The effect of salts on the electrochemical polymerization of aniline

SVIilTHETIIC IiIIiI|TlllLS ELSEVIER

Synthetic Metals 92 (1998) 149-155

The effect of salts on the electrochemical polymerization of aniline Shaolin Mu *, Jinqing Kan Department of Chemisto', Teacher's College, Yangzhou University, Yangz~hou225002, China Received 22 September 1997; accepted 2 October 1997

Abstract Aniline is electrolysed by repeated potential cycling between 0 and 1.1 V (versus SCE); a current plateau instead of a current peak is observed on the I-E curve for the first cycle during electrolysis of aniline in aqueous acid solution in the presence of NaC1. The occurrence of the current plateau plays an important role in extending the potential range of electrolysis of aniline at a high rate. The polymerization rate of aniline in a solution containing a given HCI concentration increases with increasing concentration of sodium chloride; the potential range of the intermediate generated at the disk electrode is also extended with increasing concentration of NaCI. The potential necessary for the formation of the soluble intermediate is at least 0.75 V. The hydrochloric acid concentration necessary for an autocatalytic polymerization of aniline decreases from 0.6 to 0.4 M in the presence of NaCI. The conductivity of polyaniline prepared in a solution containing 0.2 M aniline, 0.4 M HC1 and 1 M NaCI is about 30 times higher than that of polyaniline prepared in a solution containing 0.2 M aniline and 0.4 M HCI. The oxidation potential of aniline in the presence of the salts shifts toward negative potentials, depending on the ionic radius of cations and the concentration of the different salts. © 1998 Elsevier Science S.A. Kevwords: Aniline; Effect of salts; Polymerization rate; Oxidation potential; Conductivity

1. Introduction The electrochemical oxidation of aniline in aqueous sulfuric acid solution can be traced back over 100 years; its final product obtained at a platinum anode is a dark precipitate [ l ]. Among the studies of the electrochemical oxidation of aniline, the polymerization mechanism has received great attention. Mohilner et al. [2], Bacon and Adams [3], Diaz and Logan [4] and Albery et al. [5] suggested that the polymerization of aniline at a platinum anode proceeds through a free radical mechanism. In 1984, Kobayashi et al. [6] reported the electrochromic properties of polyaniline. In 1985, MacDiarmid et al. [17,8] reported the interconversion of metallic and insulating forms of polyaniline and batteries formed using polyaniline as the electrode material. Since then, many papers about the synthesis, properties and applications of polyaniline have appeared [ 9 - 2 2 ] . Studies of the electrochemical polymerization mechanism of aniline [ 2 3 31] play an important role in understanding the polymerization reaction process, controlling the polymerization rate and improving polyaniline properties. The molecular weight of polyaniline can be markedly increased by the chemical oxidation polymerization of aniline with ammonium peroxydisulfate at low temperatures in the * Corresponding author. Fax: + 86 514 734 9819. 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved PII S 0 3 7 9 - 6 7 7 9 ( 97 ) 0 4 0 8 0 - 0

presence of neutral salts [ 19,20]. The change in the molecular weight of polyaniline may affect its properties. Considering this case, we tried to synthesize polyaniline at low temperature using repeated potential cycling. The solution for electrolysis consisted of 0.2 M aniline, 1 M HCI and 1 M NaC1. A particular phenomenon, i.e., a current plateau instead of a current peak, was observed on the I - E curve for the first cycle. This experimental phenomenon may be caused by the temperature or sodium chloride. To discover the reason, the experiment was carried out to 20 °C; the experimental result was unchanged. This means that the above experimental phenomenon is caused by sodium chloride alone. The effect of sodium chloride on the electrochemical polymerization behaviour of the aniline has not been previously reported. In this paper, we report the effect of salts on both the oxidation potential and polymerization rate of aniline using the methods of repeated potential cycling, constant potential and rotating ring-disk electrode (RRDE), and the effect of sodium chloride on the polyaniline conductivity, and approach the effect of sodium chloride on the polymerization mechanism.

2. Experimental The chemicals used were all reagent grade. Aniline was distilled before use. The electrolysis cell consisted of two

S. Mu, J. Kan /Synthetic Metals 92 (1998) 149-155

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platinum foil electrodes and a saturated calomel electrode (SCE). The area of the working electrode was 4 m m × 4 mm. All potentials given are referred to the SCE. A DH- 1 potentiostat-galvanostat was used for electrolysis of aniline• Polyaniline films were prepared using the methods of constant potential and repeated potential cycling. The change in current with time during the potentiostatic electrolysis of aniline was recorded using a YEW Model 3066 pen recorder; the solution was stirred at a given rate during the electrolysis. An excellent cohesive film was formed on the platinum foil. The cyclic voltammograms during electrolysis of aniline were recorded using a YEW model 3036 X-Y recorder• The scan rate was 48 mV s - ~. The sweeping potential range was between 0 and 1.1 V. In order to prove that an intermediate was generated during electrolysis of aniline, an experiment for collecting intermediate was carried out using a model HR-103 A RRDE. The diameter of the platinum disk electrode was 7.89 mm. The ring electrode was controlled at 0.1 V, and the disk potential was scanned from 0 in a positive direction and stopped at 1.15 V. The scan rate was 48 mV s - ~ and the rotation rate was controlled at 1000 rpm. The conductivity of polyaniline was measured by the four-probe technique. Polyaniline for the determination of conductivity was obtained using a constant-potential electrolysis at 0.80 V. After synthesis, polyaniline was washed thoroughly using 0.2 M HCI solution and then was immersed in 0.2 M HCI solution for 48 h. Finally, the polyaniline was dried at 80 °C for 24 h• The dried polyaniline was used for the determination of conductivity at 15 °C; other experiments were carried out at 20 °C. 3. Results and discussion

3.1. Electrolysis of aniline using potential cycling Fig. I shows the cyclic voltammograms for electrolysis of 0.2 M aniline in 1.0 M HC1 solution at 20 °C. Fig. 2 shows

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Fig. I. Cyclic voltammograms of polyaniline film growth from 0.2 M aniline in I M HCI solution at a scan rate of 48 mV s ~ and 20 °C: ( 1 ) first cycle: (2) second cycle; (3) third cycle.

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Fig. 2. Cyclic voltammograms of polyaniline film growth from a solution containing 0.2 M aniline, I M HCI and 1 M NaC1 at 48 mV s ~ and 20 °C: ( 1 ) first cycle; (2) second cycle; (3) third cycle; and so on.

the cyclic voltammograms for electrolysis of a solution containing 0.2 M aniline, 1.0 M HC1 and 1.0 M NaC1 at 20 °C. From Fig. 1, we can see that the current begins to increase sharply at 0.78 V. A current peak appears at 0.99 V for the first scan, which is the oxidation potential of aniline. The current beyond the peak potential decreases quickly with increasing potential (curve 1 in Fig. 1). However, Fig. 2 shows that the peak shifts to 0.94 V for the first scan. This peak potential is less than that of curve 1 in Fig. 1; the current beyond the peak potential decreases a little first and then increases again with increasing potential. In particular, the current during the reverse scan from 1.10 to 0.82 V is also unchanged, i.e., a current plateau appears on curve 1. The occurrence of the current plateau indicates that the oxidation rate of an electroactive species is equal to its diffusion rate. However, the solution during electrolysis is quiescent in this case, so it should be a current peak like curve 1 in Fig. I. Thus the formation of the current plateau implies that an intermediate having electroactive species is immediately produced at the beginning of the aniline oxidation. The intermediate is continuously oxidized; otherwise it is not possible to form the current plateau. The occurrence of the current plateau plays an important role in extending the potential range of electrolysis of aniline at a high rate. The above results indicate that the presence of sodium chloride in the aniline solution promotes the formation of a current plateau at the beginning of aniline oxidation. Fig. 3 shows the cyclic voltammograms for electrolysis of a solution containing 0.2 M aniline, 1.0 M HCI and 2 M NaCl. The peak potential appears at 0.92 V for the first scan (curve 1 ) ; the current beyond the peak decreases somewhat first and

S. Mu, J. Kan/Synthetic Metals 92 (1998) 149-155

151

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Fig. 3. Cyclic voltammograms of polyaniline film growth from a solution containing 0.2 M aniline, I M HCI and 2 M NaCI at 48 mV s - ~ and 20 °C: ( 1 / first cycle; (2) second cycle: (3) third cycle; and so on.

then increases again with increasing potential. The current during the reverse scan increases with decreasing potential, and a current peak forms at 0.87 V. This is different from that of curve 1 in Fig. 2. It is clear that this difference is caused by the concentration of NaCI. The anodic peak current in Figs. 1-3 increases with the number of potential cycles, which is due to the autocatalytic polymerization of aniline [ 24,31 ]. Except for the anodic peak current of the first scan (in Fig. 3), the currents of other anodic peaks in Fig. 3 are much larger than those in Fig. 1, whereas all anodic peak currents are also larger than those in Fig. 2. This indicates that the polymerization rate of aniline increases with increasing concentration of sodium chloride. The cyclic voltammograms (omitted here) for electrolysis of a solution containing 0.2 M aniline, 1.0 M HC1 and 0.5 M NaC1 are the same as those shown in Fig. 1, except that the current peak shifts from 0.99 to 0.97 V for the first scan. We found that during electrolysis of a solution containing 0.2 M aniline, I M HC1 and NaCI, the critical concentration of NaCI for the formation of a current plateau on the I - E curve of the first scan is at least 0.8 M, otherwise only a current peak for the first scan is observed on the I - E curve at a concentration of NaC1 less than 0.8 M. Fig. 4 shows the cyclic voltammograms for electrolysis of a solution containing 0.2 M aniline, 0.4 M HCI and 1.0 M NaCI. A prominent anodic peak appears at 0.94 V for the first scan, the shapes of all cyclic voltammograms being the same as those in Fig. I. The currents of all peaks increase markedly with the number of potential cycles. This is evidence for the autocatalytic polymerization of aniline. The shapes of all cyclic voltammograms, for electrolysis of a solution containing 0.2 M aniline, 0.4 M HCI and 2 M NaC1, are the same as those in Fig. 4, i.e., no current plateau is observed for the first

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E/V(vs.SCE) Fig. 4. Cyclic voltammograms of polyaniline film growth from a solution containing 0.2 M aniline, 0.4 M HC1 and 1 M NaC1 at 48 mV s ] and 20 °C: ( 1 ) first cycle; (2) second cycle; (3) third cycle; and so on.

scan. This means that the formation of the current plateau is also dependent on the HCI concentration. Fig. 5 shows the cyclic voltammograms for electrolysis of a solution containing 0.2 M aniline and 0.4 M HCI. The currents of all peaks are much smaller than those in Fig. 4 for the corresponding scan number, except the peak current for the first scan. Even though the currents of the peaks after the first cycle increase with the number of potential cycles, the increment is very small. This small increment is attributed to the polyaniline film growth at the platinum electrode owing to the repeated cycles. The cyclic voltammograms for electrolysis of solutions containing 0.2 M aniline, 0.3 M HC1 and 1.0 or 2 M NaCI show no evidence for autocatalytic polymerization of aniline. From the above results, we can conclude that the concentra5.0

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S. Mu, J. Kan/Synthetic Metals 92 (1998) 149-155

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Fig. 6. Cyclic voltammogram of polyaniline film in 1 M HCI. The film was synthesized in a solution containing 0.2 M aniline and 1 M HCI, using repeated potential cycling between 0 and 1.1 V.

tion of HCI necessary for an autocatalytic polymerization of aniline decreases from 0.6 M in the absence of NaC1 [31 ] to 0.4 M in the presence of 1 M NaCl. The cyclic voltammogram of polyaniline synthesized in a solution containing 0.2 M aniline and 1 M HC1 is shown in Fig. 6. From it we can see that there are three pairs of oxidation-reduction peaks. Their oxidation peak potentials are coincident with those during the polymerization of aniline after three cycles (Fig. 1 ). The difference between Fig. l and Fig. 6 is that the oxidation current decreases sharply with increasing potential when the potential is over 0.75 V (Fig. 6); however, the oxidation current continues to increase quickly with further increasing potential (Fig. l ). The latter is caused by the electrochemical oxidation of aniline on the polyaniline film itself. As a result, polyaniline in the presence of aniline cannot be overoxidized when the scan potential is over 0.75 V; the evidence for this is that the current increases with increasing number of potential cycles (Fig. 1). On the contrary, polyaniline in a solution without aniline is degraded when the potential is scanned between 0 and 1.1 V. The evidence for this degradation is that the oxidation current of the peak at 0.75 V decays with increasing number of potential cycles; this is caused by overoxidation of polyaniline. The cyclic voltammograms of the polyaniline films synthesized in aniline solution in the presence of NaC1, KCI and LiCI were carried out in 1 M HC1 solutions with 1 M of the corresponding salts in the absence of aniline. Their cyclic voltammograms are similar to that shown in Fig. 6, thus they are not shown here.

3.2. Potentiostatic electrolysis of aniline In this section, the working electrode potential was set at 0.78 V, and the solution was stirred at a given rate. Curves 2

and 3 in Fig. 7 show the change in current with time during electrolysis of 0.2 M aniline in both 0.4 M HCI and 1.0 M HCI solutions, respectively. Curve 2 shows that the current decreases first and then reaches a steady state. This is a general result for electrolysis at a constant potential in a convective system. Curve 3 shows that the current decreases first and then increases with time quickly; this is due to the fact that an autocatalytic polymerization takes place. Curve 4 shows the changes in current with time during electrolysis of a solution containing 0.2 M aniline, 1.0 M HCI and 0.5 M NaCI. The change in current with time is similar to that of curve 3, but its current is larger than that of curve 3. This is caused by NaC1. Curve 6 shows the change in current with time for electrolysis of a solution containing 0.2 M aniline, 1.0 M HCI and 1.0 M NaC1; curve 7 shows the change in current with time for electrolysis of a solution containing 0.2 M aniline, 1 M HC1 and 2.0 M NaC1. The currents on curves 6 and 7 are all larger than that of a solution containing 0.5 M NaCI at the same time of electrolysis. It is clear that the current increases with increasing concentration of NaC1. Curve 5 is for electrolysis of a solution containing 0.2 M aniline, 0.4 M HC1 and 1 M NaCI; its current at the beginning of electrolysis is higher than those of the two solutions containing 1 M HCI and 1 M NaC1 (curve 6) and containing 1 M HCI and 2 M NaCI (curve 7). This is due to the fact that the protonation extent of aniline decreases with decreasing acid concentration, which leads to facile oxidation of aniline in lower acid concentrations. This result is in good agreement with the cyclic voltammograms of the first cycle shown in Figs. 2-4, in which the peak current of the first cycle (curve 1 in Fig. 4) is larger than those shown in Figs. 2 and 3. Curve 1 in Fig. 7 shows the change in current with time for the electrolysis of a solution containing 0.2 M aniline, 0.3 M HCI and 1.0 M NaCI. The current for short electrolysis duration is very large, but it decreases quickly with time. The former is caused by the low protonation of aniline owing to

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60 t/s Fig. 7. Relationship between current and time during electrolysis of aniline at a constant potential of 0.78 V and in various solutions at 20 °C: ( 1) 0.2 M aniline, 0.3 M HC1 and 1.0 M NaCI; (2) 0.2 M aniline and 0.4 M HC1; (3) 0.2 M aniline and l M HCI; (4) 0.2 M aniline, 1 M HCl and 0.5 M NaCl; (5) 0.2 M aniline, 0.4 M HC1 and l M NaCI; (6) 0.2 M aniline, 1 M HCI and I M NaCl; (7) 0.2 M aniline, l M HC1 and 2 M NaCl.

S. Mu, J. Kan / Synthetic Metals 92 (1998) 149-155

the lower acid concentration; the latter indicates that no autocatalytic polymerization of aniline takes place in this solution.

3.3. Electrolysis of aniline using a rotating ring-disk electrode The solution for electrolysis consisted of 0.2 M aniline and 1.0 M HCI. The changes in the disk current (ld) with disk potential (Ed) and the ring current (It) with time are shown in Fig. 8. For the first scan from 0 to 1.15 V, Id (curve 1 ) and lr (curve 1) increase rapidly when the disk potential is over 0.75 V, and the ld peak potential and its corresponding I~peak on the Ir--t curve occur at the same time. For the second (curve 2) and third (curve 3) scans, even though the disk current decreases markedly, the ring current is still observed. This means that a soluble intermediate was generated at the disk

153

electrode. The Id-E and Ir-t curves for electrolysis of a solution containing 0.2 M aniline, 0.4 M HCI and 1.0 M NaC1 are similar to those shown in Fig. 8, so they are omitted here. Fig. 9 shows the Id-E and Ir-t curves for electrolysis of a solution containing 0.2 M aniline, 0.4 M HC1 and 2 M NaCI. The Id-E and Ir-t curves in Fig. 9 are similar to those in Fig. 8. From Figs. 8 and 9, we can see that the potential necessary for the formation of the soluble intermediate is at least 0.75 V. Figs. 10 and 11 show the Id-E d and Ir-t curves for electrolysis of a solution containing 0.2 M aniline, 1 M HC1 and 1 M NaCI, and a solution containing 0.2 M aniline, 1 M HCI and 2 M NaCI, respectively. It is clear that the disk current ld and its corresponding lr increase with increasing concentra-

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Fig. 11. ld-Ea and l~-t curves for electrolysis of a solution containing 0.2 M aniline, 1 M HCl and 2 M NaC1. Er, ring potential at 0.1 V; It, ring current; E d, disk potential; la, disk current; Er, ring potential at 0.1 V; lr, ring current; Ed, disk potential; lj, disk current.

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S. Mu, J. Kan / Synthetic Metals 92 (1998) 149-155

tion of sodium chloride, and the anodic peak potential for the first scan shifts to 0.95 V. These results are coincident with the cyclic voltammograms shown in Figs. 2 and 3. However, there is a difference between the cyclic voltammograms in Figs. 2 and 3 and the Id-E curves in Figs. 10 and 11. In the former, a wide current plateau occurs on the cyclic voltammograms for the first scan, but in the latter a prominent peak appears on the Ij-E curve. This indicates that the soluble intermediate having electroactivity generated at the disk electrode was swept away from the disk electrode due to the fast rotation, which leads to the disk current decreasing quickly with further increasing potential.

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3.4. Effects of salts on the polymerization of aniline In the presence of more than 1 M NaC1, the potential cycling experiments show that the oxidation potential of aniline shifts from 0.99 to 0.94 V and a wide current plateau occurs on the I-E curve for the first scan; the current increases markedly with the number of potential cycles. Is this effect caused by anions, cations or the ionic concentration? In order to answer this question, some experiments for the electrolysis of the aniline solutions containing different salts, such as LiC1, KC1, MgCI2 and N a z S O 4 , w e r e carried out using repeated potential cycling. Fig. 12 shows the cyclic voltammograms for electrolysis of a solution containing 0.2 M aniline, 1 M HC1 and l M KCI. The current increases rapidly with the number of potential cycles. This indicates that an autocatalitic polymerization also takes place in the presence of KC1. A prominent peak appears at 0.94 V; no current plateau forms on the cyclic voltammogram for the first cycle. It seems that the current plateau on the cyclic voltammogram for the first cycle is caused by sodium ions. In order to prove this further, solutions containing 0.2 M aniline, ! M HC1 and 1 M MgCIz, containing 0.2 M aniline, 1 M HC1 and 1 M NazSO4, and containing 0.2 M aniline, 1 M HC1 and 1 M LiC1 were electrolysed using repeated potential cycling. The results are the same as that shown in Fig. 12, i.e., no current plateau forms on the I-E curves for the first scan. This means that the current plateau is not caused by sodium ions. We must point out that the oxidation potential of aniline in the presence of LiC1 shifts to 0.88 V for the first cycle. Considering the above experimental results, the electrolysis of a solution containing 0.2 M aniline and 2 M HC1 was done using repeated potential cycling. The cyclic voltammograms are identical in shape to those of a solution consisting of 0.2 M aniline and 1 M HC1, only the peak potential of the aniline oxidation is at 1.0 V for the first scan, and no current plateau is observed. The difference is that the currents of all peaks for electrolysis of a solution containing 0.2 M aniline and 2 M HCI are much larger than that of a solution containing 0.2 M aniline and 1 M HCI for the same cycle except the peak eurrent at 1.0 V for the first cycle. The above results show that the oxidation potentials of aniline in 1 and 2 M HC1 solutions are 0.99 and 1.0 V, respectively. This small difference is caused by the protona-

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EN(vsSCE) Fig. 12. Cyclic voltammograms of polyaniline film growth from a solution containing 0.2 M aniline, I M HCI and 1 M KC1 at 48 mV s ~ and 20 °C: ( 1) first cycle; (2) second cycle; (3) third cycle; and so on.

tion of aniline. It is well known that a protonized molecule is oxidized at a higher positive potential than an unprotonated one [32]. The results also show that the oxidation potential of aniline in the presence of the salts is less than that in their absence, although the solutions for electrolysis contain the same HC1 concentration and the same anion concentration, such as chloride ions. In order to know the reason, the pH values of 0.4 M HCI solutions with and without aniline were determined. Table 1 lists the pH values of 0.4 M HCI solutions and of 0.4 M HCI solutions with the addition of 1 M of different salts. It is clear that the pH values of the solutions in the presence of the salts decrease compared with those of the solution without the salt. This indicates that the dissociation of HCI was affected by the addition of different salts. Table 2 lists the pH values of solutions containing 0.2 M aniline and 0.4 M HCI without and with salt. We found that the pH value of 0.4 M HC1 solution in the presence of aniline is 0.84 (Table 2), and that of 0.4 M HCI is 0.59 (Table 1). The difference of pH value between the two solutions is caused by the protonation of aniline. In comparison with the pH value of the corresponding solution in Table 1 and Table 2, the pH value of the corresponding solution in the Table 1 pH values of 0.4 M HC1 solutions with and without salt

pH

HCl

HCl + KCI

HCl + NaCl

HC1 + LiC1

0.59

0.46

0.39

0.31

Table 2 pH values of solutions consisting 0.2 M aniline and 0.4 M HCI without and with salt and the oxidation potential of aniline in a solution containing 1 M HCI without and with salt

pH

E / V (vs. SCE)

HCI

HCI + KCI

HCI +NaCI

HC1 +LiCI

0.84 0.99

0.71 0.94

0.54 0.92

0.48 0.88

s. Mu, J. Kan / Synthetic Metals 92 (1998) 149-155 presence of aniline (Table 2) is higher than that of Table l" this is also caused by the protonation of aniline. The addition of different salts resulted in a decrease of the pH values of the acid solutions (Table 1 ), which should cause an increase in the degree of protonation of aniline. We found that the oxidation potential of aniline in the presence of salts shifts toward the negative direction (in Table 2), which is caused by the increase of anion concentration compared Table 1 and Table 2. From Table 2, we can find that there is a difference between the oxidation potentials of the solutions of different salts. Although this difference is small, it is very significant. The oxidation potential of aniline decreases with decreasing protonation, which is caused by the ionic radius of alkaline metals. It is clear that the protonation extent of aniline decreases with decreasing ionic radius at the same concentration of salt. The polymerization rate of aniline increases with increasing concentration of salts, which is mentioned above. This means that anions take part in the polymerization reaction, i.e., anion is also a reactant. This is understandable, since the product, polyaniline, contains counter anions. During the electrolysis of aniline, the formation of the current plateau on the cyclic voltammograms for the first cycle is only caused by sodium chloride itself. The reason for this is not clear now. 3.5. Conductivity o f polyaniline The conductivity of polyaniline prepared in a solution containing 0.2 M aniline, 0.4 M HC1 and 1 M NaC1 is 2.0 S c m - T. The conductivity of polyaniline prepared from a solution containing 0.2 M aniline, 0.4 M HC1 and 2 M NaC1 is 2.4 S c m - ~. The conductivity of polyaniline prepared from a solution containing 0.2 M aniline and 0.4 M HCI is 6.7X 10 -2 S cm ~ [31]. It is clear that the conductivity of polyaniline prepared in 0.4 M HC1 solution in the presence of NaC1 is about 30 times higher than that of polyaniline prepared in 0.4 M HCI solution in the absence of NaC1. However, the conductivity of polyaniline prepared both in the solution containing 0.2 M aniline, 1 M HCI and 1 M NaC1, and in the solution containing 0.2 M aniline, 1 M HC1 and 2 M NaCI, is 2.2 S cm ~, which is the same as that of polyaniline prepared in a solution containing 0.2 M aniline and 1 M HCI [ 31 ]. The above results indicate that once an autocatalytic polymerization takes place, the polyaniline conductivity is independent of HC1 concentration in the aniline solution for electrolysis.

4. Conclusions The experimental results show that the oxidation potential of aniline in the presence of salt shifts toward negative potentials, and the polymerization rate increases with increasing

155

concentration of the salt. The former results from the increase of anion concentration; the latter is due to the tact that anion is one of the reactants. During the electrolysis of aniline in HCI solution using repeated potential cycling, the formation of the current plateau for the first cycle is only caused by NaC1. This experimental phenomenon remains to be studied.

Acknowledgements This work was supported by the National Natural Science Foundation of China.

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