminary results on the anodic polymerization of aniline on a Pt electrode in H2S04, HCl and HC104 supporting electrolytes. Experimental The electrochemical polymerization was carried out in a conventional three-electrode cell, containing HZS04, HCl or HC104 (pH = 1) solution. A Pt plate was the counter electrode and SCE served as the reference electrode. The working Pt electrode (surface area 3 cm2) was treated successively in concentrated H2S04 and HNOs solutions, and was then rinsed with twicedistilled water. Before polymerization, the working electrode was cycled between -0.2 V and +0.8 V several times until reproducible voltammograms were obtained. Purified aniline was then added to the acid solution to form 0.1 M aniline solution. The potential cycling polymerization was performed in the potential range -0.2 V to +0.8 V with a sweep rate of 50 mV/s. The films obtained from H,SO,, HCl and HC104 are abbreviated to PAN,, PAN, and PAN,, respectively. Electrochemical polymerization and electrochemical measurements were carried out on a DH-1 Universal Potentiostat. The morphology of the PAN films was recorded on a JXA-840 Scanning Electron Microscope. 0319-6719/86/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
330
All measurements were conducted at room temperature (15 “C). The electrolytic solutions were deaerated under N2 gas and all samples taken from solutions were kept under a purified Nz atmosphere before SEM were recorded. Results and discussion Figure 1 shows the voltammograms recorded during anodic polymerization of aniline on a Pt electrode in HzS04 solution. This voltammogram is similar to that obtained by Kitani [l]. After the addition of aniline the hydrogen adsorption peak is suppressed. This indicates that adsorption of aniline may occur on the Pt electrode surface. When the potential is scanned to +0.6 V (H,S04), the oxidation of aniline begins and a high oxidation current is obtained. Similar results were obtained in the cases of HCl and HC104 (Figs. 2 and 3). Another feature that can be seen from Figs. 1 - 3 is that with potential cycling in the range -0.2 V to 0.8 V, three redox couples were observed on
Qx s *-?
+1.0
+d-
+0.8
+a3 .
+0.6
+a1 -
+04
2 E
+0.2
‘f
+al -
o
-
0
-0.l -
- 0.2
-OS -
-
0.4
-a3
-a2
0
+a2
E (VJ Fig. 1. Voltammograms recorded during the anodic polymerization of electrode. The numbers on the curves indicate the number of potential HzS04 (pH = 1) +O.l M a n i l i n e . Sweep rate: 50 mV/s. Dotted curve: voltammogram of the Pt electrode.
+a4
*a6
+a8
vs. SCE aniline on a Pt cycles. Solution: the background
Fig. 2. Voltammograms recorded during the anodic polymerization of aniline on a Pt electrode. The numbers on the curves indicate the number of potential cycles. Solution: HCl (pH = 1) +O.l M aniline. Sweep rate: 50 mV/s. Dotted curve: the background voltammogram of the Pt electrode.
-0.2
-
0.3
-
0.1
0
ro.1
+ 0.4
W.6
40.8
Erv) vs. SLE Fig. 3. Voltammograms recorded during the anodic polymerization of aniline on a Pt electrode. The numbers on the curves indicate the number of potential cycles. Solution: HC104 (pH = 1) +O.l M aniline. Sweep rate: 50 mV/s. Dotted curve: the background voltammogram of the Pt electrode.
the voltammograms. The positions and relative intensities of these three peaks were effected by the nature of the anion of the supporting electrolyte. In the case of HzS04, three redox couples are recorded at +0.075 (the first couple), +0.42 V (the second) and +0.64 V (the third) on the voltammograms. The intensities of the first and third couples increase faster than the second with increasing cycling number. In HCl solution the positions of the first and second couples are very close to those in H,S04, but the third one is 75 mV more negative. In contrast to the H,S04 system, the second peak increases with the cycling number; after 50 cycles it is still higher than the other two. Quite different results were obtained in HC104 solution. The first and the third peaks appear only as shoulders, but the second one grows considerably . Suppose that the aniline polymerization process on a Pt electrode could be separated into two stages. The first stage is polymerization on the bare Pt electrode or partially blocked electrode by polyaniline. The second stage i s polymerization on the completely covered Pt electrode by polyaniline. At the first stage the Pt electrode surface is expected to be one of the determining factors of the polymerization rate. When the deposited polyaniline film blocks the Pt electrode surface, the aniline polymerization becomes more difficult, so the rate of polymerization decreases as shown in Figs. 1 - 3. Evidently, further polymerization of aniline (second stage) would depend to
(a)
(b)
Fig. 4. Scanning electron micrographs of PAN: (a) PAN,; (b) PA-N,; (c) PAN,.
some extent on the morphology of films formed. It is expected that for a polyaniline film with a pore-rich structure, the polymerization rate would be increased. This was confirmed by SEM, as shown in Fig. 4. It is evident from Fig. 4 that the morphology of polyaniline prepared in H,S04 is granular and richer in pores than that of PAN, and PAN,. Therefore the increase of polymerization rate for PAN, is more significant than that for PAN, and PAN,. The higher polymerization rate in the case of HzS04 was further confirmed by redox charge measurements. In general the redox charge at the same cycle can be used to compare the relative average rate of film formation (or polymerization). As is shown in Table 1, the increase of redox charge in the case of HzS04 is faster than in the other two acids and increases with the cycling number. For example, at the fifth cycle the redox charge for PAN, is 1.68 times that of PAN, and 1.26 times that of PAN,, while at the 50th cycle it becomes 8.0 times and 6.3 times faster respectively. Figure 5 shows the voltammograms of PAN, and PAN, in the corresponding acids with or without aniline in solution. The aniline in solution exerts only a small influence on the voltammograms, but the effect of the anions of the supporting electrolytes is still evident It was found that the polyaniline sample taken from the electrolysis solution without any treatment shows the presence of a crystalline salt
333 TABLE 1 The effect of anions on the redox charge of deposited films at different cycles Solution
Redox charge (mc/cm2) Number of cycles
HzS04
HCl HC104
5
10
20
30
40
50
0.78 0.46 0.61
1.61 0.62 0.92
4.26 0.96 1.54
7.91 1.45 2.12
14.19 2.03 2.59
20.66 2.59 3.27
b
I. -0.2
n
I
.
*
L
1
+o.g
+o.g
-0.2
E
(V)
-02
+08
vs. SCE
Fig. 5. Voltammograms of PAN in acids (pH = 1). (a) HzS04; (b) HCl; (c) HC104. Solid curve, without aniline; dashed curve, with aniline.
formed between aniline and the acids, which disappears after rinsing with H,O and acids. On other hand, the different treatment of the polyaniline sample with H20, acid or alkaline solutions may shift the acid-base equilibrium of the protonated polyaniline formed in acid solutions. From the above results it seems reasonable to postulate that the anions of the supporting electrolyte may take part in the protonation and intercalation of polyaniline. The reduced state of protonated polyaniline could be represented by.
The intercalation of anions in the oxidized state of protonated polyaniline could be expressed as follows:
334
Detailed results on the electrochemical behaviour of polyaniline will be reported in a separate paper. Acknowledgements This work was supported by a Fund of the Chinese Academy of Sciences. References 1 A. Katani and J. Izumi, Bull. Chem. Sot. Japan, 57 (1984) 248. 2 T. Kobayashi and H. Yoneyama, J. Electroanal. Chem. Interfacial Electrochem., 16 (1984) 419. 3 N. Oyama and Y. Ohnuki, Chem. Lett., (1983) 1759. 4 A. G. MacDiarmid, J. C. Chiang and M. Halpern, Polym. Prepr., (1984) 248. 5 R. N. Adams, Electrochemistry at Solid Electrodes, Marcel Dekker, New York, 1969, p. 327. 6 D. M. Mohilner and R. N. Adams, J. Am. Chem. Sot., 84 (1962) 3618.